Microwave breakdown in a point-to-plane electrode gap

Investigates 9.4 GHz pulsed discharges in a point-to-plane electrode gap. The electrode system was established at an electric-field antinodal point in a resonant length of rectangular waveguide with air as the dielectric at NTP. The peak field strength in the gap occurs at the surface of the point electrode and this had to be deduced by an indirect method. A finite-difference solution to Laplace´s equation was used to compute the ratio of the field strength at the point to that at the plane; a perturbation method was used to find the field strength at the plane as a function of input power. Measurements of the breakdown probability per pulse show that the maximum field strength in the gap needed to achieve breakdown is very much higher than the continuous-wave value, even at the 10^-4 probability level. There is evidence that this was partly caused by the short pulse lengths used, of the order of 1 mu s, and partly by diffusion of charged particles out of the region where the field strength exceeded the breakdown value, but the main effect is concluded to be the very small volume of this region, which severely limits the probability of occurrence of an ionising electron as the breakdown threshold is approached.