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

Abstract

Electron-scale turbulence (for $$3$$ $$\lesssim$$ $$k⟂ρ_s$$ $$\lesssim$$ $12$) ) 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 ~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 $$k⟂ρ_s$$ $$\leqslant$$ $$9$$ –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. Interestingly, the observed intermittency in electron-scale turbulence during the intermittent phase cannot be explained by the linear analysis.

Authors:

Ren, Y.  ^[1];

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Smith, D. R. ^[2]; Zweben, S. J. ^[1]; Bell, R. ^[1]; Guttenfelder, W.  ^[1];

• Search DOE PAGES for author "Guttenfelder, W."
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Kaye, S. M. ^[1]; LeBlanc, B. P. ^[1]; Mazzucato, E. ^[1]; Lee, K. C. ^[3]; Domier, C. W. ^[4]; Sun, P. J. ^[5]; Yuh, H. ^[6]

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+ Show Author Affiliations

1. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
2. Univ. of Wisconsin-Madison, Madison, WI (United States)
3. National Fusion Research Inst., Daejeon (Republic of Korea)
4. Univ. of California at Davis, Davis, CA (United States)
5. Chinese Academy of Sciences, Hefei, Anhui (People's Republic of China). Inst. of Plasma Physics
6. Nova Photonics, Inc., Princeton, NJ (United States)

Publication Date:

Tue Aug 06 00:00:00 EDT 2019

Research Org.:

Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)

Sponsoring Org.:

USDOE

OSTI Identifier:

1562292

Grant/Contract Number:  

AC02-09CH11466; AC02-76CH03073; FG03-95ER54295; FG03-99ER54518

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Volume: 59; Journal Issue: 9; Journal ID: ISSN 0029-5515

Publisher:

IOP Science

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; electron-scale turbulence; ETG; L-H transition; spherical tokamak; NSTX; microwave scattering