Helicon wave absorption on energetic ions in fusion plasmas

Helicon waves can provide off-axis heating and current drive via Landau damping on electrons in tokamaks. However, minority fast ions 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 a Maxwellian, which is less realistic than a slowing down distribution. The validity of the approximation is assessed by comparing the rate of energy transfer for regimes of interest in DIII-D, NSTX-U, and future reactors. Current results consist of the characterization of the cutoff-regime—where the Maxwellian distribution leads to an orders of magnitude over-approximation of the damping rate due to an unphysically long tail at high velocities—and the non-cutoff regime, where less significant discrepancies are associated with the shapes of the distribution functions themselves. A preliminary analysis of this distinction and its dependence, as well as a description of the boundary of this regime characterization in parameter space, is likewise presented.