Near-wall sheath in a plasma with non-local fast electrons

It is demonstrated that the presence of a small number of fast, non-local electrons can dramatically change the thickness and electric field in the near-wall sheath. Even if the density of the non-local ``fast group,'' $n_f$, is much less than the density of the bulk electrons, $n_b$, ($n_f \sim 10^{-5} n_b$), the near wall potential can increase dramatically resulting in a comparable increase in the sheath thickness. Due to this low fractional density, the average energy (electron temperature, $T_e$) of all electrons is little changed from that of the bulk, yet the near-wall potential drop can increase to 10's of $T_e/e$. More importantly, due to the non-local nature of this group of electrons, the near-wall sheath potential does not depend on $T_e$ at all and is determined only by the energy of the fast group. In the local approximation, the near-wall sheath potential is essentially determined by the average electron energy and can be influenced by fast electrons only if they significantly alter $T_e$. This work was supported by The Air Force Office of Scientific Research.