EFFECT OF AIR ADMIXTURE IN HELIUM ON THE RATE OF THE BREAKDOWN CURRENT RISE

The influence of dry air admixtures upon the breakdown mechanism of helium in small gap separations was investigated, using plane parallel stainless-steel electrodes enclosed within a Pyrex chamber. The test chamber was evacuated to a limit pressure of 10(-5) torr and the electrodes degassed under a glow discharge. All experiments were carried out under atmospheric pressure, with the additions of air being made at relative partial pressures of p(r) = 0, 10(-4), 3 x 10(-4), 10(-3), and 3 x 10(-3). The initial breakdown current was measured by means of a digital oscilloscope and the semilogarithmic plots of the discharge current as a function of time (up to ca. 100muA) were found to exhibit an exponential dependence. The reciprocal value of the time constant of the breakdown current rise, tau-1, was demonstrated to vary in accordance with the linear relation tau-1 = tau(o)-1 p(r)m DELTAV for an overvoltage DELTAV = V - V(b) (for V > V(b), V(b) being the breakdown voltage) in the range between zero to ca. 35 V; a log-log plot of tau-1 versus the applied voltage, DELTAV, with relative partial pressure p(r) as a parameter was shown to lead the empirical relationship tau0(-1) = 3.11p(r)0.6 (s-1 V-1) valid within the range from ca. 10(-4) to 3 x 10(-3). The values of the breakdown voltage were determined from the extrapolated intercepts of the tau-1 versus the applied voltage, V, characteristics. The experimental rate of rise of the breakdown current was compared with the theoretical current growth, calculated using a two-dimensional model of the breakdown process. The calculations were performed at p(r) = 10(-4) with DELTAV = 33 V. It has been found that the rate of rise of the calculated current exceeds the experimental one by a factor of about 2.6, but agrees quite satisfactorily with the value that is obtained when the experiment is conducted at p(r) = 3 x 10(-4).