X-ray Diagnosis of Runaway Electron Distributions During Whistler-Wave Scattering Events in MST Tokamak Plasmas

One promising route for mitigation of runaway electrons (RE) in tokamaks stems from the excitation of whistler waves due to a RE-driven kinetic instability which scatters the fast particles and as a result limits their energy. Here, we study RE dynamics during bursts of whistler-wave activity in low-density (n_e ~ 10¹⁷ - 10¹⁸ m^-3), quiescent tokamak plasmas at low toroidal field, B_T = 0.13 T, in the Madison Symmetric Torus (MST). A soft-x-ray detector operating in pulse-height mode is used to measure emission spectra due to pitch-angle scattering of the RE. The x-ray distribution is tracked through whistler burst cycles which occur at a rate of 2 kHz and each last approximately 0.3 ms. Emission spectra show a drop in the higher-energy range, 10-15 keV, aligned with the onset of whistler waves and persisting to the end of heightened activity. Correlations between waves and x-ray emission are studied at both the individual burst scale and over the full I_P flat-top in an effort to understand the energy budget of the RE. The results are compared to Fokker-Planck simulations using the CQL3D code with a synthetic x-ray diagnostic. These results improve understanding of the evolution of RE radial and velocity-space distributions during wave-scattering events, which is potentially important for avoiding tokamak damage due to high-energy RE beams.