X-Ray Emissions from Runaway Electrons During Whistler-Wave Activity and Current Ramps in MST Tokamak Plasmas

One route for mitigation of runaway electrons (RE) stems from the excitation of whistler waves due to a RE-driven kinetic instability, which pitch-angle scatters fast electrons, limiting their parallel velocity. In the Madison Symmetric Torus (MST), using an internal B-dot probe, we observe temporal bursts of high frequency magnetic fluctuations in several bands up to the electron cyclotron frequency. These bursts are correlated with x-ray emission from runaway electrons, suggesting a kinetic instability at higher relative frequencies than previously observed. These dynamics appear in low-density (n_e ∼ 10¹⁷ m⁻³) tokamak plasmas at low toroidal field, B_T = 0.13 T. X-ray measurements are made using a soft-x-ray detector on a radial viewing chord operating in pulse-height-analysis mode. A comparison is made between observed x-ray signals and a CQL3D based synthetic diagnostic. MST ‘s programmable power supplies allow fine control over the poloidal and toroidal magnetic field circuits. We use programmed ramps in plasma current to induce transient periods of increased applied electric field, nearing the Dreicer field. The soft x-ray measurement shows an increase in flux corresponding with the increased electric field, indicating active influence over the RE population.

Work supported by U.S. DOE award DE-SC0018266 and NSF award PHY 1828159.