Helicon wave absorption on energetic ions in fusion plasmas

Helicon waves can be tuned for absorption at mid-radius, providing off-axis heating and current drive via Landau damping on electrons in tokamaks. However, fast ions from neutral beam injection are energetic enough to resonate with the injected waves, potentially reducing the current drive efficiency. A common approach to estimating the parasitic absorption on fast ions is to approximate their velocity distribution as an "equivalent Maxwellian". To determine the sensitivity of the helicon fast ion interaction to this unphysical simplification, we calculate the helicon wave absorption for a more realistic slowing down distribution for fast ions. The validity of the approximation is assessed by comparing the rate of energy transfer to that of the Maxwellian for regimes of interest. The dependence of fast ion absorption rate on relevant physical parameters aids identification of regimes of nontrivial absorption in order to later avoid them. Future work would involve comparing with DIII-D data, implementing the slowing down distribution in the GENRAY ray tracing code, and generalizing the calculation of the absorption rate to related processes.