Plasma Breakdown and Hot Electron Production on WHAM

The Wisconsin HTS Axisymmetric Mirror (WHAM) is a linear plasma device recently built at UW – Madison which had its first plasma in July of 2024. WHAM achieves breakdown and heats electrons using its electron cyclotron heating (ECH) system, which operates at 110 GHz with powers up to 500 kW for 10ms. A overview of the gyrotron system will be presented, including flattop power control up to 500kW, arbitrary polarization control, and a launch and radial aiming system. Results of breakdown using ECH will be shown for different fueling and ECH scenarios, with a discussion of the theory behind ECH breakdown in mirrors and its relation to hot electrons. The timescale and mechanism of breakdown have been measured on WHAM, and align with prior theory. A set of low density, high ECH power shots will be presented, which produce x-rays of energies exceeding 500keV and a plasma which remains confined for seconds after ECH turnoff. These shots are seen to produce sloshing electrons whose turning points lie close to the ECH contour. Such electrons can be spun up to extremely high energies and therefore have much longer confinement times compared to a typical mirror plasma in WHAM. Lastly, a set of experiments attempting 2nd harmonic breakdown will be shown which, while unsuccessful at creating high density plasmas, do corroborate simulations predicting poor X2 absorption at low density.