Title: Edge turbulence evolution and intermittency development near the density limit on the HL-2A tokamak

Abstract

The development of intermittent non-Gaussian processes is researched in the edge turbulence of ohmically heated HL-2A discharges approaching the density limit. As the density limit is approached, the $E\times B$ shear flow at the last closed flux surface (LCFS) weakens, a strong positive skewness develops in the scrape-off layer (SOL), and negative skewness develops inside the LCFS of turbulent density fluctuations. A conditional averaging analysis confirms more frequent increased amplitude positive (negative) going density fluctuation activity in the SOL (inside the LCFS) as the density limit is approached. The measured turbulent stress across the edge, LCFS, and SOL region is decomposed into diffusive and residual stress components, and the nonlinear exchange of kinetic energy between the turbulence and the low-frequency $E\times B$ shear flow is determined. Residual stress acts to amplify the $E\times B$ flow at the LCFS, while the diffusive stress acts to dissipate the flow just inside this region, at the interface between the core plasma and the LCFS. The relative strength of the flow drive associated with the residual stress weakens as the density limit is approached, while the turbulent viscosity associated with the diffusive stress increases at high density. The adiabatic parameter, $k_z^2{v}_{\mathit{th}}^2/\omega {\nu }_e$, drops significantly to about 0.5 in the SOL when the density limit is approached, indicating a transition from the adiabatic regime to the hydrodynamic regime due to increased collisionality. Such changes enhance the particle transport through the nonadiabatic electron response and hence should end in a stronger edge cooling at fixed edge plasma heat flux.

Authors:

Wang, L. ^[1]; Tynan, G. R.  ^[2];

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Hong, R.  ^[3];

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Nie, L. ^[1]; Chen, Y. ^[1]; Ke, R.  ^[1];

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Wu, T. ^[1]; Long, T. ^[1]; Zheng, P. ^[1]; Xu, M. ^[1]

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+ Show Author Affiliations

1. Center for Fusion Sciences, Southwestern Institute of Physics, Chengdu, Sichuan 610041, China
2. Center for Fusion Sciences, Southwestern Institute of Physics, Chengdu, Sichuan 610041, China; Center for Energy Research, University of California San Diego, La Jolla, California 92093, USA
3. Center for Energy Research, University of California San Diego, La Jolla, California 92093, USA

Publication Date:

2019-09-27

Research Org.:

Univ. of California, San Diego, CA (United States); Princeton Univ., NJ (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Fusion Energy Sciences (FES); National Key Research and Development Program of China

Contributing Org.:

HL-2A Team

OSTI Identifier:

1609941

Alternate Identifier(s):

OSTI ID: 1566928

Grant/Contract Number:  

FG02-07ER54912; AC02-09CH11466

Resource Type:

Journal Article: Accepted Manuscript

Journal Name:

Physics of Plasmas

Additional Journal Information:

Journal Volume: 26; Journal Issue: 9; Journal ID: ISSN 1070-664X

Publisher:

American Institute of Physics (AIP)

Country of Publication:

United States

Language:

English

Subject:

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Physics