Probing suprathermal electrons in C-2W with x-ray energy spectra

A silicon drift-detector-based pulse-height x-ray energy spectrometer has been recently deployed on C-2W to investigate the distribution of suprathermal electrons. Measurements from this spectrometer reveal x-ray photons with energies of several kiloelectronvolts from a bias-driven mirror plasma with an electron temperature of approximately 150 eV. These high-energy photons follow an exponential distribution with an effective temperature that is significantly higher than the bulk electron temperature, providing compelling evidence for the presence of suprathermal electrons. The effective temperature increases with the applied bias voltage and the fueling rate near the biased electrodes. In addition, x rays up to 6 keV were detected even though the bias voltage was less than 3 kV. These observations support a proposed bias-driven electron heating mechanism in which a stream of electrons accelerated by biased electrodes transfer energy to plasma electrons via a two-stream instability.