Evolution of the maximum ion impact energy at an electrode during sheath and presheath formation

Abstract

The evolution of the system sheath/presheath has been investigated experimentally. The ions of maximum kinetic impact energy εmax collected at the surface of an orifice electrode are thereby used as test particles. The orifice electrode cylindrically surrounds an Ar plasma dc-excited in a parallel plate configuration, at a pressure of 10.8 × 10−2 mbar. A negative rectangular low-voltage pulse is applied to the orifice electrode. The resulting time dependent ion current is extracted and investigated. A combination of pulse length and retarding field variation allows one to determine εmax and its time dependence.The time evolution of εmax is measured during sheath and presheath formation. Sheath relaxation is investigated by applying a voltage pulse to an already existing sheath/presheath system in front of the orifice electrode. This is compared to the evolution of a new sheath starting from the sheath-free electrode. Probe measurements confirm that the presheath is formed within the last part of energy increase of εmax over an energy value of kTe/2.

Computational study of processes in plasma sheaths surrounding probes of various geometries

“…For the investigation of such problems the techniques of computational physics seem to be best suited. The transport of charged particles originating from undisturbed plasma to metal substrates or probes can be studied either by fluid modelling or by particle simulation approach, the later bringing usually much more information about processes in the sheath and presheath, [2]- [4]. In our work we concentrate on an electropositive gas, argon, because the discharge mechanisms and collision processes are most transparent there.…”