Parametric Decay during HHFW on NSTX
Authors: J.R. Wilson, S. Bernabei, T. Biewer, S. Diem, J. Hosea, B. LeBlanc, C.K. Phillips, P. Ryan, and D.W. Swain
Date of PPPL Report: May 2005
Presented at: the Sixteenth Topical Conference on Radio Frequency Power in Plasmas, 11 - 13 April 2005, Park City, Utah
High Harmonic Fast Wave (HHFW) heating experiments on NSTX have been observed to be accompanied by significant edge ion heating (Ti >> Te). This heating is found to be anisotropic with Tperp > Tpar. Simultaneously, coherent oscillations have been detected with an edge Langmuir probe. The oscillations are consistent with parametric decay of the incident fast wave (ω > 13ωci) into ion Bernstein waves and an unobserved ion-cyclotron quasi-mode. The observation of anisotropic heating is consistent with Bernstein wave damping, and the Bernstein waves should completely damp in the plasma periphery as they propagate toward a cyclotron harmonic resonance. The number of daughter waves is found to increase with rf power, and to increase as the incident wave's toroidal wavelength increases. The frequencies of the daughter wave are separated by the edge ion cyclotron frequency. Theoretical calculations of the threshold for this decay in uniform plasma indicate an extremely small value of incident power should be required to drive the instability. While such decays are commonly observed at lower harmonics in conventional ICRF heating scenarios, they usually do not involve the loss of significant wave power from the pump wave. On NSTX an estimate of the power loss can be found by calculating the minimum power required to support the edge ion heating (presumed to come from the decay Bernstein wave). This calculation indicates at least 20–30% of the incident rf power ends up as decay waves.