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Seminar Speakers
2023
T.S. Hahm
Seoul National University, South Korea
"Effect of energetic particles on zonal flow growth"
ABSTRACT: Fast ions' effects on turbulence-driven zonal flow generation are investigated in the context of a simple reduced model based on the Hasegawa-Mima equation. Fast ions' much higher characteristic frequency of parallel motion in comparison to the drift wave's phase velocity along the magnetic field facilitates a derivation of the reduced model equations. Nonlinear mode coupling analyses show that the threshold amplitude of drift wave required for the zonal flow modulational instability is significantly reduced, making its generation easier. This occurs as a down-shift of the drift wave's frequency and a reduction of dispersion in the presence of the fast ions cause a decrease of the mismatch between the primary drift wave frequency and the zonal flow modulated sideband drift wave's characteristic frequency. This finding could be a common nonlinear physics mechanism behind numerous recent results on tokamak plasma confinement enhancement caused by the fast ions.
Yusuke Kosuga
Kyushu University, Japan
“The excitation of rogue waves in magnetized plasmas”
ABSTRACT: Rogue waves are originally observed in ocean. They induce sudden increase of wave amplitude, and then disappear. In this talk, we discuss that such rogue waves can arise in magnetized plasmas, which we here refer to as drift rogue waves. The excitation of drift rogue waves is modeled via the nonlinear evolution of the envelope modulation of underlying drift waves. We also present an evidence that drift rogue waves can be excited in actual experiments by using the data obtained from linear magnetized plasma experiment, PANTA. Implications on transport and magnetic confinement will be discussed.
Tatsuya Kobayashi
National Institute for Fusion Science, Japan
“Study on turbulent transport and transport barrier formation using perturbation experiment in LHD”
ABSTRACT: In high-temperature turbulent plasmas, there are complex transport events that cannot be explained by the quasi-linear diffusion model. These include nonlinear/nonlocal transport, transport bifurcation, and isotope effect. In order to experimentally assess those transport phenomena, the flux-gradient relation is directly described through a heating power modulation experiment. In the flux-gradient relation, flux variations having two different time scales are observed, by which a closed loop forms in a single heating modulation cycle. An apparent isotope effect is discovered in the threshold condition for the internal transport barrier formation, i.e., the threshold condition is eased as the ion mass becomes larger. This isotope effect can be ascribed as the susceptibility of the radial electric field on the plasma ion mass. In the last, a new experimental approach for investigating the phase-space turbulence is introduced as a potential framework that can explain the complex transport phenomena.
Makoto Sasaki
Nihon University, Japan
“Research on plasma turbulence by data-driven approach”
ABSTRACT: Magnetic confined plasma turbulence involves the formation of structures such as zonal flows and streamers, which determine the global transport characteristics. Although the nonlinear interaction of turbulence has been analyzed based on Fourier analysis, the correspondence with the real-space structure is not clear. Therefore, we propose a method of interaction analysis based on singular value decomposition (SVD). SVD is a data-driven mode decomposition method, by which an orthogonal basis is obtained. Analysis with a small number of degrees of freedom is possible, and the correspondence with the real-space structure is clear. In particular, we introduce a method for quantifying nonlinear interactions and transport.
2022
Steffi Diem
Department of Engineering Physics, University of Wisconsin-Madison, USA
“Exploring Transformative Startup Solutions for Magnetically Confined Fusion Plasmas”
ABSTRACT: The potential to use fusion as a carbon-free, fuel-abundant energy source to meet the world’s growing energy demands has motivated significant US and international research. One research path towards the realization of fusion energy involves tokamaks that magnetically confine hot plasmas in the shape of a torus. Almost every tokamak fusion reactor in the world relies on magnetic induction from a central solenoid to drive the current necessary to create a fusion grade plasma. Minimizing or completely eliminating the need for a central solenoid in a tokamak would greatly simplify the construction and reduce the cost of these devices, increasing their viability for commercial energy production. Solenoid-free startup techniques such as helicity injection (HI) and radiofrequency (RF) wave injection offer the potential of reducing the technical requirements of, or possibly the need for, a central solenoid. A major upgrade is underway for the spherical tokamak facility, Pegasus-III at the University of Wisconsin-Madison. The new facility will provide a dedicated US platform to study innovations in plasma startup techniques, allowing for studies of both HI and RF during plasma initiation, ramp-up and sustainment. Experimental plans for RF heating and current drive in the microwave range of frequencies will be presented. The new capabilities of the Pegasus-III facility will provide a bold test of the viability of a non-solenoidal compact tokamak using relevant power plant techniques.
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Tess Bernard
General Atomics, California, USA
“Effects of Neutral Interactions in Gyrokinetic Simulations of Magnetized Boundary Plasmas”
ABSTRACT: Interactions between the plasma and neutral particles play a crucial role in determining the exhaust characteristics of tokamak plasmas. We present the coupling of a continuum full-f gyrokinetic turbulence model with a 6D continuum model for kinetic neutrals, carried out using the Gkeyll code. Our objective is to improve the first-principles understanding of the role of neutrals in plasma fueling, detachment, and their interaction with blobby transport and edge plasma profiles. Our model incorporates electron-impact ionization, charge exchange, and wall recycling boundary conditions. We have carried out a simulation for a scrape-off layer (SOL) in simplified geometry with NSTX parameters and compared it to a case without neutrals. The former exhibits density profile flattening, reduced normalized density fluctuations and reduced plasma flow magnitudes. We show that most of the differences between the simulations can be attributed to the increased fueling rate. An analysis of blobby turbulence demonstrates that the case with neutrals has slower blobs that are more uniform in shape, which coincides with measurements from seeded blob simulations. The effect of different collisionalities is also presented. In general, these results highlight the importance of coupling first-principles turbulence models with neutrals in predictive SOL modeling.
Lucas Rovige
Laboratoire d'Optique Appliquée, CNRS, Ecole Polytechnique, ENSTA Paris, Institut Polytechnique de Paris, France
“Kilohertz Laser-Wakefield Acceleration with Near-Single Cycle Pulses: Toward Applications and Beyond Ponderomotive Force”
ABSTRACT: Laser-wakefield acceleration, achieved by focusing ultra-high intensity (I>1018 W/cm²) laser pulses in a plasma, allows the acceleration of electrons at relativistic velocities over micrometric lengths thanks to the huge electric fields (>100 GV/m) created by the plasma wave in the laser wake. The high-repetition rate regime of laser-wakefield [1] acceleration is characterized by the use of particularly short laser pulses (<5fs) approaching a single optical cycle, and an energy per pulse of the order of a few milijoules, allowing the acceleration of electrons up to a few MeV. I will present our results on the continuous operation of our kHz laser-plasma accelerator accumulating more than 18 million consecutive shots [2], made possible by the use of a newly designed type of asymmetric shocked gas jet. This enhanced stability allowed us to perform a first application experiment of our accelerator in radiobiology. Moreover, the interaction of an ultra-intense single-cycle laser pulse with an underdense plasma can lead to an asymmetry in the plasma response that depends on the carrier-envelope phase (CEP) of the laser driver [3]. I will discuss our new experimental results where we observe for the first time that the accelerated electron beam pointing oscillates in the laser polarization plane, in phase with the CEP. This effect is significant, with an oscillation amplitude as high as 15 mrad. Particle-in-Cell simulations explain this observation through highly localized, off-axis injection of sub-fs, ultralow emittance electron bunches triggered by the CEP-dependent asymmetry in the plasma wake [4]. These observations imply that we achieve sub-cycle control on the injection and subsequent dynamics of the electron beam through the waveform of the laser.
References
[1] J. Faure et al, 2019 Plasma Phys. Control. Fusion 61 014012
[2] L. Rovige, J. Huijts, I. Andriyash et al, 2020, Phys. Rev. Accel. Beams 23, 093401
[3] E.N.Nerush and I.Yu.Kostyukov, Phys. Rev. Lett. 103,035001 (2009)
[4] J. Huijts, L. Rovige et al, Phys. Rev. X 12, 011036 (2022).
Emily Belli
General Atomics, San Diego, California, USA
“Understanding the Hydrogen Isotope Effect Mystery in Tokamaks”
ABSTRACT: Understanding the scaling of the plasma energy confinement time with hydrogenic isotope mass is of critical importance for fusion research, as most tokamaks operate with deuterium (D) as the main ion species, while ITER calls for dominant hydrogen (H) operation in the first phase, transitioning to 50:50 deuterium-tritium (DT) fuel composition at reactor-level operation. Experimental observations often show confinement improving with increasing ion mass. Naive gyroBohm-scaling theoretical arguments (that ignore electron dynamics), however, predict that the turbulent ion energy flux scales with the square root of the ion mass, with the implication that the global confinement degrades with increasing ion mass. This contradiction between experiments and theoretical predictions is a long-standing problem known as the "isotope effect". In this work, a theoretical framework for understanding the role of kinetic electrons in the hydrogenic isotope mass dependence of turbulent transport, which is responsible for most thermal and particle losses in tokamaks, is presented. Using nonlinear gyrokinetic simulations of DIII-D, we illustrate a remarkable transition in the isotope mass scaling of the turbulent energy flux from gyroBohm-like in ion-dominated core transport regimes to a reversal in electron-dominated edge transport regimes. The favorable reversal in the mass scaling in the edge is due to the nonadiabatic response of the electrons, and a new analytic scaling law is developed to characterize the finite electron-to-ion mass ratio corrections to the turbulent energy flux. For particle transport in mixed DT plasmas, unfavorable DT particle flow separation due to ion finite Larmor radius effects is also discussed. Overall, these results may have favorable implications for the global energy and particle confinement and power threshold requirements for achieving H-mode regimes in a reactor like ITER in the transition from H to D to DT plasmas.
Hui Li
Los Alamos National Laboratory, New Mexico, USA
“Are There Waves in Magnetized Compressible Turbulence in Space and Astrophysical Plasmas?”
ABSTRACT: Turbulence is ubiquitous throughout magnetized plasmas in the Universe. Past research has made great progress on understanding the nature of MHD turbulence, including its anisotropy and its impact on regulating the transport of energetic particles. When the plasma beta is low and turbulent Mach number becomes large, ranging from subsonic to transonic to supersonic, turbulence becomes compressible, characterized by enhanced density fluctuations. Using extensive 3D MHD simulations and our newly developed 4D (spatio-temporal) FFT analysis, we examine in detail the nature of compressible MHD turbulence (Gan, Li, et al. 2022, ApJ, 926:222). We use two approaches to determine the presence of eigenwave modes – Alfven, Fast and Slow waves, namely the mode decomposition based on spatial variations only and spatio-temporal 4D fast Fourier transform (4D FFT) analysis of all fluctuations. The latter method enables us to quantify fluctuations that satisfy the dispersion relation of fast modes with finite frequency. Overall, we find that the fraction of fast modes identified via the spatio-temporal 4D FFT approach in total fluctuation power is either tiny with nearly incompressible driving or ∼2% with highly compressible driving. We discuss the implications of our results for understanding the compressible fluctuations in space and astrophysical plasmas, and its implications for heating and accelerating charged particles in such compressible turbulence.
