Plasma Walls Beyond the Perfect Absorber Approximation for Electrons

Plasma walls accumulate electrons more efficiently than ions, leading to wall potentials which are negative with respect to the plasma potential. Theoretically, walls are usually treated as perfect absorber for electrons and ions, implying perfect sticking of the particles to the wall and infinitely long desorption times for particles stuck to the wall. For electrons, we question the perfect absorber model and calculate, specifically for a planar dielectric wall, the electron sticking coefficient s_e and the electron desorption time τ_e. For the uncharged wall, we find s_e ≪ 1 and τ_e≈ 10^-4 s. Thus, in the early stage of the build-up of the wall potential, when the wall is essentially uncharged, the wall is not a perfect absorber for electrons. For the charged wall, we find τ_e^-1≈ 0. Thus, τ_e approaches the perfect absorber value. But s_e is still only of the order of 10^-1. Calculating s_e as a function of the wall potential and combining this expression with the quasi-stationary balance equations for the electron and ion surface densities, we find the self-consistent wall potential, including surface effects, to be 30% of the perfect absorber value.