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 tau(e). For the uncharged wall, we find s(e) << 1 and tau(e) approximate to 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 tau(-1)(e) approximate to 0. Thus, tau(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.