Experimental Study of Parametric Dependence of Electron-scale Turbulence in a Spherical Tokamak

Ren, Y ; Kaye, S M ; Mazzucato, E ; Bell, R E ; Diallo, A ; Domier, C W ; LeBlanc, B P ; Lee, K C ; Smith, D R

American Institute of Physics

关键词: Diffusion;    Drift Instability;    Confinement;    Fluctuations;    High-beta PlasmasSpherical Torus;    Stability;    Microinstability;    Tokamaks;    NSTX Transport Coefficients;    Transpsort Phenomena;    Turbulence;    Wave Scattering;   

DOI  :  10.2172/1056835
RP-ID  :  PPPL-4781
PID  :  OSTI ID: 1056835

学科分类：原子、分子光学和等离子物理

美国|英语

来源: SciTech Connect

【 摘 要 】

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 tangential 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?????s < 10 (??s 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 fleld, so that electron gyroradius (??e ), electron beta (??e ) and safety factor (q95 ) are kept approximately constant. More than a factor of two change in electron collisionality, ?? *e, was achieved, and we found that the spectral power of electron-scale turbulence appears to increase as ?? *e is decreased in this collisonality scan. However, both linear and nonlinear simulations show no or weak dependence with the electron-ion collision frequency, ?? e/i . Instead, other equilibrium parameters (safety factor, electron density gradient, for example) affect ETG linear growth rate and electron thermal transport more than ?? 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.