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Past Seminars

March 27, 2018

Kshitish Barada

Associate Project Scientist, 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.

March 15, 2018

Jerry Hughes

Principal Research Scientist, 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.

March 8, 2018

Won-Ha Ko

National Fusion Research Institute, Daejeon, Korea

"LH Transition with Resonant Magnetic Perturbations on KSTAR*"

ABSTRACT: Significantly low H-mode power threshold (P_TH) 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 (δ_TF=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/B_{0, 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)