Measurement of the growth of cavities and of EHD instabilities during the negative-point breakdown of silicone fluids

The initiation and development of discharges from a point cathode have been studied as a function of fluid viscosity. Optical studies show that the initial stage involves the growth of a vapor cavity adjacent to the point electrode. The initial growth of cavities and their transition from a small, roughly spherical region to a multibranched geometry have been recorded for a range of fluid viscosities. All these measurements have been made with low-voltage pulses (9 to 12 kV) in relatively short gaps (0.6 mm). Plots of cavity size vs. time have been used to derive velocity-vs.-growth curves. These velocities are compared with values of instantaneous velocity derived from image smear, and a knowledge of the effective light flash duration. It is confirmed that cavity growth follows an inertia-limited model for fluid viscosities <100 cSt