Spontaneous multi-keV electron generation in a low-RF-power axisymmetric mirror machine

X-ray emission shows the existence of multikilo-electron-volt electrons in low-temperature, low-power, capacitively coupled RF-heated magnetic-mirror plasmas that also contain a warm (300 eV) minority electron population. Though these warm electrons are initially passing particles, we suggest that collisionless scattering - μ nonconservation in the static vacuum field - is responsible for a minority of them to persist in the mirror cell for thousands of transits during which time a fraction is energized to a characteristic temperature of 3 keV, with some electrons reaching energies above 30 keV. A heuristic model of the heating by a Fermi-accelerationlike mechanism is presented, with μ nonconservation in the static vacuum field as an essential feature.