Results of High Harmonic Fast Wave Heating Experiments on NSTX
Authors: J.C. Hosea, R.E. Bell, M. Bitter, P. Bonoli, M. Carter, D. Gates, B.P. LeBlanc, R. Majeski, T.K. Mau, J. Menard, D. Mueller, M. Ono, S. Paul, C.K. Phillips, R. Pinsker, A. Rosenberg, P. Ryan, S.A. Sabbagh, D. Stutman, D. Swain, Y. Takase, J. Wilgen, and J.R. Wilson
Date of PPPL Report: August 2001
Presented at: the 28th EPS Conference on Controlled Fusion and Plasma Physics, Madeira, Portugal, June 18-22, 2001.
The study of high harmonic fast wave (HHFW) heating and current drive is being conducted on the National Spherical Torus Experiment (NSTX) device to determine the physics of applying radio-frequency (rf) waves at high harmonics (~10-20) of the ion cyclotron frequency in this high-beta plasma regime and to extend the performance of the NSTX plasma. The magnetic field of this low aspect ratio device is lower (less than or equal to 0.35 T for this work) than that for the typical moderate aspect ratio tokamak regime by about an order of magnitude and the plasma densities achieved are typically in the mid 1019 m-3 range. Thus, the dielectric constant of the plasma, e always equals wpe2/wce2, is of order ~50-100 resulting in wave physics properties which favor electron heating by TTMP and Landau damping. Radio-frequency power is applied on NSTX at 30 MHz using an antenna array with 12 current straps aligned in the poloidal direction. The antenna can be phased to launch waves with toroidal wave numbers, kT between 2 m-1 and 14 m-1 and can be phased for current drive with peak toroidal directionality at 7 m-1. To date most of the HHFW experiments have been carried out using kT = 14 m-1 with 0-p-0-p-... phasing of the strap currents. The diagnostic complement on NSTX includes a 30-Hz, 10-spatial-channel Thomson scattering (MPTS) system for measuring profiles of electron temperature Te and density ne every 33 msec, and a charge-exchange recombination spectroscopy (CHERS) system for measuring profiles of the impurity ion temperature Ti and toroidal rotation VT during a neutral beam blip. Strong electron and ion heating are observed in helium discharges, whereas the heating efficiency is noticeably reduced for deuterium discharges. A detailed comparison between helium and deuterium discharge responses at kT = 14 m-1 is presented here. Also, initial results for different radio-frequency phasing and start-up assist experiments will be discussed briefly.