THEORY OF A COLLISION-DOMINATED SPACE-CHARGE SHEATH ON AN EMITTING CATHODE

A diffusion model of sheaths with frozen ionization and recombination on emitting cathodes is considered. Governing equations are solved asy mptotically and numerically. It is found, in particular, that the elec tric field in the sheath is directed from the cathode surface into the plasma at low current densities and in the opposite direction at high current densities. Special attention is paid to values of the electri c field at the cathode surface, which can contribute to electron emiss ion, and of the charged-particle density at the sheath edge, which pro vides a boundary condition for equations describing the distribution o f the charged particles in the quasineutral region. On the basis of as ymptotic and numerical results, approximate analytical formulae for th ese quantities are constructed.