Stabilization of electron-scale turbulence by electron density gradient in NSTX

Abstract

Microturbulence is present in all magnetic confinement fusion devices, and is believed to play a major role in driving anomalous transport levels that exceed neoclassical theory predictions. In NSTX, electron thermal transport is found to dominate energy loss. Theory and experiments have shown that electron temperature gradient (ETG) turbulence on the electron gyro-scale, kḻpe </~ 1, can be responsible for anomalous electron thermal transport in NSTX. Electron scale (high-k) turbulence is diagnosed with a high-k microwave scattering system [92] in NSTX. Here we report on the stabilization effects of the electron density gradient on electron-scale density fluctuations in a set of neutral beam injection (NBI) heated H-mode plasmas. We found that the absence of high-k density fluctuations from measurements is correlated with large equilibrium density gradient, and this correlation will be shown to be consistent with linear stabilization of ETG modes due to density gradient by using the analytical ETG linear threshold in [94] and linear gyrokinetic simulations with GS2 [95]. We also found that the observed power of the electron-scale turbulence (when it exists) is anti-correlated with the equilibrium density gradient. Thorough analysis of electron density fluctuations from the high-k scattering diagnostic at NSTX shows that larger equilibrium density gradient leads to higher values of the wavenumber corresponding to the maximum in the fluctuation wavenumber spectrum. Higher equilibrium electron density gradient also gives rise to a lower value of the plasma frame frequency of the detected density fluctuations. Linear gyrokinetic simulations using GS2 are in agreement with experimental results, and show a clear correlation between the wavenumber value at the maximum linear growth rate and the local value of the electron density gradient. Higher values of the electron density gradient are also shown to reduce the value of the real frequency of instability.