We investigated the breakdown in low-pressure helium gas both experimentall
y 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 agree
ment (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 heliu
m at low pressures. These processes are: (1) the dependence of the lion-ind
uced) secondary electron emission yield on the ion energy, (2) the appearan
ce 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.