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Distinct turbulence sources and confinement feature in spherical tokamak plasma regime

Author: Weixing Wang
Requested Type: Poster Only
Submitted: 2015-01-19 14:46:03

Co-authors: S. Ethier, Y. Ren, S. Kaye, J. Chen, E. Startsev, Z. Lu (UCSD)

Contact Info:
Princeton Plasma Physics Lab
PPPL., Princeton University
Princeton, NJ   08543
USA

Abstract Text:
Highly distinct features of spherical tokamaks (ST) such as NSTX/U result in a different fusion plasma regime with unique physics features compared to conventional tokamaks. Nonlinear global gyrokinetic simulations critical for addressing turbulence and transport physics in ST regime have led to new insights. The drift wave Kelvin-Helmholtz instability, driven by strong toroidal shear flow, is found to be unstable in NSTX L-mode plasmas. For the first time, the shear flow mode characterized by finite k// is identified to be relevant to realistic fusion plasmas. While the strong rotation shear leads to a reduction in the lowest-k turbulence, the remaining low-k fluctuations can produce a significant ion thermal transport comparable to experimental levels in the outer core region (with no “transport shortfall”). Low-k fluctuations in L-mode also drive a large anti-gradient residual stress mainly due to zonal flow shear induced asymmetry in k//-spectrum. Another new, important turbulence source identified in NSTX is the dissipative trapped electron mode (DTEM), which is believed to play little role in conventional tokamak regime. Due to the high trapped electron fraction in NSTX, long wavelength DTEMs are destabilized by collisions with steep electron temperature and density gradients achieved there, and produce significant particle, ion energy and toroidal momentum transport in agreement with experimental levels in NSTX H-modes. DTEM-driven transport is shown to increase with electron collision frequency, providing a possible source for the scaling of confinement time observed in NSTX H-modes, which is inversely proportional to electron collisionality [S. M. Kaye et al., Nucl. Fusion 47, 499 (2007)]. More interestingly, there exists a turbulence-free regime in the collision-induced CTEM to DTEM transition. This collisionality regime is likely relevant to NSTX-U and ST burning plasma regime, and may lead to a natural access to electron transport barrier.

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