Electron-Scale-Driven Turbulence by Negative-Density-Gradient in NSTX

Gyrokinetic simulations of an NSTX spherical-tokamak plasma reveal novel negative-density-gradient (NDG) drift waves at electron-gyroradius-scale that drive turbulent transport comparable to the experimental power flow. Linear simulations indicate that the dominant instability is a trapped-electron electron-scale tearing-parity mode driven mainly by negative electron density gradient, $a/L_{ne} \equiv - a/n_e \, (dn_e/dr) < 0$. Although thermal transport is mainly carried by transverse magnetic, $\delta A_{\parallel}$, fluctuations, the growth rate is sensitive to compressional magnetic fluctuations, $\delta B_{\parallel}$, differentiating these from standard microtearing modes (MTMs). Linear sensitivity analysis also reveals that the modes are most unstable at lower collisionality, consistent with the trapped-electron character. Electron-scale and multi-scale nonlinear simulations show that these modes can account for several megawatts of experimental power, with the latter suggesting that experimental gradients may lie at a bifurcation between distinct turbulence regimes.