Elizabeth Tolman
Institute for Advanced Study, New Jersey, USA
“Electric Field Screening in Pair Discharges and Generation of Pulsar Radio Emission”
ABSTRACT: Pulsar radio emission may be generated in pair discharges which fill the pulsar magnetosphere with plasma as an accelerating electric field is screened by freshly created pairs. In this talk, we present a simplified analytic theory for the screening of the electric field in these pair discharges and use it to estimate total radio luminosity and spectrum. The discharge has three stages. First, the electric field is screened for the first time and starts to oscillate. Next, a nonlinear phase occurs. In this phase, the amplitude of the electric field experiences strong damping because the field dramatically changes the momenta of newly created pairs. This strong damping ceases, and the system enters a final linear phase, when the electric field can no longer dramatically change pair momenta. Applied to pulsars, this theory may explain several aspects of radio emission, including the observed luminosity, 1028 erg s-1, and the observed spectrum, ω-1.4+-1.0.
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Thomas Klinger
Max Planck Institute for Plasma Physics, Germany
“The Optimized Stellarator Wendelstein 7-X Ready for the Next Operation Campaigns”
ABSTRACT: The optimized superconducting stellarator device Wendelstein 7-X (major radius 5.5 m, minor radius 0.5 m, 30m3 plasma volume) is due to restarted operation after the assembly of all-water-cooled in-vessel components including the 10 new high-heat-flux island divertor modules. In addition, we upgraded and extended significantly the heating systems.
The talk summarizes in a first part the main findings from the previous operation phase with inertially cooled in-vessel components [1]. Three major milestones were achieved: (1) Electron cyclotron resonance heated (ECRH) plasmas with power up to 7 MW in X-mode polarization were routinely obtained with densities 3-8∙1019 m-3 and electron temperatures 2-10 keV. Wall conditioning with boronization turned out to be decisive to control the impurity content (O and C). With O-mode polarization overdense plasmas with up to 1.4∙1020 m-3 were achieved. (2) With ECRH O-mode overdense heating, controlled and stable divertor detachment was successfully demonstrated for up to 30 s duration, limited by the wall load capacity only. (3) With repetitive hydrogen ice pellet injection, improved confinement with plasma density of 8∙1019 m-3 at temperature Te≅Ti=3.4 keV and 1.1 MJ diamagnetic energy was accomplished. This is due to ion temperature gradient (ITG) turbulence suppression caused by density peaking.
In the second part of the paper, we report on the current technical status of Wendelstein 7-X with a focus on needs and requirements for high-power (10 MW) long-pulse operation (100 – 1800 s). To achieve full plasma performance, the heating systems of Wendelstein 7-X are upgraded, namely the extension of the ECRH system, the extension of the neutral beam injection system (NBI), and the new ion cyclotron resonance heating (ICRH) system. The latter is mainly used for fast-ion confinement studies. Commissioning of the device, the peripheral and the control systems takes nine months before the next operation phase starts in fall 2022.
Finally, we outline the main open science questions to be adressed during the coming operation campaigns.
[1] T. Klinger et. al, Nuclear Fusion 59 (11) 112004 (2019).
Presented on behalf of the Wendelstein 7-X team.
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Peiyun Shi
West Virginia University, USA
“Electron Heating and Non-Maxwellian Distributions Observed during Electron-Only Magnetic Reconnection in PHASMA”
ABSTRACT: Electron-only reconnection is a novel type of magnetic reconnection that has recently garnered much attention since it was observed and confirmed in bow shocks of the turbulent magnetosheath [Phan et al., Nature 557, 202 (2018)]. Without significant coupling of ions, the electron-only reconnection can have higher reconnection rate and outflow speed, thought to play an important role in the turbulence energy cascade. How energy is converted from the magnetic fields to the electrons in electron-only reconnection is still not well understood observationally, theoretically, or experimentally. Direct measurements, at the kinetic scale, of particle velocity distribution functions (VDFs) have played an important role in identifying important kinetic processes explaining the energy conversion during reconnection in spacecraft observations and numerical simulations. However, complementary measurements are still lacking in laboratory reconnection experiments.
In the PHAse Space MApping (PHASMA) device at West Virginia University, we employ the incoherent Thomson scattering diagnostics to directly measure electron VDFs (EVDFs) during electron-only reconnection [Shi et al., Phys. Rev. Lett. 128, 025002 (2022)]. In this seminar, I will present the local electron heating measurement around the separatrix, corresponding to 70% of the incoming Poynting flux. Moreover, the non-Maxwellian EVDFs, composed of a warm bulk population and a cold beam, were also observed in a laboratory reconnection plasma for the first time. The oppositely directed electron beams found on either side of the X-point are clear evidence of magnetic reconnection. In the spirit of elucidating kinetic physics of observed EVDFs, efforts involving two-dimensional particle-in-cell simulations and recent upgrades of the Thomson scattering system to measure 2D and 3D EVDFs will also be introduced.
Dmitri A. Uzdensky
Center for Integrated Plasma Studies, University of Colorado Boulder, Colorado, USA
“Extreme Plasma Astrophysics”
ABSTRACT: The basic physical conditions in plasma environments around exotic astrophysical relativistic compact objects like neutron stars and black holes can be very extreme and quite different from those in more familiar, traditional heliospheric and laboratory plasmas. The physics of these extreme astrophysical plasmas is much richer than traditional plasma physics and needs to include the effects of special (and sometimes general) relativity, pair-plasma composition, strong coupling between plasma particles and high-energy photons, and, in the most extreme cases, QED effects like pair production and annihilation. Understanding how these “exotic” physical effects modify various fundamental collective plasma processes — e.g., waves, instabilities, magnetic reconnection, collisionless shocks, turbulence — is the scope of Extreme Plasma Astrophysics — an exciting and challenging frontier of modern plasma physics. I will review the rapid progress in exploring this frontier, motivated by numerous recent spectacular astrophysical discoveries and facilitated by recent computational advances such as the development of novel kinetic plasma codes that include radiation reaction and pair creation, in conjunction with concerted theoretical efforts. Examples of this progress include new breakthroughs in our understanding of radiative relativistic turbulence and magnetic reconnection with pair creation in the context of accreting black holes and neutron star magnetospheres. I will also outline promising future directions of this burgeoning field, including for laboratory studies.
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Feiyu Li
New Mexico Consortium, New Mexico, USA
“Hybrid Simulation of Alfvén Wave Parametric Decay Instability in Laboratory Plasmas”
ABSTRACT: Nonlinear Alfvén waves are subject to parametric instabilities. The parametric decay instability (PDI) is of special interest because it can produce backward propagating Alfvén waves which may trigger turbulence development. The PDI also generates compressible waves which may heat charged particles. Despite the significant implications of PDI to several heliophysics phenomena, laboratory verification of the PDI process has been challenging due to the long-wavelength nature of Alfvén waves. Recently, progress has been made on the Large Plasma Device (LAPD), where nonlinear wave-wave interactions were identified and a modulational-like instability observed [1]. However, the PDI was missing despite its significant growth rates under the conditions investigated. To resolve the puzzle, we are developing hybrid simulation capabilities to investigate the LAPD-type experiments with realistic geometry and physics conditions. As a first step, we relax the usual periodic boundary conditions adopted in the literature and study the PDI of an Alfvén wave packet in a large system under an absorption boundary [2]. The waves show different decay dynamics, including reduced energy transfer, and localized density cavitation and ion heating. These dynamics are influenced by several factors relating to the PDI, including the growth rate, central wave frequency, and unstable bandwidth. A final steady state of the wave packet may be achieved when the instability does not have enough time to develop within the residual packet, and the packet size shows well-defined scaling dependencies on the growth rate, wave amplitude, and plasma beta. We further consider the LAPD-like wave injection and outline the conditions required for observing the PDI in a laboratory plasma [3]. Current studies are based on a quasi-1D geometry. An extension to full 3D kinetic Alfvén waves is underway to quantitatively understand the experimental results. The outcome is also expected to shed light on several problems in the heliosphere such as corona/solar-wind heating and spacecraft observations.
References: [1] S. Dorfman and T. A. Carter, Observation of an Alfven Wave Parametric Instability in a Laboratory Plasma, Physical Review Letters, 116, 195002 (2016) [2] F.-Y. Li, X.-R. Fu, and S. Dorfman, Parametric decay of Alfvénic wave packets in nonperiodic low-beta plasmas, arXiv: 2108.10913; The Astrophysical Journal (in press) [3] F.-Y. Li, et al. under preparation.
2021
Xiaodi Du
General Atomics, California, USA
“Multi-scale Chirping Modes Driven by Thermal Ions in a Plasma with Reactor-relevant Ion Temperature”
ABSTRACT: A thermal ion driven bursting instability with rapid frequency chirping, assessed to be an Alfvénic ion temperature gradient mode [1], has been observed in plasmas having reactor-relevant temperature in the DIII-D tokamak [2].The modes are excited over a wide spatial range from macroscopic device size to micro-turbulence size and the perturbation energy propagates across multiple spatial scales. The radial mode structure is able to expand from local to global in ~ 0.1 ms, and it causes magnetic reconnection in the plasma edge, which can lead to a minor disruption event. The ηi (=\partial lnT_i/\partial lnn_i) exceeds the theory-predicted threshold for the destabilization of Alfvénic continuum due to compressibility of core ions. The most unstable modes belong to the strongly coupled kinetic ballooning mode and β-induced Alfvénic eigenmodes branch [3]. The key features of the observation are successfully reproduced by linear analysis solving the electromagnetic gyrokinetic equations (CGYRO code) [4]. Since the mode is typically observed in high ion temperature >10 keV and high-β plasma regime, the manifestation of the mode in future reactors should be studied with development of mitigation strategies, if needed.
*Collaborators: R.J. Hong, W.W. Heidbrink, J. Xiang, H. Wang, N.W. Eidietis, M.A. Van Zeeland, M.E. Austin, Y. Liu, N.A. Crocker, T.L. Rhodes, K. Särkimäki, A. Snicker. Supported by the U.S. DOE under award no. DE-FC02-04ER54698.
[1] F. Zonca, L. Chen and R.A. Santoro, Plasma Phys. Controlled Fusion 38 2011 (1996).
[2] X.D. Du, R.J Hong, W.W. Heidbrink, X. Jian et al., Phys. Rev. Lett. 127, 025001 (2021).
[3] I. Chavdarovski and F. Zonca, Phys. Plasmas 21, 052506 (2014).
[4] J. Candy, E. Belli and R. Bravenec, J. Comput. Phys. 324, 73 (2016).
Minjun Jhong Choi
Korea Institute of Fusion Energy, Daejeon, Korea
“Characteristics of Edge Fluctuation and Transport in the Edge Localized Mode Suppressed Plasmas by the Resonant Magnetic Perturbation Field”
ABSTRACT: The resonant magnetic perturbation (RMP) field has been utilized to suppress the edge localized mode (ELM) in tokamak plasmas. Understanding the characteristics of turbulence and transport in the RMP applied edge plasma is important to identify the RMP ELM suppression mechanism as well as to improve a transport model for the divertor heat load. Besides conventional spectral analyses, we adopted the Complexity-Entropy analysis to understand or distinguish a state of plasma turbulence and transport in this work. This statistical analysis has been known to be useful to tell a deterministic chaotic signal from a stochastic signal. We found that the Te fluctuation near the pedestal top becomes less deterministic (more stochastic) with a transition from the ELM mitigation to the suppression. This change of statistical property of the Te fluctuation is well correlated with the RMP field penetration events indicated by temperature drops, which suggests that our statistical analysis on the Te fluctuation can reflect the stochastic fields effect and provide an estimate of the width of the stochastic fields region near the pedestal top. Correlation between opening of the localized stochastic region near the pedestal top and the ELM mitigation-suppression transition supports the formation of a narrow stochastic layer at the pedestal top for the ELM suppression. On the other hand, we analyzed the particle flux at the divertor striking point in the RMP applied plasmas and found that it becomes less deterministic (more stochastic) with the increasing RMP field. This implies that a transport model including the stochasticity would be necessary to correctly describe the divertor heat load in plasmas with a strong RMP field.
