Effect of mirror and confinement vessel fields on the power flows from biased electrodes to the FRC electrons

In TAE Technologies’ experimental device, C-2W [1], record-breaking, advanced beam-driven field-reversed configuration (FRC) plasmas are produced. Long-lived, hot FRCs with large excluded flux radius are sustained in steady-state in the central confinement vessel by utilizing several advanced subsystems. These subsystems include variable energy neutral beams, advanced divertors, open field-line biased electrodes, variable axial magnetic fields with mirrors, a fueling setup, and an active plasma control system. In this presentation, we discuss dynamic biasing experiments used to infer power flows into FRC electrons from the biasing system at different mirror strengths and confinement vessel fields. The ionization of gas near the end electrodes creates high-energy electrons in the midplane due to the large axial potential between the electrodes and the confinement region. The presence of high-energy electrons is supported by the observation of high-energy x-rays in the confinement vessel that can couple energy to bulk electrons likely via beam-plasma/two-stream instability. With an increase in mirror strength, the fraction of the electrons passing through the mirror would drop, resulting in reduction in the power coupling to the FRC electrons.

[1] Gota et al., "Overview of C-2W: High-temperature, steady-state beam-driven field-reversed configuration plasmas", Fusion 61, 106039 (2021).