Resonant pitch-angle scattering of runaway-electrons by externally-launched helicon waves in the DIII-D tokamak

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Abstract
Resonant wave–particle interactions between externally launched helicon waves (also known as whistler waves) and runaway electrons (REs) have been demonstrated on the DIII-D tokamak. In this work we extend the initial results reported in (Choudhury H. et al 2026 Phys. Rev. Lett. 136 025101) by exploring the effects of antenna alignment with the edge magnetic field, toroidal wave propagation direction, and coupled power on RE scattering in the quiescent RE experimental scenario. Two distinct experimental configurations have been investigated: one in which the antenna’s Faraday cage aligns well with the edge background magnetic field, known as the ideal antenna configuration, and one with misalignment, known as the non-ideal case. Previously, it had been found that helicon power in the ideal antenna configuration prevented RE growth despite the normalized toroidal electric field remaining high enough to drive exponential RE growth in the absence of helicon power. In this paper, we show that scattering via the normal Doppler resonance ( n = 1 ) effectively limits the growth of the RE population in both the ideal and non-ideal antenna configurations, with the effect of average power and energy deposited investigated. In contrast, launching waves that favor the anomalous Doppler resonance ( n = − 1 ) is observed to enhance rather than reduce the RE population. In addition, fast magnetic measurements reveal rising-tones in the 30–60 MHz range during helicon-off periods, which are not observed prior to helicon power. Finally, the challenges of using launched helicon waves to scatter post-disruption RE beams are discussed. Collisional damping and a large vacuum gap between the plasma and antenna on the outboard side present significant obstacles to helicon waves propagating into the plasma core.
Funding Information
  • U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences
  • . Department of Energy Office of Fusion Energy Sciences (89233218CNA000001)
  • Office of Science of the U.S. Department of Energy (DE-AC02-05CH11231)
  • DIII-D National Fusion Facility
  • DOE Office of Science (DE-AC05-00OR22725)
  • U.S. Department of Energy National Nuclear Security Administration (89233218CNA000001)
  • United States Government
  • NERSC (FES-ERCAP0032298)

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