Effect of different elementary processes on the breakdown in low-pressure helium gas

We investigated the breakdown in low-pressure helium gas both experimentally and by computer simulations. At low breakdown voltages (V-BR less than or equal to 1000 V) the experimental and simulation results show a good agreement (differences are within 20%), while at higher voltages the simulations and experiments agree qualitatively. Our simulations indicate that several processes contribute to the particular shape of the Paschen curve in helium at low pressures. These processes are: (1) the dependence of the lion-induced) secondary electron emission yield on the ion energy, (2) the appearance of ion impact ionization of the gas at high electric fields and (3) the secondary electron emission from the cathode due to fast neutral atoms.