Ionization in helium at high pressures

An apparatus for the measurement of steady-state pre-breakdown ionization currents in helium at high pressures is described, and the sampling procedure necessary to obtain with it reproducible results discussed. The primary and secondary ionization coefficients determined from these measurements are given for 3 3 < E/p0 < 5.0 v cm-1 mmHg-1 and for 150 < p0 < 560 mmHg. The values of ? are discussed in relation to previous theoretically computed values and the experimental values obtained at lower gas pressures but with similar values of E/p0. The values of the generalized secondary ionization coefficient are found to be relatively high (similar 0.1) and to increase as E/p0 decreases.