Electron-scale turbulence spectra and plasma thermal transport responding to continuous E × B shear ramp-up in a spherical tokamak

Microturbulence is considered to be a major candidate in driving anomalous transport in fusion plasmas, and the equilibrium E × 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 × B shear ramp-up in an NSTX centre-stack limited and neutral beam injection-heated L-mode plasma. It is found that while linear stability analysis shows that the maximum electron temperature gradient mode linear growth rate far exceeds the observed E × B shearing rate in the measurement region of the high-k scattering system, the unstable ion temperature gradient (ITG) modes are susceptible to E × B shear stabilization. We observed that as the E × B shearing rate is continuously ramped up in the high-k measurement region, the ratio between the E × 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 magnetohydrodynamic 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. (paper)