XOOPIC Simulation of Ion Motion and Sheath Behavior Caused by a Sudden Negative Pulse Applied to an Electrode

It is well known that a plasma produces a transient ion-matrix sheath when a sudden negative step bias is applied to a grid-shaped electrode. During this sheath forming process, ions are expected to be rushing to the grid electrode. Due to the grid structure, rushing ions are trapped and oscillate in the negative potential well formed around the grid. As soon as the bias is removed from the grid, ions propagate with a ballistic mode, forming a pseudowave. In this research, we studied the characteristics of the trapped ion motion by using the XOOPIC plasma particle simulation program. The energy distribution of ions and their dependence on the negative pulse shape and grid spacing, together with transient sheath expansion phenomena, were studied. We observed that the voltage rise time of the pulse did not affect the pseudowave properties. On the other hand, the pulse fall time was crucial to the pseudowave characteristics because ions escaping from the sheath were decelerated during the fall time. Grid spacing and the pulse width effects were also considered.