Norman Cao
Courant Institute of Mathematical Sciences, New York University, USA
“Rossby Waves on the Edge of Chaos in Zonally-Dominated Flows”
ABSTRACT: The spontaneous emergence of structure is a ubiquitous process observed in fluid and plasma turbulence. These structures typically manifest as flows which remain coherent over a range of spatial and temporal scales, resulting in turbulent states which resist statistically homogeneous description. This work considers the stochastically forced barotropic beta-plane quasigeostrophic equations, a prototypical two-dimensional model for the Jovian atmosphere with connections to magnetized plasma flows. A picture for the dynamics of coherent large-scale flows observed in this system is proposed based on a transition to chaos in deterministic wave-induced Lagrangian flows. First, analysis of direct numerical simulations demonstrate that a significant fraction of the flow energy is organized into coherent large-scale Rossby wave eigenmodes, comparable to the total energy in the zonal flows. A characterization is given for Rossby wave eigenmodes as nearly-integrable perturbations to Lagrangian flow trajectories, establishing a link between the observed flows and finite-dimensional Hamiltonian chaos in the plane. Poincaré section techniques and measurements of the Lyapunov exponent of the wave-induced Lagrangian flow provide evidence that the observed amplitude of the large-scale Rossby waves combined with zonal flows is just large enough to produce a flow which is transitory between integrable and chaotic. These results suggest that the large-scale wave activity exhibits a form of self-organized criticality where the system evolves towards states associated with the onset of chaos, manifesting as Rossby wave breaking without critical layers. Finally, extensions of the developed methods are proposed for other plasma and fluid systems.
Livia Casali
Department of Nuclear Engineering, University of Tennessee Knoxville, USA
“Core–Edge Integration in Tokamaks”
ABSTRACT: Developing a core-edge solution for power handling while maintaining high core performance in tokamaks is a critical path toward practical fusion energy. This requires dissipating the heat and particles flowing towards the wall without reducing the performance of the core. Core-edge compatibility scenarios will be discussed with emphasis on the integration of high radiative scenario with detachment in different configurations. Since the ability to understand and control impurities in the plasma is vital for the operation of future devices and the achievement of a core-edge solution, we will examine how to balance mitigated divertor heat flux against core contamination. We will analyze the effect of different radiators on detachment, pedestal and core confinement in both experiments and computational modeling. Insights on the mechanism leading to impurity leakage out of the divertor and core impurity accumulation will be presented. The results will show that improved core-edge compatibility can be obtained by choosing appropriate radiative species for power exhaust, optimizing plasma shape, divertor geometry and plasma flows for particle entrainment.
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Zhihong Lin
University of California Irvine, USA
“Gyrokinetic Simulations of Neoclassical and Turbulent Transport in 3D Toroidal Plasmas”
ABSTRACT: Neoclassical and turbulent transport are intrinsically coupled in 3D toroidal plasmas such as stellarators and tokamaks with resonant magnetic perturbations (RMP). I will highlight recent progress on the 3D physics including the discovery of a new helically trapped electron mode (HTEM) in the W7-X from the first global gyrokinetic simulations with kinetic electrons, the zonal flow dynamics in the W7-X and LHD, and the generation of neoclassical ambipolar electric field and its effects on microturbulence in the W7-X and DIII-D with RMP. Plan for coupled neoclassical and turbulence simulation will also be described.
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Archie Bott
Department of Astrophysical Sciences, Princeton University, USA
“Investigating Turbulent-Dynamo Processes With Laser-Plasma Experiments”
ABSTRACT: The phenomenon of magnetic-field amplification due to the motion of turbulent plasma has been investigated in a series of experiments carried out at various high-energy laser facilities during the last few years. Plasma jets driven by intense laser irradiation pass through asymmetric grids, the collide head on, leading to developed turbulence. Thomson-scattering, soft-X-ray-imaging and proton-radiography diagnostics have allowed for a thorough characterization of the plasma state, including measurements of temperature, flow velocities, turbulent spectra, and magnetic fields. Our key finding is that at sufficiently large magnetic Reynolds numbers, magnetic fields are amplified very efficiently, attaining dynamical strengths. The robustness of this conclusion has been confirmed subsequently via several extensions of the original experimental configuration. Our results lend support to theoretical expectations that plasma turbulence is responsible for the magnetic fields universally observed in various astrophysical environments, from stars to the intra-cluster medium.
Florence Marcotte
Inria Sophia Antipolis-Méditerranée & Laboratoire Jean Alexandre Dieudonné, Université Côte d'Azur, France
“Dynamo Action in Radiative Stellar Layers”
ABSTRACT: Observations have revealed that many stellar cores spin much slower than predicted by theoretical models, and that rotation profiles across radiative zones are considerably flatter than they ought to be. This important mismatch between observational constraints and stellar evolution models betrays the existence of a powerful mechanism for angular momentum transport in radiative layers. I will describe how intense magnetic fields can be generated by dynamo action in differentially rotating radiative zones. The resulting magnetic fields are shown to trigger magnetohydrodynamic turbulence and achieve efficient transport of angular momentum, gradually flattening rotation profiles. This work is a collaboration with Ludovic Petitdemange (LERMA) and Christophe Gissinger (ENS).
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Saskia Mordijck
College of William & Mary, Virginia, USA
“Electron Density: Role of Fueling Versus Transport”
ABSTRACT: The fusion gain in a tokamak is directly linked to the density of the plasma. However, due to the high temperatures, it impossible to fuel the core of the plasma directly and increase the core density. Without any direct fueling in the core of a tokamak, the plasma density is fully controlled by transport perpendicular to the confining magnetic field surfaces. In this talk, I will show how cross-field transport is dominated by turbulence in the plasma core by comparing experiments with existing models. These models capture how various types of turbulence influence transport and thus the density profile. While the density profile in the core is fully determined by turbulent transport, at the plasma edge, the picture is more complicated. At the edge of the tokamak, turbulent transport effects intermingle directly with fueling through ionization of the surrounding gas. To better understand the impacts of turbulence on the particle flux, we perform a series of experiments on LAPD varying the neutral density and electron density gradient. While some trends follow linear predictions of resistive drift wave turbulence, other phenomena cannot be explained using linear predictions.
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Peter Read
University of Oxford, UK
“Zonal Jets on Jupiter and Saturn: New Developments and Open Questions”
ABSTRACT: The banded structure of clouds and zonal winds on the gas giant planets, Jupiter and Saturn, has intrigued scientists since the time of Galileo. Observational and theoretical research over the past 40 years has provided much valuable insight into how planetary rotation and the spherical shape of a planet can influence its atmospheric dynamics and cause it spontaneously to form strong zonal jets. Until recently the focus has been on qualitatively capturing key features of the flow observed near the tops of the visible clouds, though the extent to which the observed cloud-level circulation extends coherently to deeper levels has been highly uncertain. The Juno and Cassini missions, however, have shifted the observational focus towards these deeper layers, offering the prospect of new insights into the three-dimensional structure of the atmospheric circulation throughout the electrically neutral outer fluid envelope of each planet. In this talk I will present a brief overview of our current understanding of the dynamics of the zonally banded jets on Jupiter and Saturn, including new insights coming especially from the Juno and Cassini missions. Despite these new advances, however, many other uncertainties remain and I hope to highlight and discuss some of these in the latter part of the talk.
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Zach Hartwig
MIT, USA
“High-Field Superconducting Magnet Technology for SPARC, a Compact Net-Energy Fusion Tokamak”
ABSTRACT: Large-scale high temperature superconductor (HTS) magnets that exceed 20 tesla offer an accelerated path to commercial fusion energy in the 2030s. However, in a high-field fusion device, magnets must survive extreme mechanical, electrical, and thermal conditions and possess simple, low-resistance, and manufacturable electrical joints, high thermal stability, and robust strategies to handle thermal runaway quench events. Furthermore, the technology to solve these challenges must be qualified at the scale and maturity necessary to begin rapid manufacturing of magnets to meet aggressive timelines. Since 2018, the MIT Plasma Science and Fusion Center and Commonwealth Fusion Systems have jointly been developing high-field HTS magnet technology for SPARC, a compact fusion tokamak seeking to generate >50 megawatts of fusion power with a gain factor of >2 in the mid-2020s. This seminar will provide an overview of (1) the SPARC tokamak and the underlying physics motivating the high-field approach; (2) demonstration of 50 to 100 kiloamp class HTS superconducting cables for large-scale magnets; and (3) the SPARC Toroidal Field Model Coil, the largest HTS magnet to date by several orders of magnitude that will demonstrate 20 tesla operation in the summer of 2021.
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Steven Tobias
University of Leeds, UK
“Magnetic Helicity Fluxes and Large-Scale Dynamos”
ABSTRACT: The generation of large-scale magnetic field by dynamo action at high magnetic Reynolds number is a central question for astrophysics. Theoretical and numerical considerations indicate that the near reversibility of the system may lead to large-scale field only being generated on a long (ohmic) timescale. In this talk I shall discuss whether this is really a problem for astrophysics and whether loss of magnetic helicity through boundaries or inhomogeneities can remedy some of the issues.
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Siyao Xu
Institute for Advanced Studies, New Jersey, USA
“Small-Scale Turbulent Dynamo in Astrophysical Environments: Nonlinear Regime”
ABSTRACT: The small-scale turbulent dynamo is an important mechanism for amplifying magnetic fields in diverse astrophysical media. Earlier analytical studies focus on the kinematic regime where the magnetic back reaction on turbulence is insignificant. In the nonlinear regime, the turbulent diffusion of strong magnetic fields is enabled by the turbulent magnetic reconnection, which dominates over other non-ideal MHD effects in determining the diffusion of magnetic fields and thus the growth rate of magnetic energy during the nonlinear stage. The analytical theory on nonlinear turbulent dynamo that I will discuss naturally explains the inefficient dynamo growth seen in nonlinear dynamo simulations, as well as the large correlation length of turbulent magnetic fields measured in observations.
