High-harmonic Fast-wave Heating in NSTX

Abstract

High-Harmonic Fast-Wave (HHFW), a radio-frequency technique scenario applicable to high-beta plasmas, has been selected as one of the main auxiliary heating systems on the National Spherical Torus Experiment (NSTX). The HHFW antenna assembly comprises 12 toroidally adjacent current elements, extending poloidally and centered on the equatorial plane. This paper reviews experimental results obtained with a symmetrical (vacuum) launching spectrum with k|| = 14 m(superscript ''-1'') at a frequency of 30 MHz. We describe results obtained when HHFW power is applied to helium and deuterium plasmas, during the plasma-current flattop period of the discharge. Application of 1.8-MW HHFW pulse to MHD quiescent plasmas resulted in strong electron heating, during which the central electron temperature T(subscript ''eo'') more than doubled from approximately 0.5 keV to 1.15 keV. In deuterium plasmas, HHFW heating was found less efficient, with a central electron temperature increase of the order of 40% during a 1.8-MW HHFW pulse, from approximately 400 eV to approximately 550 eV. (At HHFW power of 2.4 MW, central electron temperature increased by 60%, reaching 0.625 keV.) HHFW heating in presence of MHD activity is also discussed. A short neutral-beam pulse was applied to permit charge-exchange recombination spectroscopy (CHERS) measurement of the impurity ion temperature T(subscript ''i''). Preliminary CHERS analysis show that ion temperature approximately equals electron temperature during HHFW heating. Of special interest are deuterium discharges, where the application of HHFW power was done during the current ramp-up. We observe the creation of large density gradients in the edge region. In the latter case, the density rose spontaneously to n (subscript ''eo'') less than or equal to 8 x 10 (superscript ''13'') cm (superscript ''-3'').