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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Download PPPL-5203 (pdf 2.6 MB 14 pp)
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