SHEATH DYNAMICS OF ELECTRODES STEPPED TO LARGE NEGATIVE POTENTIALS

Results are presented of particle simulations of the time-dependent behavior of the plasma sheath surrounding an electrode whose potential is abruptly changed from 0 to some negative value. The surrounding plasma is assumed isotropic and collisionless, and the electrode is a sphere or infinite cylinder. Potential steps up to - 1000kT(e)/e are considered. The time histories and frequency spectra of sheath and plasma oscillations initiated by the potential step are studied. The energy in sheath oscillations (frequency < omega(p)) escapes by nonlinear coupling to plasma oscillations (frequency omega(p)). The time scale for sheath-plasma coupling depends strongly on the ion-to-electron mass ratio. Both linear and nonlinear ion acoustic waves are excited by the response to the potential step. Near a small (radius 1lambda(D)) cylindrical electrode, trapped ions are shown to make an important contribution to the sheath ion density. The height of the transient peak in collected current for a cylindrical electrode displays strong dependence on the ion-to-electron temperature ratio, for sufficiently large potential steps.