Mechanism of Breakdown of Laboratory Gaps

Upon application of an impulse voltage, of such value as not to cause sparkover, a nonuniform field gap, of the proportions frequently encountered in engineering work, the field at first corresponds to that which would be expected from the conventional electrostatic solution. The fields in the vicinity of the electrodes may exceed the critical field momentarily but when this field is exceeded and a free electron appears in the region of the overstressed field, an electron avalance is triggered that develops into aspace charge. For rod-rod gaps the space charge develops from both electrodes but for rodplate gaps from the rod only. The flow of the charge into the intervening gap is at a rate of about 0.001 c which corresponds approximately to the electron drift; so that for a 10-cm gap the charge has diffused through the entire gap in about 0.3 ¿sec and for a 200-inch gap in 9 ¿sec. The current feeding the space charge rises very rapidly and decreases somewhat along an exponential curve so that a substantial portion of the space charge is established in slightly less time than these values. A certain critical average gradient exists for gaps which will produce ultimate sparkover of the gap with prolonged application of the voltage. There is some evidence to indicate that when the space charge is fully developed across the gap the electric gradient in the gap between the electrodes is approximately uniform.