Modes and the alpha-gamma transition in rf capacitive discharges in N2O at different rf frequencies

This paper reports current-voltage characteristics and pressure-voltage transition curves from the weak-current α-mode to the strong-current γ-mode for rf capacitive discharges in N2O at frequencies of 2MHz, 13.56MHz, and 27.12MHz. At 2MHz the rf discharge is mostly resistive whereas at 13.56MHz and 27.12MHz it is mostly capacitive. The weak-current α-mode was found to exist only above a certain minimum gas pressure for all frequencies studied. N. Yatsenko [Sov. Phys. Tech. Phys. 26, 678 (1981)] previously proposed that the α−γ transition corresponds to breakdown of the sheaths. However, we show that this is the case only for sufficiently high gas pressures. At lower pressure there is a smooth transition from the weak-current α-mode to a strong-current γ-mode, in which the sheaths produce fast electrons but the sheath has not undergone breakdown.