Runaway Electron Emission During Whistler-Wave Activity and Current Ramps in MST Tokamak Plasmas

One route to mitigation of runaway electrons (RE) in tokamaks stems from the excitation of whistler waves due to a RE-driven kinetic instability. This instability scatters the fast electrons into the perpendicular direction, limiting their parallel velocity. We study RE dynamics in low-density, n_e ∼ 10¹⁷ m⁻³, tokamak plasmas at low toroidal field, B_T = 0.13 T, in the Madison Symmetric Torus (MST). A soft-x-ray detector in pulse height analysis mode is used to measure radial emission. Whistler-frequency emissions with peak intensity around f ∼ 3.2 GHz, 1.3 GHz, and 40 MHz are measured with a high-frequency B-dot probe inserted to r/a = 0.8. After plasma initiation, whistlers and x-rays appear with ∼0.3 ms of activity followed by ∼0.3 ms of inactivity, a pattern which repeats quasiperiodically. Time-resolved energy emissions show a peak around 4 keV only for periods of high x-ray flux. Additionally, programmable power supplies allow for control over the poloidal and toroidal magnetic fields. By programming plasma current ramps, we create spikes in loop voltage, e.g., from 2 V up to 6 V or down to -2 V. The soft x-ray detector shows a corresponding change in x-ray flux indicating some control over the RE.