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Robert J. Goldston
Princeton University, USA
“A Model for the Tokamak Scrape-Off Layer and its Implications for H-Mode Physics”
ABSTRACT: A simple non-turbulent model can be constructed for the width of the scrape-off layer just outside the edge of the confinement zone of a tokamak plasma. This model has been shown to match experimental data well in the relatively quiescent H-Mode condition, at the densities and collisionalites normally encountered with modest gas fueling. It has now been generalized to address a wider range of density and collisionality and then used to predict the ExB shearing rate (omega_s) just outside the separatrix. This can be compared with the ideal, short-wave-length interchange growth rate, (gam_int), and there appears to be a good correlation in ASDEX-Upgrade between H-Mode conditions and omega_s/gam_int > 0.4. This is the point where, theoretically, the interchange eigenmode disappears. As the density and collisionality increase, the generalized scrape-off layer model predicts that omega_s/gam_int falls. In ASDEX-Upgrade as omega_s/gam_int is predicted to drop below 0.4 with rising density, the H-Mode condition is lost. This suggests that the H-Mode may at least have aspects of a “tail-wags-dog” scenario, where the suppression of interchange turbulence in the region just outside the edge of the confined plasma facilitates the formation of a transport barrier just inside.
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Laszlo Bardoczi
General Atomics, USA
“Neoclassical Tearing Mode Seeding by Nonlinear Three-Wave Interactions in Tokamaks”
ABSTRACT: We report experimental observation of seed magnetic island formation by nonlinear three-wave coupling of magnetic island triplets. In this experiment, disruptive 2,1 islands are seeded by coupling of 4,3 and 3,2 tearing modes to a central 1,1 sawtooth precursor. Three-wave interactions between these modes are conclusively identified by bi-spectral analysis, indicating fixed phase relationships in agreement with theory. This new seeding mechanism has important implications for future reactors that must operate in stable plasma equilibria, free of disruptive 2,1 islands.
Ethan T. Vishniac
Johns Hopkins University & American Astronomical Society, USA
“Magnetic Fields in Accretion Disks”
ABSTRACT: Magnetic fields play a crucial role in the accretion of plasma onto collapsed objects, producing some of the most luminous and energetic objects in the universe. These magnetic fields may be accreted from the environment or self-generated due an internal dynamo. The latter is a particular interesting example of a large-scale dynamo in a highly conducting environment. Amusingly, there are famous no-go arguments against both processes. I will discuss how these arguments can be evaded, and what it tells us about magnetic field dynamics in disks and in other astrophysical objects.
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Matthew W. Kunz
Princeton University, USA
“Cosmic Magnetism and Plasma Microphysics (or, I get by with help from my little friends)”
ABSTRACT: The Universe is magnetized. While magnetic-field strengths of just ~10^{-18} G are required to achieve this both in our Galaxy and in clusters of galaxies, observations of Faraday rotation, Zeeman splitting, and synchrotron emission all make the case of ubiquitous ~μG fields. That these systems are not content with hosting weaker fields is surprising, at least until one realizes that the energy density of a ~μG field is comparable to that of the observed turbulent motions. It is then natural to attribute the amplification and sustenance of (at least the random component of) the interstellar and intracluster magnetic fields to the fluctuation (or "turbulent") dynamo. In this talk, we will explore the various ways in which plasma microphysics makes magnetic-field amplification in weakly collisional plasmas by macroscale turbulent motions both possible and efficient, with application to the intracluster medium of galaxy clusters. Results from hybrid-kinetic, Braginskii-MHD, and high-resolution MHD simulations will be featured alongside analytical arguments. The predictions of this work are consistent with (i) deep Chandra X-ray observations of turbulence in the Coma cluster that suggest an anomalously low viscosity, (ii) recent LOFAR observations of diffuse radio emission in massive clusters when the Universe was only half of its present age, (iii) Faraday rotation maps of intracluster magnetic fields revealing structure on scales ~1-10 kpc, and (iv) plasma kinetics measured directly by in situ spacecraft in the solar wind.
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Nicola Bonanomi
Max Planck Institute for Plasma Physics, Garching bei München, Germany
“Gyro-kinetic Analysis of the L-mode Edge Turbulent Transport in Tokamaks”
ABSTRACT: The plasma edge region determines the confinement regime in a tokamak. The edge turbulence level regulates the transition from low- (L-mode) to high-confinement (H-mode) regimes and is the responsible for the characteristics of high-confinement regimes such as the I-mode and the EDA H-mode. Understanding the properties of the edge turbulent transport is then crucial for the improvement and the control of the different plasma regimes and for the prediction of future devices plasmas such as ITER and DEMO. We analyzed the edge turbulent transport combining observations from ASDEX Upgrade and JET-ILW L-mode plasmas and related gyro-kinetic turbulence simulations. We focus on the role of the ion isotope mass as well as on the role of the temperature and density profiles in driving or stabilizing the edge turbulent transport. The impact of the main ion mass is observed to be important in many tokamak experiments, and especially it strongly determines the heating input power needed for the low- to high- confinement transition. This transition is believed to be associated to the development ofa strong shear in the drift velocity related to the edge radial electric field, this field being strongly related to the ion temperature and density profiles. Our gyro-kinetic results reproduce quantitatively the experimental fluxes and show that the edge high collisionality favors instabilities strongly affected by the parallel electron dynamics. The corresponding linear term in the gyro-kinetic equation, proportional to the electron to ion mass ratio, leads to increased transport at lower mass, an effect which is magnified when electromagnetic effects are included. The analysis suggests a path towards edge turbulence stabilization in which the evolution of the ion temperature, connected with an increased ion heat flux, leads to increased velocity shearing without driving the turbulence. It shows also differences in the turbulence response to a change in density or temperature profiles and gives new indications for the development of reduced models for the edge turbulent transport.
Richard E. Wirz
Plasma & Space Propulsion Laboratory, University of California Los Angeles, USA
“Persistent Sputtering Yield Reduction in Plasma-Infused Materials for Plasma Thrusters and Fusion”
ABSTRACT: Material sputtering provides challenges for plasma devices from space propulsion to fusion. In this talk, Prof. Wirz will discuss plasma-material interactions for plasma thrusters and recent developments towards fusion plasma materials. Plasma thrusters are predominantly driven by thruster component lifetime through erosion and ion and electron induced “facility effects” during ground testing that obfuscate the thruster’s anticipated on-orbit performance. Recent developments in these areas have revealed material approaches that may help fusion devices, which must address both component lifetime and plasma performance via reduction of contaminants.
In particular, we have shown that metallic foams provide geometric trapping and interface/sheath manipulation that can exhibit persistent sputtering yield reductions of 40% to over 80% compared to a flat surface under 100 to 300 eV argon plasma bombardment. These results show a strong yield dependence based on the relationship between foam geometry and plasma sheath size. For foam pore sizes near or larger than the sheath thickness, the plasma infuses the foam and transitions the plasma-surface interactions from superficial to volumetric phenomena. By defining a plasma infusion parameter, the sputtering behavior can be separated into the plasma-facing and plasma-infused regimes. While plasma infusion leads to a larger effective sputtering area, geometric recapture of ejected particles facilitates an overall reduction in yield. For a given level of plasma infusion, the reductions in normalized yield are more pronounced at lower ion energies since angular sputtering effects enable more effective geometric recapture of sputterants. Opportunities for plasma-infused materials in space propulsion and fusion will be discussed.
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Max Austin
Institute for Fusion Studies, University of Texas, USA
“Negative Triangularity Plasmas in the DIII-D Tokamak: A Novel Solution for Good Confinement and Power Handling in a Fusion Reactor”
ABSTRACT: Plasma discharges with an unconventional “inverse D” cross sectional shape, as opposed to the typical “D” shape, have been developed in the DIII-D tokamak. These inverse D or negative triangularity shaped plasmas have achieved significant normalized plasma pressure values (beta_N) of up to 3.0 and high normalized confinement factors, H98y2 = 1.1, all while robustly maintaining a low-confinement-type edge that is free of the edge localized mode (ELM) instability. The high confinement in these discharges is attributed to the shape which is theoretically predicted to weaken the turbulent modes that cause anomalous transport. The reduced fluctuation levels are also observed experimentally. The diminished turbulent losses in turn make the global energy confinement sufficient for a fusion device despite its low-confinement-type (L-mode) edge. At the same time, this L-mode edge precludes the development of ELMs that can eject possibly damaging levels of energy and particles on to a reactor’s walls. Moreover, the geometry of this configuration is amenable to a divertor system placed on the outboard side of the tokamak, which offers larger area for heat dissipation and the divertor would be more feasible to build and maintain. Additionally negative triangularity discharges have demonstrated high values of self-generated bootstrap current, relatively wide scrap-off-layer widths and low impurity retention. These properties of negative triangularly make it an attractive option for a next step fusion device on the road to developing a reactor.
William Irvine
University of Chicago, Illinois, USA
“The Life of Vortex Knots and Links and the Conservation of Helicity”
ABSTRACT: What happens if you take a vortex loop — akin to a smoke ring in air — and tie it into a knot? The possibility of such knottiness in a fluid has fascinated physicists and mathematicians ever since Kelvin’s ‘vortex atom’ hypothesis, in which the atoms of the periodic table were hypothesized to correspond to closed vortex loops of different knot types. More recently, helicity — a measure of knottiness in fluids and plasmas — has re-emerged in fluid dynamics because, as a conserved quantity, it offers the potential for fundamental insights and control. I will tell of how to make a vortex knot and link in water (in experiment), in the wave function of a superfluid (on a computer) and of what happens thence, in particular how the fabric of helicity provides a lens on its evolution in classical and quantum fluids.
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Susanna Cappello
National Research Council – RFX Consortium, Italy
“Studies of Helical Self-Organization Processes in the Reversed-Field Pinch”
ABSTRACT: We will review the physical understanding and key results from visco-resistive 3D nonlinear MHD modeling of Reversed-Field Pinch helical self-organization processes. Magnetic transport-barrier formation and nearly periodic reconnection events are found to be at play [1-4], akin to the experimental observation of thermal transport barriers and residual “back-transition” cycles when approaching helical regimes in high current discharges [5-8]. A quasi laminar dynamo effect is made by one preferred dominant MHD mode, producing the typical helical kinking of the plasma core. We will describe the recent successful technique to “channel” the system towards chosen “stimulated” macroscopic helical shapes by applying suitable (either Resonant or Non-Resonant) Magnetic Perturbations at the edge of the plasma, as predicted by nonlinear MHD modeling and observed in recent RFX-mod experiments [2]. In so doing, we are able to modify the transport properties of the configuration, with the two-fold objective of developing “handles” for the understanding of transport barrier formation processes and exploring new routes for optimization of pinch configurations. We have found that the magnetic chaos healing effect by helical structure development [9]appears to be more robust in the case of Non-Resonant helical regimes [2]. This line of research will be further explored in the upgraded RFX-mod2-device [10] in Padova-Italy, expected to start operation in 2022.
