  We consider in more detail a model previously proposed for the hard X-ray
(>10 keV) emission observed from the supernova remnant Cas A, whereby electrons
are accelerated by lower-hybrid waves and radiate bremsstrahlung. We consider
both cold and thermal plasma limits of the modified two-stream instability that
generates the lower-hybrid waves, and by studying time dependent ionization
balance for various components of the Cas A ejecta and shocked circumstellar
medium, find locations within the shell where one or other of the instabilities
may occur. Either instability can be effective, with the cold plasma
instability imposing fewer constraints on the shocked reflected ion population
responsible for exciting the waves. The instability must be located in the
ejecta shocked at the earliest times and therefore closest to the contact
discontinuity where magnetic fields are expected to be the strongest. The
energy deposited in this ejecta by collisions between accelerated and ambient
electrons is broadly consistent with that required to reheat this ejecta to
observed temperatures.
