  Generation of anomalously energetic suprathermal electrons was observed in
simulation of a high- voltage dc discharge with electron emission from the
cathode. An electron beam produced by the emission interacts with the
nonuniform plasma in the discharge via a two-stream instability. Efficient
energy transfer from the beam to the plasma electrons is ensured by the plasma
nonuniformity. The electron beam excites plasma waves whose wavelength and
phase speed gradually decrease towards anode. The short waves near the anode
accelerate plasma bulk electrons to suprathermal energies. The sheath near the
anode reflects some of the accelerated electrons back into the plasma. These
electrons travel through the plasma, reflect near the cathode, and enter the
accelerating area again but with a higher energy than before. Such particles
are accelerated to energies much higher than after the first acceleration. This
mechanism plays a role in explaining earlier experimental observations of
energetic suprathermal electrons in similar discharges.