References:
[1] S. Cappello et al., Nuclear Fusion 51, 103012(2011); S. Cappello PPCF 46, B313 (2004)
[2] M. Veranda, D. Bonfiglio, S. Cappello, et al., Nuclear Fusion 57, 116029 (2017)
[3] F. Pegoraro, D. Bonfiglio, S. Cappello, G. DiGiannatale, M V Falessi, D. Grasso and
M. Veranda,PPCF 61, 044003 (2019)
[4] Veranda M. et al Magnetic reconnection inthree-dimensional quasi-helical pinches. Rend. Fis. Acc. Lincei (2020)
https://link.springer.com/article/10.1007/s12210-020-00944-4
[5] D. Bonfiglio, M. Veranda, S. Cappello, et al., Phys. Rev. Lett 111, 085002 (2013)
[6] R. Lorenzini, et al., Nature Physics 5, 570(2009)
[7] P. Piovesan, M. Zuin, et al., Nuclear Fusion 49,085036 (2009)
[8] J.S. Sarff, et al., Nuclear Fusion 53, 104017(2013)
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Mikhail V. Medvedev
University of Kansas & MIT, USA
“Magnetic Monopole Plasma on Cosmic Scales”
ABSTRACT: Existing theoretical and observational constraints on the abundance of magnetic monopoles are limited. In this talk, we demonstrate that an ensemble of monopoles forms a plasma whose properties are well determined and whose collective effects place new tight constraints on the cosmological abundance of monopoles. In particular, the existence of micro-Gauss magnetic fields in galaxy clusters and radio relics implies that the monopoles may contribute less than 0.1% of the total matter content in the Universe, which rules them out as a dark matter candidate. Future detection of gigaparsec-scale coherent magnetic fields could improve this limit by a few orders of magnitude. If the monopoles exist, we also predict the existence of magnetic Langmuir waves and turbulence which may appear on the sky as “zebra patterns” of an alternating magnetic field with non-vanishing (k.B), which can be excited near an accretion shock of a galaxy cluster and may be an efficient mechanism for generating the observed intra-cluster magnetic fields.
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John P. Palastro
Laboratory for Laser Energetics, University of Rochester
“Laser–Plasma Interactions Driven by Spatiotemporally Structured Light Pulses”
ABSTRACT: The substantial bandwidth of modern laser pulses combined with the creative use of optical elements presents a new paradigm for optimizing or realizing laser–plasma interactions–spatiotemporal pulse shaping. In the far field, a conventional laser pulse has separable space and time dependencies that severely limit how the pulse can be structured. Spatiotemporal pulse shaping provides the flexibility to structure a pulse with advantageous space–time correlations that can be tailored for a desired interaction. As an example, stretching the region over which a laser pulse focuses and adjusting the relative timing at which those foci occur provides control over the velocity of an intensity peak independent of the group velocity and maintains the high intensity of that peak over distances unconstrained by diffraction. Here we will review techniques for spatiotemporal pulse shaping; how it promises to advance applications such as plasma channel formation, photon acceleration, vacuum laser acceleration, and laser wakefield acceleration; and how it can be used to study fundamental plasma physics such as inverse Compton scattering and Fermi acceleration. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856 and the U.S. Department of Energy Office of Fusion Energy Sciences under contract no. DE-SC0016253.
View Dr. Palastro's Laboratory for Laser Energetics profile.
John E. Rice
Plasma Science and Fusion Center, MIT, USA
“Understanding Ohmic Confinement Phenomenology in Tokamaks”
ABSTRACT: Phenomenology of Ohmic energy confinement saturation in tokamaks is reviewed. Characteristics of the linear Ohmic confinement (LOC) and saturated Ohmic confinement (SOC) regimes are documented and transformations in all transport channels across the LOC/SOC transition are described, including rotation reversals, "non-local" cut-off and density peaking, in addition to dramatic changes in fluctuation intensity. Unification of results from nearly 20 devices indicates that the LOC/SOC transition occurs at a critical value of the product of the density, edge safety factor and device major radius, and that this product increases with toroidal magnetic field. Comparison with gyro-kinetic simulations suggests that the effects of sub-dominant TEMs are important in the LOC regime while ITG mode turbulence dominates with SOC.
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John A. Goree
Dept. of Physics and Astronomy, The University of Iowa, USA
“Shock Waves Observed Experimentally at the Particle Level in a Dusty Plasma”
ABSTRACT: Many kinds of plasmas exhibit shock waves, but efforts to understand them at the particle level are usually limited to simulations. For experiments, it is impractical to observe at the particle level in most plasmas, with the notable exception of dusty plasmas. A dusty plasma includes small solid particles, which gain their electric charges by collecting more electrons than ions. When these solid particles are larger than a micron, they scatter light efficiently, so that their motion can be tracked using video microscopy as a diagnostic. In this talk I describe shock-wave experiments, with spatially and temporally resolved measurements at the particle level. In the experiments, an exciter is moved at a controlled supersonic speed to produce a shock, and this is done either steadily to make a continuously driven shock, or impulsively to make a blast wave. Three results will be presented. First, the thickness of the shock is found to be a few interparticle spacings. Second, the Mach numbers of shock and exciter have a relationship that is obtained, and this result reflects the equation of state of the strongly coupled dusty plasma. Third, it is found that the shock wave's amplitude is sustained despite the presence of strong collisional damping, and this is explained by an input of energy from a Buneman-like plasma instability, driven by ion flow. Supported by U.S Department of Energy grant DE-SG0014566 and Army Research Office award W911NF1810240, subaward No. A006827502.
Pavel Kovtun
University of Victoria, Canada
“Pushing the Limits of Hydrodynamics”
ABSTRACT: Hydrodynamics is a well-established field with a venerable history. In this talk, I will focus on foundational aspect of hydrodynamics which came to light in recent years. Do the equations of hydrodynamics even make sense? To what degree can the crudeness of hydrodynamics be improved? What about the phenomena that hydrodynamics should describe but fails to? And what about the phenomena that hydrodynamics shouldn't describe, but does?
2020
Guilhem Dif-Pradalier
CEA, France
“Plasma–Boundary Interplay: Incidence on Barrier Formation and Turbulence Organisation”
ABSTRACT: For over three decades, the observation of rapid core confinement improvement upon favourable modifications of edge operating conditions has been a nagging source of puzzlement for experimentalists investigating conditions for a lasting source of fusion energy in tokamaks [1]. The transport properties of drift-wave/interchange turbulence and the interaction of the confined plasma with its material boundaries have long been recognised as essential to the resolution of this conundrum [2]. Key aspects of the turbulent dynamics in the plasma edge are poorly quantified, owing to the disparity of temporal and spatial scales and the inadequacy of performing scale separations. "First-principles" approaches are only really useful insofar as they test basic ideas or open avenues for basic understanding. It is in that spirit that, relaxing oft-made scale separation assumptions in the edge a narrow region at the interface between open and closed magnetic field lines seems central to explaining the transport properties of turbulence, on more global scales. This `not local’ influence is mediated through localised interactions with the material boundaries and is responsible for the emergence of a stable, localised and persistent transport barrier in the plasma edge. These results suggest a framework of understanding where turbulence is not only locally driven by the local gradients but centrally influenced by fluxes of turbulence activity, primarily though not exclusively coming from the edge [3,4]. In particular, we discuss vorticity-related mechanisms and their observed impact on transport barrier formation. These questions are certainly important in the perspective of accessing high confinement regimes, a central question for fusion.
References
[1] F. Wagner et al., Phys. Rev. Lett. 49, 1408 (1982)
[2] B.B. Kadomtsev, Tokamak Plasma: A Complex Physical System (London: IoP Publishing) (1992)
[3] X. Garbet et al., Nucl.Fusion 34, 963 (1994)
[4] R. Singh and P.H. Diamond, Phys. Plasmas 27, 042308 (2020)
Lothar Schmitz
Department of Physics & Astronomy, UCLA, USA
“Reducing the L-H Transition Power Threshold in ITER — What Can We Learn from Microscopic Transition Physics?”
ABSTRACT: High confinement mode (H-mode) operation is essential for the International Thermonuclear Experimental Reactor (ITER) and future burning plasmas, yet no predictive physics-based model exists so far for the required L-H transition threshold power. We demonstrate here on DIII-D that fast (0.1-1 ms) electric field transients, quantitatively consistent with the radial polarization current, initiate the transition once the transient E×B shearing rate exceeds the plasma frame turbulence decorrelation rate [1]. Edge turbulence is subsequently suppressed as the Reynolds stress increases, allowing the edge transport barrier to form.
Significant differences in transition dynamics are observed between Helium and Hydrogen plasmas (important for pre-nuclear physics operation in ITER). In recent experiments, we have demonstrated that the observed high power threshold in hydrogen (a substantial challenge for ITER) could be reduced by Helium dilution. Initial experiments also indicate that the L-H power threshold can be reduced at low ion collisionality via Neoclassical Toroidal Viscosity (NTV) from applied n=3 non-resonant magnetic fields (NRMF).
[1] L. Schmitz et al., Phys. Rev. Lett. 108, 155002 (2012).
Nuno F. Loureiro
Nuclear Science & Engineering, MIT, USA
“New Insights into Plasma Turbulence”
ABSTRACT: The current understanding of plasma turbulence in the MHD regime suggests that turbulent eddies become progressively more elongated structures in the plane perpendicular to the local mean magnetic field. Using recent results from reconnection theory, we argue that such eddies must inevitably fall prey to the tearing instability. As a result, there is a transition, in the inertial range, to a tearing (reconnection)-mediated turbulent cascade, where the spectral scaling k^{-11/5} is predicted. These results can be extended to collisionless plasmas; in the different regimes of interest that have been explored, the magnetic energy spectral scaling is invariably found to be in the range k^{-8/3} to k^{-3}, consistent with many observations.
To learn more, visit Prof. Loureiro's faculty profile and group website.
David Hughes
Leeds University, UK
“Inertialess Dynamos in Rotating Convection”
ABSTRACT: One of the great challenges in planetary magnetohydrodynamics is to understand how magnetic fields are regenerated (the dynamo process) in a convecting fluid that has little inertia or viscosity (more precisely, small Rossby number and small Ekman number). Here I shall consider dynamos driven by formally inertialess convection; this approach has the twin advantages of being in the relevant physical regime and also allowing a useful decomposition of the momentum equation. I shall show how two very distinct types of dynamo are possible (weak and strong field dynamos), how the crucial balance of forces differ in the two cases and what this might tell us about the Taylor constraint.
Martin Greenwald
Plasma Science and Fusion Center (PSFC), MIT, Massacheusetts, USA
“SPARC and the High-field Path to Fusion Energy”
ABSTRACT: The SPARC tokamak is currently under design as a mid-sized, DT burning magnetic confinement experiment. By employing novel high-temperature superconducting magnets, it will achieve 12.2 T, 8.7 MA in a modest-sized device with R = 1.85 m and a = 0.57 m. The SPARC physics mission is to create and confine a plasma that produces net fusion power and retire risks on the high-field path to commercial fusion energy. The performance to satisfy that mission has been defined as Q > 2 and PFusion > 50 MW which would be comfortably more than the 25 MW of ICRF input power. Achieving this goal, we believe, would be a sufficient demonstration to place fusion firmly into the world’s energy plans. Significant margin against uncertainties in performance assumptions has been built into the design such that well-established physics predicts that SPARC could produce more than 140 MW of fusion power with Q > 10. Successful operation of SPARC would inform and enable the construction of an ARC-class fusion pilot plant – a device with a major radius on the order of 3 m, producing over 500 MW of fusion power.
Evdokiya Kostadinova
Baylor University, Texas, USA
“More is Different. Anomalous Is Normal. That’s Why Turbulence.”
