THEORETICAL AND EXPERIMENTAL INVESTIGATIONS ON THE PROCESS OF HIGH-PRESSURE DISCHARGE DEVELOPMENT

The voltage breakdown behavior of a plane-parallel gap of 0.48-mm length filled with helium was examined at atmospheric pressure with admixtures of dry air at relative pressures of 0, 10(-4), 3 x 10(-4), and 10(-3). The initial stages of the breakdown were investigated by means of a quantitative model consisting of the electron, ion, and excited-particle conservation equations and the Poisson equation. The system of equations was solved for an applied voltage of 180 V, corresponding to a field of 3750 V cm-1, at one single partial pressure of the impurities, p(r) = 10(-4). Two numerical routines were used for the solution: A commercial IMSL subroutine TWODEPEP, and a newly developed method of solution in several fractional steps. The results were compared up to about 15-mu-s, at which time the growing instability of the commercial subroutine began to require an excessively long computer time: At this time, the ratio of the total numbers of electrons and ions obtained by the first and second method was 0.8 and 0.9, respectively. The axial profiles of the density of electrons, ions, and excited particles were obtained up to a time of 16.75-mu-s, reaching maximum values of 6.8 x 10(10) cm-3, 4.9 x 10(11) cm-3, and 4.3 x 10(12) cm-3, respectively. The maximum value of the space-charge field attained at the cathode was 10.2 kV cm-1. The experimental setup was designed to measure the initial rise of the breakdown current. Several series of measurements were performed at various overvoltages at each of the given impurity partial pressures. The rate of rise of the current curve obtained experimentally was compared with the calculated curve; the value of the theoretical slope was found to fall between the experimental slopes, corresponding to the relative partial pressures of p(r) = 3 x 10(-4) and 10(-3). This is interpreted as an indication that the Penning ionization coefficient was overestimated by approximately a factor of 3.