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

Journal Article · 27 September 2019 · Physics of Plasmas

DOI: https://doi.org/10.1063/1.5100176 · OSTI ID:1609941

Wang, L. ^[1]; Tynan, G. R.  ^[2]; Hong, R.  ^[3]; Nie, L. ^[1]; Chen, Y. ^[1]; Ke, R.  ^[1]; Wu, T. ^[1]; Long, T. ^[1]; Zheng, P. ^[1]; Xu, M. ^[1]

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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

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.

• Research Organization: Univ. of California, San Diego, CA (United States); Princeton Univ., NJ (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES); National Key Research and Development Program of China
• Contributing Organization: HL-2A Team
• Grant/Contract Number: FG02-07ER54912; AC02-09CH11466
• OSTI ID: 1609941
• Alternate ID(s): OSTI ID: 1566928
• Journal Information: Physics of Plasmas, Vol. 26, Issue 9; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English