Characterization of Fast Ion Absorption of the High Harmonic Fast Wave in the National Spherical Torus Experiment

Ion absorption of the high harmonic fast wave in a spherical torus is of critical importance to assessing the viability of the wave as a means of heating and driving current. Analysis of recent NSTX shots has revealed that under some conditions when neutral beam and RF power are injected into the plasma simultaneously, a fast ion population with energy above the beam injection energy is sustained by the wave. In agreement with modeling, these experiments find the RF‐induced fast ion tail strength and neutron rate at lower B‐fields to be less enhanced, likely due to a larger β profile, which promotes greater off‐axis absorption where the fast ion population is small. Ion loss codes find the increased loss fraction with decreased B insufficient to account for the changes in tail strength, providing further evidence that this is an RF interaction effect. Though greater ion absorption is predicted with lower k∥, surprisingly little variation in the tail was observed, along with a small neutron rate enhancemen...