  X-ray emission shows the existence of multi-keV 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 - mu non-conservation 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 are 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-acceleration-like mechanism is presented, with mu
non-conservation in the static vacuum field as an essential feature.
