Townsend-type breakdown as a criterion for high-frequency vacuum arc interruption at submillimeter gaps

In vacuum circuit breakers, multiple reignitions give rise to HF current arcing ( approximately=500 A; approximately=200 kHz). Due to the small contact distance and the very large current gradient, at every arc-current zero pressures of several tens of millibars can be expected. Very soon thereafter ( approximately=30 ns) this gap is dielectrically stressed by the first component of the restriking voltage ( approximately=10 MHz), originating from parasitic impedances. The combination of the associated high electric field and the relatively high neutral density may cause Townsend-type breakdown, leading to another half-sine of continued arcing. Both dielectric stress and residual neutral density are expressed as a function of di/dt, yielding values of interruptible di/dt as a function of the contact distance with the Townsend breakdown criterion. Comparison with experiments shows fair agreement in the range of di/dt of 100-1000 A/ mu s and distances of 0.1-0.5 mm for two different circuits.<
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