THE RADIO FREQUENCY SHEATH IN SELF SUSTAINED PLASMOIDS

Abstract
Capacitive RF discharges are sustained by the AC current which traverses the sheaths and superimposes an AC voltage to the DC potential between plasma and electrodes/walls. When frequency is low enough, the sheath impedance is high and the peak value of the AC voltge across the sheath can be larger than the DC thermal voltage drop : this opens a gate in the potential barrier which keeps the electrons from leaving the discharge, and increases the positive space charge. Steady state is reached when the potential barrier is restituted, i.e when the plasma potential is increased up to the peak value of the AC voltage across the sheath. This effect is similar to rectification and clamping in electronic circuitry. In connection to this increase in plasma potential, sheath thickness is also increased, according to Child's law.