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

The evolution of the system sheath/presheath has been investigated experimentally. The ions of maximum kinetic impact energy ε max 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 ε max and its time dependence. The time evolution of ε max 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 ε max over an energy value of kT e /2.