X-Ray Energy Spectra of Plasmas in C-2W

TAE Technologies' current experimental device, C-2W (also called "Norman"), sustains a field-reversed configuration (FRC) plasma utilizing a combination of neutral beam injection, edge biasing, and a robust real-time plasma control system [1]. > 500 eV x-rays emitted from plasmas in C-2W are routinely observed by beryllium-filtered bolometers. The observed x-rays correlate with bulk electron temperatures, ionization rates near the edge biasing electrodes, and edge biasing voltages [2]. These correlations support a bias-driven electron heating mechanism, where fast electrons accelerated by the edge biasing system transfer energy to bulk plasma electrons via either a two-stream instability or Coulomb collisions. Probing the energy distribution of these fast electrons is valuable for a better understanding of how energy is transferred from the fast electrons to the bulk plasma electrons. Toward that goal, an x-ray spectroscopy diagnostic is being developed to spectrally resolve 1 – 20 keV x-rays with an energy resolution of < 200 eV. Preliminary x-ray spectra measured by this diagnostic show high-energy tails that can be modeled as Maxwellian distributions with suprathermal effective temperatures. The effective temperatures of these high-energy tails correlate strongly with edge biasing voltages. This observed correlation demonstrates that the x-ray spectra are physically meaningful and provides further experimental evidence for the bias-driven electron heating mechanism.