Diffusion limited growth of negative point, pre-breakdown cavities in silicone fluids

Measurements have been made of pre-breakdown cavity growth and of the accompanying discharge pulses in a range of silicone fluids, 0.65, 10, 100 and 1000 cSt viscosity. At high viscosities the average time between pulses, Δt, is proportional to fluid viscosity, but in the low viscosity limit the dependence is roughly η^½. To explain this viscosity dependence we make use of the fact that a pulse cannot occur until charge from the previous pulse has decayed. We consider two possible mechanisms of charge dissipation: (i) ion detrapping and drift in the applied field; (ii) diffusion of an electronegative impurity species to the cavity surface, charge exchange to create a mobile ion, and its subsequent drift in the field. In the former case the rate limiting step, ion drift, leads to Δt∝η, where in the latter, diffusion limited case, Δt∝η^1/3. This and other evidence inclines us to the diffusion-limited discharge model in the low viscosity limit