Dual mode operation of a helical resonator discharge

Measurements of the resonance characteristics of a helical resonator plasma generator over a wide range of frequencies and gas pressures are described. It was found that, analogous to an inductive discharge, it can operate both in a capacitive and an inductive mode. However, in the helical resonator either mode can be selected for one gas pressure and at similar power simply by changing the operating frequency. This will produce an extended plasma for the capacitive mode and a plasma concentrated in the resonator for the inductive mode. The inductive mode is enhanced at higher power levels and higher pressures. The pressure range investigated ranged from 0.05 to 10 Torr. Electron densities were measured with a microwave interferometer both inside the resonator and downstream and reached 3×1010 cm−3 for 0.5 Torr argon at 0.075 W/cm−3 rf power. The resonance characteristics were affected by gases such as N2 and SF6.