Discovery and characterization of a population of Fermi-like accelerated electrons from an electrostatic oscillation in the PFRC-II run as a low-power tandem mirror*

Using Silicon Drift Detector (SDD) X-ray pulse-height spectrometers and Langmuir probes, we have discovered a population of electrons with an effective temperature of 3 keV and individual electrons above 30 keV in the PFRC-II device when run as a low-power tandem mirror. These electrons have density around 10^8/cm^3 vs bulk density 10^11/cm^3 and temperature ~5 eV. We have detected an electrostatic oscillation, ~100 MHz and ~10 V, at a mirror nozzle of the central cell, proposed to be caused by a spontaneously generated beam of electrons entering this cell. Like the apparatus of Alexeff et. al. in the 1960s and current experiments on the GOL-3 mirror machine, this beam-induced fluctuation causes heating of electrons. These machines were assumed to break resonance boundaries and cause electrons to become stochastic via a turbulent wave spectrum. In the PFRC-II, our calculations indicate the effect is caused by a Fermi-like acceleration. They indicate that mirror-bounce motion combined with a small potential fluctuation is sufficient to break resonances and cause stochastic electron motion. Magnetic moment non-adiabaticity is essential.