Inductive and capacitive discharge modes in helical resonator plasmas

Abstract

Summary form only given. In order to observe the inductive and capacitive modes at lower powers, a medium-size helical resonator (11 diam., 12.5 length, 11 turn coil with 6.25 diam.), with a nominal resonance frequency of 13.56 MHz and a characteristic impedance of 970 Ohms, was used as a plasma generator at RF input powers from 27 to 150 W. Gas pressure ranged from 0.005 to 1 Torr. Measuring forward and reflected power and standing wave ratio as a function of frequency, it was found that the capacitive mode resonance was lower in frequency than the inductive mode resonance by approximately 5 MHz. The inductive mode resonance was suppressed for low power and a low transformer ratio (3:11 and lower). At high transformer ratios (up to 1:22) the capacitive mode was suppressed, although at low pressures another mode appeared at much higher frequencies. As has been observed for inductive discharges, operation in the capacitive mode is characterized by the discharge extending outside the cavity, possibly to some grounded part; in the inductive mode the discharge is better confined to the cavity volume and has an annular profile across the tube diameter.