Generation of a long uniform low-temperature RF discharge in helium up to atmospheric pressure

Abstract

A long uniform low-temperature discharge is achieved inside a 1 mm radius fused silica tube using plane parallel linear electrodes. At a driving frequency of 13.56 MHz, a pure helium discharge is generated from 10 Torr to atmospheric pressure without any gas flow. Depending on the applied power and pressure, the discharge occurs either in the Ω or in the γ mode. As long as the gas flow is zero, the discharge shows no gradient along the electrodes. Consequently, the length of the discharge is only limited by the axial dimension of the electrodes. As the pressure increases, the Ω to γ transition occurs at higher and higher power densities. The breakdown voltage is minimum at 75 Torr with a power density down to 0.1 W cm−3. However, below 20 Torr, charged particle losses become very important so that the breakdown and maintaining voltages increase drastically. Regardless of the pressure, the gas temperature is found to always lie between 300 and 520 K.