ABSTRACT: In a famous 1972 publication, Philip Anderson argued that the behavior of complex systems cannot be reduced to the interactions of elementary entities. Instead, at each level of complexity entirely new properties emerge due to the many-body correlations involved. Simply put, more is different. While non-interacting particles move in a random fashion, called normal diffusion, correlated particles move in a less random way, called anomalous diffusion. Anomalous diffusion is so common in the natural world that scientists often conclude: anomalous is normal. The marriage between increasing complexity and anomalous transport often results in turbulent dynamics of the many-body system. Dusty plasmas are ideal media for the investigation of these phenomena.
Here we study turbulence in a dusty plasma by computing the spectrum of energies available to the dust particles as a function of random disorder and properties of nonlocal interactions mediated by the plasma. We argue that at critical scales within the system, anomalous dust diffusion, guided by nonlocal interactions, leads to enhancement of energy transport and increased probability for turbulent dynamics. These theoretical predictions are compared against results from many-body simulations and dusty plasma experiments conducted on board the International Space Station.
All authors acknowledge the joint ESA / Roscosmos “Experiment Plasmakristall-4” onboard the International Space Station. This research is funded by NSF-1903450, NSF- 1707215, NASA- 1571701, DE-SC0021284.
Arianna Gleason
SLAC/Stanford University, California, USA
“New Light on the Frontier of Matter in Extreme Conditions”
ABSTRACT: The study of matter under extreme conditions is a highly interdisciplinary subject with broad applications to materials science, plasma physics, geophysics and astrophysics. Understanding the processes which dictate physical properties in warm dense plasmas and condensed matter, requires studies at the relevant length-scales (e.g., interatomic spacing) and time-scales (e.g., phonon period). Experiments performed at XFEL light sources across the world, combined with dynamic compression, provide ever-improving spatial- and temporal-fidelity to push the frontier. This talk will cover a very broad range of conditions, intended to present an overview of important recent developments in how we generate extreme environments and then how we characterize and probe matter at extremes conditions– providing an atom-eye view of transformations and the fundamental physics dictating materials properties. Examples of case-studies closely related to Earth and planetary science relevant materials will be discussed.
Richard J. Buttery
General Atomics, San Diego, CA, USA
“The Advanced Tokamak Path to a Compact Fusion Pilot Plant”
ABSTRACT: The Advanced Tokamak concept provides an attractive path to develop a compact and inexpensive “pilot plant” to demonstrate net electricity and resolve nuclear technology and breeding. It works through a fortuitous alignment of high-pressure operation with strong self-driven ‘bootstrap’ current and low turbulent transport. Here, great research progress in transport, pedestal, stability and energetic particle physics has identified the key principles behind a solution, which will be explained in this talk. Furthermore, new “full physics” simulations show the trade-offs and path to optimize the approach: raising pressure increases fusion performance, but increasing the density has greater leverage, raising bootstrap current and decreasing auxiliary current drive demands from expensive RF systems. The efficient solutions found have high energy confinement, reducing the necessary fusion performance, heat and neutron loads for a net electric goal. Viable devices are predicted with a compact major radius of ~4m radius giving 200MW electricity at ~6T using conventional superconductors, or better still using high Tc superconductors at 7T, which provides greater performance margins and permits easier maintenance for the nuclear research mission. The plasma exhaust is managed by a combination of core radiation, flux expansion and radiative divertor, although the challenges continuous operation requires further configuration research to reduce erosion. Overall this Compact Advanced Tokamak (CAT) approach provides an attractive and robust path to fusion energy, with high performance and stability. Further exciting research is underway at the DIII-D National Fusion Facility and elsewhere to validate the approach and test key technologies to make such a device a reality.
Edward Thomas, Jr.
Department of Physics, Auburn University — Auburn, Alabama, USA
“Using Magnetic Fields and Microgravity to Explore the Physics of Dusty Plasmas”
ABSTRACT: Over the last three decades plasma scientists have learned how to control a new type of plasma system known as a “complex” or “dusty” plasma. These are four-component plasma systems that consist of electrons, ions, neutral atoms, and charged, solid, nanometer- to micrometer-sized particles. The presence of these microparticles allow us to “tune” the plasma to have solid-like, fluid-like, or gas-like properties. This means that dusty plasmas are not just a fourth state of matter – they can take on the properties of all four states of matter.
From star-forming regions to planetary rings to fusion experiments, charged microparticles can be found in many naturally occurring and man-made plasma systems. Therefore, understanding the physics of dusty plasmas can provide new insights into a broad range of astrophysical and technological problems. This presentation introduces the physical properties of dusty plasmas – focusing on how the small charge-to-mass ratio of the charged microparticles gives rise to many of the characteristics of the system. In particular, dusty plasmas can be used to study a variety of processes in non-equilibrium or dissipative systems such as self-organization and energy cascade as well as a variety of transport and instability mechanisms. This presentation will discuss results from our studies of dusty plasmas in high (B ≥ 1 T) magnetic fields using the Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University and in microgravity experiments using the Plasmakristall-4 (PK-4) laboratory on the International Space Station.
Martin Lemoine
Institut d'Astrophysique de Paris — CNRS, Sorbonne University, Paris, France
“Shock Waves of the Very High Energy Universe”
ABSTRACT: The acceleration of particles to very high energies in powerful astrophysical sources with relativistic outflows, such as pulsar winds, microquasars, active galactic nuclei and gamma-ray bursts represents a central question in modern high-energy, multi-messenger and relativistic plasma astrophysics. In a generic model, particles are accelerated at a shock wave, then radiate in the ambient magnetic or radiation fields. It is understood, however, that the physics of particle acceleration is intimately related to the physics of the relativistic collisionless shock itself. This seminar proposes to zoom in on the kinetic scales of these shock fronts to discuss the physical processes at work. In particular, I will present a recent theoretical model of Weibel-mediated relativistic collisionless shocks, which we have corroborated by numerical particle-in-cell simulations, and discuss consequent astrophysical applications.
Roger Blandford
Kavli Institute for Particle Astrophysics and Cosmology & Stanford University, USA
“On the Electrodynamics of Fast Radio Bursts”
ABSTRACT: Fast Radio Bursts are short spikes of radio emission, typically lasting less than a second, which occur about once a minute over the whole sky. Recent observations strongly support the association of these radio bursts with magnetars — young neutron stars endowed with 10-100 GT magnetic fields. Explanations of their emission and transmission invoke the electrodynamics of Maxwell, Lorentz, Einstein and Feynman in new and interesting ways. These will be discussed alongside prospects for learning more from future observations and simulations.
Stas Boldyrev
Department of Physics, University of Wisconsin–Madison, USA
“Electron Temperature of the Solar Wind”
ABSTRACT: As the solar wind plasma expands from the hot solar corona, its temperature declines with the heliospheric distance. However, the temperature decline is not as fast as it would be predicted by the adiabatic law. The non-adiabatic temperature profile is a long-standing problem of space plasma physics. I will discuss the kinetic model of electron heating in the solar wind developed in [1]. In this model, the heating of the solar-wind electrons results from the energy exchange between the fast electrons streaming from the corona along the magnetic field lines and the electrons trapped between the electric potential and magnetic mirror walls. An analogous mechanism of electron heating was considered previously in relation to expander regions of mirror machines. The developed theory predicts that due to weak Coulomb collisions, the electron temperature declines with the heliospheric distance according to the power law T(r)~ r^-2/5, in good agreement with solar wind observations in the inner heliosphere.
[1] S. Boldyrev, C. Forest, and J. Egedal, Electron temperature of the solar wind. Proceedings of the National Academy of Sciences, 117, 9232-9240 (2020).
Emily Lichko
University of Arizona, USA
"Magnetic Pumping Model for Energizing Superthermal Particles Applied to Observations of the Earth's Bow Shock"
ABSTRACT: Energetic particle generation is an important component of a variety of astrophysical systems, from seed particle generation in shocks to the heating of the solar wind. It has been shown that magnetic pumping is an efficient mechanism for heating thermal particles, using the largest-scale magnetic fluctuations. Here we show that when magnetic pumping is extended to a spatially-varying magnetic flux tube, magnetic trapping of superthermal particles renders pumping an effective energization method for particles moving faster than the speed of the waves and naturally generates power-law distributions. We validated the theory by spacecraft observations of the strong, compressional magnetic fluctuations near the Earth’s bow shock from the Magnetospheric Multiscale mission. Given the ubiquity of magnetic fluctuations in different astrophysical systems, this mechanism has the potential to be transformative to our understanding of how the most energetic particles in the universe are generated.
Lichko, E., Egedal, J. Magnetic pumping model for energizing superthermal particles applied to observations of the Earth's bow shock. Nat Commun 11, 2942 (2020). doi:10.1038/s41467-020-16660-4.
Katsumi Ida
National Institute for Fusion Science (NIFS), Japan
"Magnetic Island as Turbulence Spreading Barrier in Toroidal Plasma"
ABSTRACT: Magnetic island is a closed magnetic flux surface bounded by a separatrix, isolating it from the rest of the space with nested magnetic flux surface. The separatrix is called X-point, while the center of the magnetic island is called O-point. The magnetic island is identified by the flattening of temperature at the O-point due to a lack of heat flux not due to the enhancement of transport. The magnetic island has been found to play a role in transport barrier because of the low thermal diffusivity inside magnetic island. Because the locally driven turbulence is negligible due to the flattening of the profile, the turbulence inside the magnetic island is dominated by the spreading turbulence. Therefore, the magnetic island is an ideal region in the plasma for turbulence spreading study in the experiment. Recently, turbulence spreading from X-point to O-point of the magnetic island has been identified in experiment. Turbulence spreading decreases with increasing ExB flow shear, which is often observed at the boundary of magnetic island. The interplay between the penetration of turbulence spreading into magnetic island and ExB flow shear at the boundary of magnetic island causes the bifurcation of turbulence and transport states inside magnetic island This transport bifurcation is in contrast to the transport bifurcation at the lams edge, where the interplay between the locally driven turbulence and ExB flow shear due to mean flow and zonal flow is important. In this presentation, the role of magnetic island on turbulence and transport, especially as a barrier of turbulence spreading, is discussed. The idea of turbulence spreading barrier gives a new insight to the space coupling of turbulent transport in magnetic fusion plasma.
Gianluca Gregori
University of Oxford, UK
"Turbulent Magnetic Fields and Particle Transport in Laboratory and Astrophysical Plasmas"
ABSTRACT: Turbulent magnetic fields are ubiquitous in the Universe. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter and contributing to the generation and transport of high energy particles. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. We also show that the deflection of high energy protons as they traverse stochastic, spatially intermittent magnetic field can be used to determine the relevant transport coefficients and that diffusion is unaffected by the spatial intermittency of the magnetic field. Our experimental data mimic ultrahigh-energy cosmic rays (UHECRs) propagation through the Milky-Way. Finally, our experimental results indicate that the presence of a sufficiently strong turbulent magnetic can also significantly alter thermal conduction and suppress heat on scales comparable to the coherence length of the field.
