High-Harmonic Fast-Wave Heating in NSTX
Authors: B.P. LeBlanc, R.E. Bell, P.T. Bonoli, J.C. Hosea, D.W. Johnson, T.K. Mau, J. Menard, D. Mueller, M. Ono, F. Paoletti, S. Paul, C.K. Phillips, R. Pinsker, A. Rosenberg, P.M. Ryan, S.A. Sabbagh, D. Stutmam, D.W. Swain, Y. Takase, J.B. Wilgen, and J.R. Wilson
Date of PPPL Report: June 2001
Presented at: the 14th Topical Conference on Applications of Radio Frequency Powers in Plasmas held in Oxnard, CA, May 7-9, 2001. Proceedings to be published by the American Institute of Physics (AIP).
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 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|| 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 Teo more than doubled from approximately 0.5 keV to 1.15 keV. In deuterium plasmas, HHFW heating was found less efficient, with a Teo 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, Teo 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 Ti. Preliminary CHERS analysis show that Ti approximately equaled Te 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 neo less than or equal to 8 x 1013 cm-3.