One-dimensional simulations of current-driven plasma sheaths

Abstract

A one-dimensional model that treats both electron and ion dynamics with the particle-in-cell method is used to study nonequilibrium electron effects in current-driven plasma sheaths. These simulations show two distinct phases of operation: a low impedance phase, where the diode voltage is small, followed by a phase of rapidly increasing diode voltage and impedance. Early in the low impedance phase, the initial plasma electrons are accelerated toward the anode to conduct the discharge current. Emitted electrons are drawn into the anode–cathode gap by the uncovered positive space charge, leading to the formation of virtual cathodes in the diode. Trapping and heating of the emitted electrons occur as a result of nonequilibrium dynamics in the time-varying virtual cathode potential structure associated with the rising circuit current. The virtual cathode potential structure is characterized by a large potential hill with a large potential drop occurring in the anode sheath. The potential drop in the anode sheath is comparable to the potential rise in the cathode sheath and is responsible for keeping the net diode voltage low during the low impedance phase. The virtual cathode electric field structure accelerates ions toward both electrodes, resulting in a significant depletion of ion charge by the end of the low impedance phase. During the transition to high impedance rapid growth of the cathode sheath is observed. This growth occurs to provide the ions necessary to shield the electric field of the cathode sheath from the bulk plasma. The substantial loss of ions at the anode during the low impedance phase causes the cathode sheath and voltage to grow faster than predicted by quasistatic models. This effect is more pronounced in the higher density simulation, where the loss of ions during the low impedance phase is more significant.