Alexander Migdal
New York University, New York, NY, USA
"Statistical Field Theory of Turbulence"
ABSTRACT: The Turbulence in incompressible fluid is represented as a Field Theory in 3 dimensions. There is no time involved, so this is intended to describe stationary limit of the Hopf functional. There are two basic ingredients: the Generalized Beltrami Flow (GBF, stationary solutions of the Navier-Stokes equations) and the new form of the energy flow equation derived in my recent paper. The basic field is the Clebsch field, parametrizing velocity and vorticity, and its gauge invariance (canonical Hamiltonian transformations, or symplectomorphisms) play central role in this theory. Explicit formulas for invariant Clebsch measure in space of GBF compatible with steady energy flow are presented. We introduce a concept of Clebsch confinement and study Clebsch instantons: singular vorticity sheets with nontrivial helicity. These singular solutions are involved in enhancing infinitesimal random forces at remote boundary leading to critical phenomena. The resulting exponential distribution for PDF of velocity circulation fits the numerical simulations within their systematic errors. This description of Turbulence as statistical field theory of vortex sheets is similar to the duality in QFT where gauge fields in strong coupling phase are dual to weakly fluctuating surfaces. The Clebsch fields provide internal degrees of freedom living on these surfaces. I know—it is so different from the Kolmogorov dogmas that it would hurt the feelings of some experts. But so were the matrix models of gravity until they were finally accepted.
Xi Chen
General Atomics, San Diego, CA, USA
"Higher Off-Axis Electron Cyclotron Current Drive Via ‘Top Launch’ Approach"
ABSTRACT: For the first time, experiments on the DIII-D tokamak have demonstrated electron cyclotron current drive (ECCD) with more than double the efficiency of the conventional outside launch by using a novel top launch geometry, as predicted by linear ray tracing and quasi-linear Fokker-Planck simulations. The development of efficient off-axis current drive is crucial for economic, steady-state tokamak fusion power plants, and "top launch" ECCD is predicted to drive strong off-axis currents by injecting EC waves nearly parallel to the vertical resonance plane with a large toroidal steering. Recent DIII-D experiments using a fixed-aiming top launcher and 2nd harmonic damping have tested the main tenets of top launch: a long absorption path, large Doppler shift damping on high energy electrons, and substantially increased ECCD efficiency at mid-radii. The longer interaction zone is confirmed by top launch measurements of broader power deposition profiles, while shifted O-mode deposition relative to X-mode verifies the predicted longer vertical path for O-mode due to weaker damping. Changing the separation between the ray path and vacuum resonance by scanning magnetic field varies the wave-electron interactions in velocity space, with experiments finding that wave absorption decreases for extreme Doppler shifts where the wave interacts with too few tail electrons. At optimal conditions with strong damping on high v|| electrons far from the trapping boundary, the experimental ECCD at ~0.5, determined from the change in the magnetic field pitch angles measured by motional Stark effect polarimetry, is greatly enhanced using top launch compared to the outside launch, and is consistent with the predictions from TORAY and quasi-linear Fokker-Planck code CQL3D. Simulations of FNSF, CFETR and DEMO support top launch ECCD as an improved efficiency current drive technique for future fusion reactors. *Supported by US DOE under DE-FC02-04ER54698.
Chris McDevitt
Dept. of Nuclear Engineering, University of Florida, Florida, USA
"Tokamak Disruptions: Some Open Physics Questions"
ABSTRACT: Tokamak disruptions have been the subject of extensive research due to the myriad of multiphysics issues that arise as well as the threat they pose to reactor-scale tokamak devices. These events, which result in the sudden release of nearly all of the device's magnetic and thermal energy, are capable of causing significant damage to the confinement vessel via thermal and electromagnetic loads, along with the formation of a relativistic population of electrons. This last issue is of particular concern, both due to the damage that can be caused by a large population of relativistic electrons to plasma facing components and due to the large gaps in our understanding of the physical mechanisms responsible for their generation and evolution during tokamak disruptions. In this seminar I will discuss several unresolved physics issues that enter when seeking to identify a means of mitigating these disruption events. Particular emphasis will be placed on discussing critical transport issues that emerge in the study of tokamak disruptions.
Hui Li
Los Alamos National Laboratory, Los Alamos, New Mexico, USA
“All Hands-on Deck: What Observations, Theory, Simulation and Laboratory Experiments are Teaching Us About the Powerful AGN Jets”
ABSTRACT: Powerful jets from active galactic nuclei (AGNs) are observed on scales from Mpc down to astronomical units, from essentially all observable wavebands and via multi-messengers (high-energy particles, photons and neutrinos). These enigmatic sources could hold the key to several long-standing mysteries such as ultra-high energy cosmic rays and more recently the origin of high-energy ~ PeV neutrinos. Understanding such sources has strongly influenced the development of laboratory jet experiments. We will describe different laboratory plasma jet experiments and how they have attempted to help understanding jets. We will also discuss how the behavior of magnetic fields can shift the framework within which we interpret the AGN jets/lobes. Future prospects of progress in simulations, experiments and observations will be discussed as well.
Marco Velli — joint seminar with UCI and UCLA
Earth, Planetary, and Space Sciences, University of California, Los Angeles, USA
“Parker Solar Probe: Early Results From the First Four Perihelia”
ABSTRACT: Parker Solar Probe (PSP) launched on August 12th, 2018, its mission is to carry out the first in situ exploration of the outer solar corona and inner Heliosphere. Observations of solar-wind plasma at a distance of ~ 36 RS, well inside the radius at which stream interactions become important, have shown that Alfvén waves organize into structured velocity spikes up to minutes long that are associated with propagating S-like bends in the magnetic-field lines. These are associated with measured magnetic field patches of large, intermittent reversals with enhanced Poynting flux interspersed with a smoother and less turbulent flow with a near-radial magnetic field. This slow, Alfvénic solar wind emerged from a small, rapidly expanding equatorial coronal hole, and the wind was still accelerating at this distance. The measured circulation of the wind around the Sun, peaking at 35-50 km/s, exceed classical predictions of a few km/s, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age. Plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities associated with with plasma heating and thermalization processes. Not many energetic particle events were observed over the first encounter, while the white light imager observed pseudostreamer and streamer stalks emitting small intermittent blobs, potential evidence of reconnection.
Andrea Garofalo
General Atomics, San Diego, California, USA
“Integrating Core Burning Plasma Performance with Edge Stability for ITER”
ABSTRACT: As the world’s first magnetically confined burning plasma, ITER is expected to demonstrate sustained thermal fusion power 10 times higher than the power used to heat the plasma (fusion gain Q~10). This goal requires operation in the high confinement (H-mode) regime. However, H-mode confinement is usually associated with MHD instabilities called edge localized modes (ELMs) that perform the important task of expelling plasma impurities, but also could lead to unacceptable tokamak wall erosion. Thus, for more than a decade, fusion scientists around the world have focused on ways to control ELMs while maintaining sufficient impurity transport. The challenge of ELM-controlled, high confinement operation is compounded by other ITER requirements: operation at low safety factor (winding ratio of magnetic field lines) for high fusion power density, and with plasma toroidal rotation much slower than normal operation in existing tokamaks; both conditions are expected to reduce MHD stability. So far, ELM-stable Q~10 performance has been achieved with two different ELM control approaches: ELM suppression by application of resonant magnetic perturbations (RMPs) and the naturally ELM-free regime of Quiescent H-mode (QH-mode). Challenges that arise when the externally injected torque is reduced seem connected with features of the heating methods, e.g. loss of ELM suppression due to details of the radial profile of the neutral beam torque, or global kink instabilities associated with neutral beam fueling of the plasma core. Particle recycling at the wall also seems to play a key role. Therefore, comparing experiments over a range of heating schemes and wall properties is critical for understanding edge stability. Higher safety factor operation would improve MHD stability but requires improved confinement quality to meet the Q~10 goal. Examples of promising options will be discussed. Supported by the U.S. DOE under grant award no. DE-FC02-04ER54698.
Peter Manz
Max Planck Institute for Plasma Physics, Garching, Germany
“The Key to the I-Mode Confinement Regime”
ABSTRACT: The I-mode [1] is an improved energy confinement regime of tokamak plasmas usually achieved by using magnetic configurations with the ion grad B drift pointing away from the active X-point, i.e. the so-called unfavorable configurations in terms of H-mode access. The I-mode is ELM-free whilst not suffering from high impurity content making it an attractive confinement regime for future devices, such as DEMO or ARC. I-modes are characterized by a temperature pedestal but no density pedestal. A transport barrier induced by a strong E x B shear flow as in the H-mode should reduce both, particle and heat transport. Thus, other mechanisms of turbulence suppression must be important. We present such an alternative mechanism leading to the observed selective suppression of electron heat transport which bases on drift-wave turbulence at low collisionality. Compared to density fluctuations electron temperature fluctuations are additionally dissipated by conductivity [3], which is more effective at higher conductivity and hence at higher temperatures. This allows the seemingly decoupling of heat and particle transport at rising temperatures. However, to achieve this regime, the ion temperature gradient has to be rather flat, which is actually measured around the separatrix in ASDEX Upgrade. With the presented mechanism we can explain the small window of operation at small magnetic fields [2] and the difficulties to go into detachment in this regime. Simulations in such a regime of edge turbulence have been carried out with the global three-dimensional gyrofluid electromagnetic turbulence code GEMR [4] at ASDEX Upgrade parameters. Turbulence suppression at larger and smaller scales are investigated in detail. The most prominent feature of I-mode turbulence, the so-called weakly coherent mode [5] is clearly revealed in the simulations.
References:
[1] F. Ryter et al., Plasma Phys. Control. Fusion 40, 725 (1998), D.G. Whyte et al., Nucl. Fusion 50, 105005 (2010).
[2] A.E. Hubbard et al., Nucl. Fusion 56, 086003 (2016), T. Happel, et al., Plasma Phys. Control. Fusion 59, 014004 (2017).
[3] B.D. Scott, Plasma Phys. Control. Fusion 39, 1635 (1997).
[4] A. Kendl, B.D. Scott, and T.T. Ribeiro, Phys. Plasmas 17, 072302 (2010).
[5] A.E. Hubbard et al., Phys. Plasmas 18, 331 (2011), P. Manz et al., Nucl. Fusion 55 083004 (2015).
Matt Kriete
University of Wisconsin-Madison, now with Auburn Univeristy, USA
“Influence of Applied Magnetic Perturbations on Turbulence-Flow Dynamics Across the L-H Transition in the DIII-D Tokamak”
ABSTRACT: Applying resonant magnetic perturbations (RMPs) to tokamak plasmas raises the L-H transition power threshold, PLH, potentially inhibiting H-mode access in next-step, reactor-scale tokamaks. Detailed 2D turbulence measurements on the DIII-D tokamak show how RMPs alter the turbulence-flow dynamics that are thought to trigger the L-H transition, thereby raising PLH. Long-wavelength density fluctuations are measured using the beam emission spectroscopy (BES) diagnostic. Velocimetry analysis is applied to images of these density fluctuations to infer the 2D turbulent flow field. Detailed tests of velocimetry analysis are performed using synthetic turbulence images and nonlinear gyrokinetic simulations to validate the technique and optimize it for DIII-D experimental parameters. The turbulence-flow measurements show that RMPs simultaneously raise the turbulence decorrelation rate, ΔωD, and reduce the flow shear rate, ωShear, in the stationary L-mode state preceding the L-H transition, thereby disrupting the turbulence shear suppression mechanism. This implies significantly more transient turbulence suppression is needed to trigger the L-H transition, which requires more heating power. RMPs also reduce the Reynolds stress drive for poloidal flow, contributing to the reduction of ωShear. On the fast, ~100 μs timescale of the L-H transition, RMPs reduce Reynolds-stress-driven energy transfer from turbulence to flows by an order of magnitude, challenging the energy depletion theory for the L-H trigger mechanism. In contrast, non-resonant magnetic perturbations, which do not significantly affect PLH, do not affect ΔωD and only slightly reduce ωShear and Reynolds stress-driven energy transfer.
