Runaway electron x-ray emission during the startup of MST tokamak plasmas

Energetic electron emission has been observed during the startup (∼0-10 ms) of reproducible tokamak plasmas (I_p = 40-60 kA, n_e < 0.05-0.2 x 10¹⁹ m^-3) produced at the Madison Symmetric Torus (MST). The multi-energy soft x-ray (ME-SXR) pinhole camera provides brightness measurements of the bremsstrahlung radiation given off by the suprathermal runaway electrons (RE’s) with space (Δr/a≈2%), time (∼1 ms), and energy resolution. Motivated by an ITPA Joint Analysis task, the effects of prefill gas pressure, applied electric field, and current ramp-up rate on the startup of MST tokamak plasmas were investigated with a specific focus on the RE emission. Resonant magnetic perturbations (RMPs) with m=1, 2, and 3 fields were applied with different amplitudes, phases, and times to explore the suppression of these RE’s during startup. Preliminary results suggest that RE acceleration is decreased at sufficiently high pressures, but the consistent presence of RE emission indicates that RE generation is a robust feature of MST tokamak plasma startup. A new gating capability was tested using the versatile PILATUS3 detector to improve the time-resolution of the x-ray measurement from 0.5 kHz to 2 kHz and 4 kHz, providing a more complete picture of the RE emission during the breakdown and startup of the plasma.