Experimental Study of Parametric Dependence of Electron-scale Turbulence in a Spherical Tokamak
Authors: Y. Ren, W. Guttenfelder, S. M. Kaye, E. Mazzucato, R.E. Bell, A.Diallo, C.W. Domier, B. P. LeBlanc, K.C. Lee, D.R. Smith, and H. Yuh
Abstract:
Electron-scale turbulence is predicted to drive anomalous electron thermal transport. However, experimental
study of its relation with transport is still in its early stage. On the National Spherical Tokamak eXperiment (NSTX),
electron-scale density fluctuations are studied with a novel tangen- tial microwave scattering system with high radial
resolution of ±2 cm. Here, we report a study of parametric dependence of electron-scale turbulence in NSTX H-mode plasmas.
The dependence on density gradient is studied through the observation of a large density gradient variation in the core
induced by an ELM event, where we found the first clear experimental evidence of density gradient stabilization of electron-gyro
scale turbulence in a fusion plasma. This observation, cou- pled with linear gyro-kinetic calculations, leads to the identification
of the observed instability as toroidal Electron Temperature Gradient (ETG) modes. It is observed that longer wavelength
ETG modes, k⊥ρs < 10 (ρs is the ion gyroradius at electron temperature and k⊥ is the wavenumber perpendicular to local
equilibrium magnetic field), are most stabilized by density gradient, and the stabilization is accompanied by about a factor
of two decrease in electron thermal diffusivity. Comparisons with nonlinear ETG gyrokinetic simulations shows ETG turbulence
may be able to explain the experimental electron heat flux observed before the ELM event. The collisionality dependence of
electron-scale turbulence is also studied by systematically varying plasma current and toroidal field, so that electron gyroradius
(ρe ), electron beta (βe ) and safety factor (q95 ) are kept approximately constant. More than a factor of two change in
electron collisionality, ν ∗e, was achieved, and we found that the spectral power of electron-scale turbulence appears to increase as ν ∗e is decreased in this
collisonality scan. However, both linear and nonlinear simulations show no or weak dependence with the electron-ion collision
frequency, ν e/i . Instead, other equilibrium parameters (safety factor, electron density gradient, for example) affect ETG
linear growth rate and electron thermal transport more than ν e/i does. Furthermore, electron heat flux predicted by the simulations is
found to have an order-of-magnitude spatial variation in the experimental mea- surement region and is also found to be much smaller
than experimental levels except at one radial location we evaluated. The predicted electron heat flux is shown to be strongly
anti-correlated with density gradient which varies for a factor of three in the measurement region, which is in
agreement with the density gradient dependence study reported in this paper.
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Submitted to: AIP (February 2012)
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Download PPPL-4781 (pdf 6.19 MB 38 pp)
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