The radiofrequency breakdown in low pressure argon

The Monte Carlo simulation technique, previously used by the author (1996 Nuovo Cimento D 18 919) for computing, in the case of an ideal gas and plane-parallel electrodes, the threshold radiofrequency (rf) voltage for breakdown versus the gas pressure in the transition region from diffusion to multipacting controlled conditions, has been applied to the case of a real gas, argon. For this purpose the Monte Carlo algorithm has been modified to include the elastic and inelastic argon cross sections for electron collisions, the finite size of the electrodes and also a surface reflection coefficient for the primary electrons impinging on the electrodes. Furthermore, the so-called null-collision method has been introduced in order to handle the velocity dependency of the electron-atom collision frequency and, for a better description of the secondary emission characteristics of the electrodes, Vaughan's empirical formula has been preferred to the previously used Dionne's formula. The modified algorithm has been used for determining the threshold breakdown voltages for the experimental set-up conditions of Hoehn et al (1997 Phys. Plasmas 4 940). With the agreement with the experimental data being satisfactory, breakdown voltage values have also been determined for low-pressure conditions of practical interest in the case of argon rf capacitive reactors. In particular, indicative limits for the existence of the discharge at very low pressures are given and the behaviours of the pressure-dependent curves, providing the breakdown voltage versus the ratio of gap width over wavelength, are discussed.