Investigation of the Paschen curve for helium in the 100-1000 kV range

The left branch of the Paschen curve for helium gas is studied both experimentally and by means of particle-in-cell/ Monte Carlo collision (PIC/ MCC) simulations. The physical model incorporates electron, ion, and fast atom species whose energy-dependent anisotropic scattering on background neutrals, as well as backscattering at the electrodes, is properly accounted for. For the range of breakdown voltage 15 kV <= V-br <= 130 kV, a good agreement is observed between simulations and available experimental results for the discharge gap d =1.4 cm. The PIC/ MCC model is used to predict the Paschen curve at higher voltages up to 1MV, based on the availability of input atomic data. We find that the pd similarity scaling does hold and that above 300 kV, the value of pd at breakdown begins to increase with increasing voltage. To achieve good agreement between PIC/ MCC predictions and experimental data for the Paschen curve, it is essential to account for impact ionization by fast atoms (produced in charge exchange) and ions and for anisotropic scattering of all species on background atoms. With the increase of the applied voltage, energetic fast atoms progressively dominate in the overall ionization rate. The model makes this clear by predicting that breakdown would occur even without electron-and ion-induced ionization of the background gas, due to ionization by fast atoms backscattered at the cathode, and their high production rate in charge exchange collisions. Multiple fast neutrals per ion are produced when the free path is small compared to the electrode gap. Published by AIP Publishing.