Distinct turbulence sources and confinement features in spherical tokamak plasma regime
Authors: W. X. Wang, S. Ethier, Y. Ren, S. Kaye,
J. Chen, E. Startsev, and Z. Lu
Abstract: New turbulence contributions to plasma
transport and confinement in spherical tokamak (ST) regime are
identified through nonlinear gyrokinetic simulations. The drift
wave Kelvin-Helmholtz (KH) mode characterized by intrinsic mode
asymmetry is shown to drive significant ion thermal transport in
strongly rotating NSTX L-modes. Long wavelength, quasi-coherent
dissipative trapped electron mode (TEM) is destabilized in NSTX
H-modes despite the presence of strong E x B shear, providing a
robust turbulence source dominant over collisionless TEM.
DTEM-driven transport in NSTX parametric regime is shown to
increase with electron collision frequency, offering one possible
source for the confinement scaling observed in experiments. There
exists a turbulence-free regime in collision-induced CTEM to DTEM
transition for ST plasmas. This predicts a natural access to a
minimum transport state in the low collisionality regime that
future advanced STs may cover.
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Submitted to: Nuclear Fusion
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