Experimental observation of electron-scale turbulence evolution across the L–H transition in the National Spherical Torus Experiment

Electron-scale turbulence (for 3 less than or similar to k(perpendicular to)rho(s) less than or similar to 12) was measured during the L-H transition in the National Spherical Torus Experiment (NSTX) (Ono et al 2000 Nucl. Fusion 40 557) using a coherent microwave scattering system. The measurements were carried out at a radial region adjacent to the edge transport barrier (ETB) (at smaller radius than ETB). The observed L-H transition occurred during current flattop, which facilitated the measurement of electron-scale turbulence. The measured electron-scale turbulence is observed to be quasi-stationary before the L-H transition, and an intermittent phase for electron-scale turbulence is observed after the start of the L-H transition with a gradual decrease in overall turbulence density fluctuation spectral power with intermittent large relative variations (on similar to 0.5-1ms time scale) in the total spectral power. A turbulence-quiescent phase is observed following the intermittent phase, and a significant reduction in the electron-scale turbulence spectral power is only observed at lower wavenumbers, namely k(perpendicular to)rho(s) <= 9-10, which is also seen in different operational NSTX scenarios due to different stabilization mechanisms. A recovery phase is seen after the quiescent phase, where the electron-scale density fluctuation power starts to gradually increase. Simultaneous ion-scale turbulence measurements at larger radius than the electron-scale turbulence measurement location show similar temporal behavior in ion-scale turbulence as in the measured electron-scale turbulence. These observations demonstrate that the suppression of turbulence during the L-H transition is not just limited to the ETB region. None of the measured electron-scale turbulence and ion-scale turbulence from edge into core is found to be obviously leading in the response to the L-H transition, and the overall turbulence suppression after the start of the L-H transition at different radii seems to start at the same time and is a gradual process happening on a tens-of-ms time scale. The trend of decrease in electron-scale turbulence during the L-H transition is found to be consistent with a decrease in the maximum electron-temperature-gradient linear growth rate from linear gyrokinetic stability analysis. However, the observed intermittency in electron-scale turbulence during the intermittent phase cannot be explained by the linear analysis.