Electron-scale Turbulence Spectra and Plasma Thermal Transport Responding to Continuous ExB Shear Ramping-up in a Spherical Tokamak
Authors: Y. Ren, et. al.
Abstract:
Microturbulence is considered to be a major candidate in driving anomalous transport in fusion
plasmas, and the equilibrium E x B shear generated by externally driven flow can be a powerful tool
to control microturbulence in future fusion devices such as FNSF and ITER. Here we present the
first observation of the change in electron-scale turbulence wavenumber spectrum (measured by a
high-k scattering system) and thermal transport responding to continuous E x B shear ramping-up
in an NSTX center-stack limited and NBI-heated L-mode plasma. It is found that while linear
stability analysis shows that the maximum ETG mode linear growth rate far exceeds the observed
E x B shearing rate in the measurement region of the high-k scattering system, the unstable ITG
modes are susceptible to E x B shear stabilization. We observed that as the E x B shearing rate is
continuously ramped up in the high-k measurement region, the ratio between the E x B shearing
rate and maximum ITG mode growth rate continuously increases (from about 0.2 to 0.7) and the
maximum power of the measured electron-scale turbulence wavenumber spectra decreases. Meanwhile,
electron and ion thermal transport is also reduced in the outer half of the plasmas as long
as MHD activities are not important and the L-mode plasmas eventually reach H-mode-like confinement.
Linear and nonlinear gyrokinetic simulations are presented to address the experimental
observations.
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Submitted to: Nuclear Fusion (December 2012)
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Download PPPL-4903 (pdf 275 KB 27 pp)
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