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

Journal Article (Journal Article)

Electron-scale turbulence (for 3 k s12) 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 electronscale 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 ~0.5-1 ms 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 ks9-10, 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 erature-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.

Full Text

Duke Authors

Cited Authors

  • Ren, Y; Smith, DR; Zweben, SJ; Bell, R; Guttenfelder, W; Kaye, SM; Leblanc, BP; Mazzucato, E; Lee, KC; Domier, CW; Sun, PJ; Yuh, H

Published Date

  • August 6, 2019

Published In

Volume / Issue

  • 59 / 9

Electronic International Standard Serial Number (EISSN)

  • 1741-4326

International Standard Serial Number (ISSN)

  • 0029-5515

Digital Object Identifier (DOI)

  • 10.1088/1741-4326/ab2f4f

Citation Source

  • Scopus