High power, high recovery rate water switch

Abstract

The high dielectric strength of water, which may reach values of 1 MV/cm makes it an attractive switching medium for high power, low inductance switches. For water switches that operate at voltages of tens of kV, and currents of less than 1 kA, the dielectric recovery time is approximately 1 ms. The recovery process is determined by the decay of a vapor bubble due to the energy deposition in the liquid during breakdown and conduction. To reduce the recovery time, and consequently increase the recovery rate, we flowed water through the switch volume to remove the vapor bubble on a time scale less than the recovery time in static water. Flowing the water transversely through the rod-pin electrode gap with a flow rate of 0.56 1/min allows us to shorten the recovery time to 700 /spl mu/s. Using axial laminar flow with a flow rate of 0.41/min through a nozzle-pin electrode system yields similar results. The recovery rate, the inverse of the recovery time, was, in this case, increased from 1 kHz in static water 1.4 kHz. Increasing the flow rate only causes an increase in recovery rate as long as the flow is laminar; beyond the transition from laminar into turbulent flow, the hold-off voltage decreases. Improvements of the design of the flow-through system such that the transition from laminar into turbulent flow occurs at higher flow velocity will lead to increases in recovery rate. Further increase in recovery rate can be achieved by operating the switch at a reduced breakdown voltage. Using axial flow and operating the switch at a breakdown voltage of 82% of its full dielectric strength allows us to reach recovery rates of 2 kHz.