THE GROWTH OF PREBREAKDOWN CAVITIES IN SILICONE FLUIDS AND THE FREQUENCY OF THE ACCOMPANYING DISCHARGE PULSES

Measurements have been made of prebreakdown cavities in silicone fluid s, and of the current pulses that accompany cavity growth. These exper iments were carried out in silicone fluids of 0.65, 10, 100 and 1000 c S viscosity. Cavity growth, driven by the electrostatic field, is limi ted at low viscosities by inertia, and at high viscosities by viscous drag. The electrostatic force on the cavity wall is related to the loc al field and to the space charge density in the liquid adjacent to the cavity. We are concerned with the relationship between the electrosta tic force and the cavity growth, and with the discharges that accompan y cavity growth. Discharges occur in well defined pulse trains: the fi rst pulse in a train generates the cavity, and subsequent pulses are d ue to discharges within the cavity. Knowing the scaling laws for cavit y growth we can use the time between the first and second pulses to es timate the cavity size when the first cavity discharge occurs; this gi ves a cavity diameter of similar to 5 to 7 mu m. The next pulse cannot occur until the charge from the previous discharge has dispersed. We find that the time between pulses is strongly viscosity dependent; at high viscosities the average time between pulses Delta t is proportion al to fluid-viscosity but in the low viscosity limit the dependence ap proaches eta(1/3) To explain this viscosity dependence we consider thr ee mechanisms: (1) a decrease in charge density due to increase in cav ity size; (2) ion detrapping from the cavity wall and drift in the app lied field; and (3) diffusion of an impurity species to the cavity sur face, charge exchange to create a mobile ion, and its subsequent drift in the field. Our experimental results are consistent with the cavity expansion model, but there is evidence of diffusion effects in low vi scosity liquids, and with ion-drift at high viscosities.