Comparison of Moderate to High Ion Cyclotron Absorption on Energetic Ions in NSTX and DIII-D

Strong absorption of fast waves (FWs) on injected deuterons at ion cyclotron harmonic numbers in the 4-10 range is observed on both DIII-D and NSTX. The results from fast ion $D_\alpha$ spectroscopic measurements from the two devices differ significantly: deposition on fast ions peaks near the cyclotron harmonic layer closest to the magnetic axis in the conventional-aspect-ratio DIII-D, while results from the low-aspect-ratio NSTX show a broader deposition profile [1]. One root of the difference stems from the absorbing fast ions sampling more harmonic layers in NSTX than in DIII-D. We investigate cyclotron absorption in cases with multiple harmonic layers within a single ion gyroradius and related phenomena numerically and analytically by examining the response of individual charged particles to rf fields in various field configurations.\par \vskip8pt \noindent [1] M. Podesta {\em et al.}, {\em RF Power in Plasmas} (Proc.18th Top.\ Conf., Gent, Belgium, 2009), to be published.