Taik Soo Hahm
Seoul National University
“Residual zonal flows for non-Maxwellian equilibrium distribution function”
Residual zonal flow level RZF [1] is one of the key relevant quantities which determine turbulence and transport of tokamak plasmas [2]. While there have been various theoretical extensions of the original work in Ref. [1] including the isotopic dependence [3], most previous works have assumed Maxwellian equilibrium distribution function F0 with rare exceptions, for instance Refs [4,5]. Neoclassical polarization shielding determines the long term behavior of zonal flows and it can be derived in the context of modern gyrokinetic [6] and bounce-kinetic theories [7]. This approach not only elucidates the underlying physics of residual zonal flows, is but also applicable to an arbitrary F0. Using this method, we show that the long wavelength, high aspect ratio result, RZF =1/1+1.63q^2/√ϵ derived for a Maxwellian F0 in Ref. [1] remains valid for any F0 which is isotropic in velocity space. In addition, it is found that presence of high energy ions such as fusion product α-particles described by slowing-down F0 can enhance RZF considerably in the intermediate wavelength regime krρT i ∼ 0.1 [4]. This presentation will cover the physics behind the neoclassical polarization shielding and long term asymptotic behavior of zonal flows and the effects of the fusion product α-particles on these.
References:
[1] M.N. Rosenbluth and F.L. Hinton, Phys. Rev. Lett. 80 724 (1998)
[2] P.H. Diamond et al., Plasma Phys. Control. Fusion 47 R35 (2005)
[3] T.S. Hahm et al., Nucl. Fusion 53 072002 (2013)
[4] Y.W. Cho and T.S. Hahm, Nucl. Fusion 59 066026 (2019)
[5] Z.X. Lu et al., Plasma Phys. Control. Fusion 61 044005 (2019)
[6] T.S. Hahm, Phys. Fluids 31 2670 (1988)
[7] B.H. Fong and T.S. Hahm, Phys. Plasmas 6 188 (1999)
Gyung Jin Choi
Department of Physics & Astronomy, UC Irvine
“Collisionless damping of residual zonal flows in tokamak plasmas by resonant magnetic perturbations”
ABSTRACT: Crash of Edge Localized Mode (ELMs) in H-mode plasmas induces large transient heat load to divertor plate of tokamaks. This will be a serious problem in ITER and future magnetic fusion devices as amount of the heat load would increase with machine size. Resonant magnetic perturbation (RMP) has been highlighted as one of the most promising candidates for mitigation or suppression of the dangerous ELMs. By breaking axisymmetry of tokamak magnetic field, RMP sacrifices some of plasma performance in many aspects to obtain better sustainability. It has been observed in various devices that H-mode transition power threshold increases with RMP. In this talk, I present an analytic theory of zonal flows to provide an explanation of this phenomenon, as it is widely accepted that ExB zonal flow is the main contributor triggering H-mode transition. Calculation of linear zonal flow response using gyrokinetic equations reveals that RMP induces long-term collisoinless damping of residual zonal flows by velocity space phase mixing. This additional damping other than collisional and nonlinear ones makes increase of required zonal flow source and thus external power input for H-mode transition. It is emphasized that RMP effect on zonal flows in tokamaks is different from 3D magnetic field effect on zonal flows in stellarators. Last part of this talk consists of issues which should be discussed to clarify limitations and make future extension of this work.
2019
Robin Heinonen
Department of Physics and CASS & UC San Diego
“Subcritical Turbulence Spreading and Avalanching”
ABSTRACT: Turbulence in confined plasma is known to self-propagate via nonlinear scattering. This phenomenon of “turbulence spreading” is of interest because it decouples the relationship between the local driving gradient and the local fluctuation intensity, in particular allowing linearly stable regions to be contaminated with fluctuations. This process has been traditionally modeled using a Fisher-KPP equation, a supercritical reaction-diffusion equation. However, such an approach suffers from a number of drawbacks. For one, it begs the question of why the turbulence hasn’t already saturated due to linear instability. Moreover, the Fisher-KPP fails to predict any but the weakest of penetration into stable regions, which is dubiously consistent with clear observations of fluctuations in such regions. As a final reason to reconsider the older model, we note that a growing body of numerical and analytical work suggests the possibility of nonlinear instability and subcritical turbulence, neither of which are described by Fisher-KPP. In this work, we resolve the above issues by introducing a new subcritical model for turbulence spreading, featuring nonlinear instability drive. In addition to predicting stronger penetration of turbulence into stable regions via ballistically propagating fronts, this model predicts the possibility of bursty, intermittent propagation of turbulence similar to avalanches. We show that such an avalanche can be triggered when a threshold is exceeded, say due to noise, and estimate the threshold with a simple physical argument. These predictions provide avenues to test the model in experiment or simulation. This work appears in an article in Physics of Plasmas and is supported by the Department of Energy under Award Number DE-FG02-04ER54738.
Tatsuya Kobayashi
National Institute for Fusion Science & Toki, Japan
“Interplay between magnetic island and turbulence in DIII-D plasmas”
ABSTRACT: Interplay between magnetic island and turbulence is experimentally investigated by means of modulation heating experiments in the DIII-D tokamak. There are two different quasi steady states in the island-turbulence interactions: the high accessibility state and the low accessibility state. The former corresponds to the state that the applied heat pulse can penetrate deeper into the magnetic island while the latter corresponds to the opposite state. Transition dynamics is clearly described and a possible physical picture is deduced. In order to investigate how the heat pulse and turbulence packet behave inside the magnetic island, the turbulence measurement is performed by the beam emission spectroscopy. It was turned out that the turbulence propagates prior to the heat pulse inside the O-point of the magnetic island. Turbulence packet spreading from the X-point to the O-point is speculated.
2018
Kshitish Barada
Department of Physics and Astronomy, University of California, Los Angeles, USA
“A Unique Predator-Prey System of Coupled Turbulence, Drive, and Sheared ExB Flow in the Pedestal of High Performance DIII-D Plasmas”
ABSTRACT: Long-lived (3 to 12 energy confinement times) predator-prey type limit cycle oscillations (LCOs) are observed to follow immediately after the coherent edge harmonic oscillations (EHOs) disappear in wide-pedestal ELM-free QH-mode plasmas. The edge parameters such as electron temperature, divertor Dα light intensity, radial electric field, and Langmuir probe ion saturation current density all start to oscillate at the LCO frequency after EHO cessation. Local density fluctuations (ñ), ExB velocity (V), and ExB velocity shear (V') measured with Doppler backscattering (DBS) at eight pedestal locations show periodic oscillations at LCO frequency. The periodic oscillations in V' lag those in ñ thus exhibiting the characteristics of a predator-prey cycle with V', the predator and ñ, the prey. A wavelike inward propagation of periodic perturbations in ExB velocity (due to changes in profile gradients by ñ driven transport) induces a temporal delay. This delay in ExB velocity produces the necessary temporal variations in ExB shear which are found to be important for ñ regulation and for the sustenance of the quasi-stationarity of this LCO regime. The temporal dynamics can be explained as follows: ñ modifies transport → transport modifies gradients (∇Te) → gradients modify ExB velocity → spatiotemporally varying ExB velocity modifies ExB shear → ExB shear modifies ñ → ñ modifies transport → … and the cycle repeats. This unique system reported for the first time is long-lived, has complex coupling amongst multiple parameters (ñ, V', V, ∇Te), and exhibits spatiotemporal behavior of ExB velocity which is key to the evolution of shear, V'. The ExB velocity, although poloidally and toroidally symmetric, is found to be driven by pressure gradients and not by ñ and so is inconsistent with being of zonal flow type which were observed transiently during L-H transition. Observations of oscillations in edge transport relevant parameters including that of Langmuir probe ion saturation current indicate a potentially significant contribution of this LCO mechanism to modulated pedestal transport in wide-pedestal QH-mode. The frequency of these LCOs is found to scale inversely with pedestal density and also the period of the cycles can be actively controlled by applying electron cyclotron heating (ECH) which also improves confinement of this regime.
Work supported by USDOE Grant# DE-FG02-08ER54984 and DE-FC02-04ER54698.
Jerry Hughes
MIT Plasma Science and Fusion Center, Cambridge, Massachusetts, USA
“Life on the Edge in High Magnetic Field Fusion Devices”
ABSTRACT: The boundary plasma in a magnetic fusion device has an enormous impact on its ability to achieve both good fusion performance in the core and sufficient mitigation of waste heat conducted to plasma facing materials. This is due to the integration of a number of transport and stability phenomena, over a relatively small boundary layer, which serves to regulate the flow of heat and particles out of the confined plasma, and which determines how that flow interacts with material surfaces. How these processes play out at parameters relevant to burning plasmas will have a significant impact on reactor viability. Experimental work on the Alcator C-Mod tokamak has explored critical boundary physics issues at parameters relevant to the ITER device, which is under construction. These parameters include toroidal and poloidal magnetic field, absolute plasma density, and heat flux to material surfaces. The device has demonstrated an edge transport barrier having a plasma pressure very near that required for ITER, and has shown success in mitigating heat flux to an ITER-like divertor. Interest is growing quickly in the prospects for compact high magnetic field devices for fusion, largely due to recent advances in the performance of high temperature superconductors, which could enable steady state operation at twice the ITER field. The impact of boundary physics must be examined in this context, and C-Mod results at up to 8T provide a path forward for projecting to very high field devices.
Won-Ha Ko
National Fusion Research Institute, Daejeon, Korea
"LH Transition with Resonant Magnetic Perturbations on KSTAR*"
ABSTRACT: Significantly low H-mode power threshold (PTH) has been observed in KSTAR in comparison with other conventional devices. Such a favorable finding is attributable to an order of magnitude lower intrinsic error field (<δB/B0>m/n=2/1 ~ 1x10-5 [1]) and toroidal field ripple (δTF=0.05% [2]), which has been corroborated by high pedestal rotation in KSTAR [3]. A thorough study of LH transition under the influence of low non-axisymmetric field (NF) has been conducted in KSTAR with low intrinsic error field. It shows that LH power threshold depends on the resonant NFs and the line-averaged density-LH power threshold curve agrees well with the power law scaling [4] as the resonant NF (δB/B0, n=1) applied up to 2.7x10-4 in KSTAR. However, LH power threshold is independent of non-resonant NFs with n=1 and n=2 which reduced only toroidal rotation by 30% in L-mode. Minimized resonant NFs or intrinsic error fields are desirable to access low H-mode power threshold in ITER and future reactors.
*This work was supported by the Korean Ministry of Science, ICT and Future Planning of Republic of Korea.
References:
[1] Y. In et al, Nucl. Fusion 55 043004 (2015).
[2] S.W. Yoon et al, IAEA-FEC (2014).
[3] W.H. Ko, et. al, Nucl. Fusion 55 083013 (2015).
[4] Y. R. Martin, et. al, Journal of Physics 123 012033(2008)