Fast response of electron-scale turbulence to auxiliary heating cessation in National Spherical Torus Experiment

Journal Article · 03 November 2015 · Physics of Plasmas

DOI: https://doi.org/10.1063/1.4935113 · OSTI ID:1254752

Ren, Y.  ^[1]; Wang, W. X. ^[1]; LeBlanc, B. P. ^[1]; Guttenfelder, W.  ^[1]; Kaye, S. M. ^[1]; Ethier, S. ^[1]; Mazzucato, E. ^[1]; Lee, K. C. ^[2]; Domier, C. W.  ^[3]; Bell, R. ^[1]; Smith, D. R. ^[4]; Yuh, H. ^[5]

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1. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
2. National Fusion Research Institute, Daejeon (South Korea)
3. Univ. of California, Davis, CA (United States)
4. Univ. of Wisconsin-Madison, Madison, WI (United States)
5. Nova Photonics, Inc., Princeton, NJ (United States)

In this letter, we report the first observation of the fast response of electron-scale turbulence to auxiliary heating cessation in National Spherical Torus eXperiment [Ono et al., Nucl. Fusion 40, 557 (2000)]. The observation was made in a set of RF-heated L-mode plasmas with toroidal magnetic field of 0.55 T and plasma current of 300 kA. It is observed that electron-scale turbulence spectral power (measured with a high-k collective microwave scattering system) decreases significantly following fast cessation of RF heating that occurs in less than 200 μs. The large drop in the turbulence spectral power has a short time delay of about 1–2 ms relative to the RF cessation and happens on a time scale of 0.5–1 ms, much smaller than the energy confinement time of about 10 ms. Power balance analysis shows a factor of about 2 decrease in electron thermal diffusivity after the sudden drop of turbulence spectral power. Measured small changes in equilibrium profiles across the RF cessation are unlikely able to explain this sudden reduction in the measured turbulence and decrease in electron thermal transport, supported by local linear stability analysis and both local and global nonlinear gyrokinetic simulations. Furthermore, the observations imply that nonlocal flux-driven mechanism may be important for the observed turbulence and electron thermal transport.

• Research Organization: Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC02-76CH03073; FG03-95ER54295; FG03-99ER54518
• OSTI ID: 1254752
• Alternate ID(s): OSTI ID: 1224930
• Report Number(s): PPPL-5196; PHPAEN
• Journal Information: Physics of Plasmas, Vol. 22, Issue 11; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English