Fast response of electron-scale turbulence to auxiliary heating cessation in National Spherical Torus Experiment
Authors: Y. Ren, W.X. Wang, B. P. LeBlanc, W.
Guttenfelder, S. M. Kaye, S. Ethier,
E. Mazzucato, K.C. Lee, C.
W. Domier, R. Bell, D. R. Smith, and H. Yuh
Abstract: In this letter, we report the first
observation of fast response of electron-scale turbulence to
auxiliary heating cessation in National Spherical Torus eXperient
(NSTX) [M. 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
signi ficantly following fast cessation of RF heating that occurs
in less than 200 us. 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 con nement time of abut 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 profi les 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. The
observations imply that nonlocal flux-driven mechanism may be
important for the observed turbulence and electron thermal
transport.
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Submitted to: Physics of Plasmas
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