A Nanosecond, High Repetition Rate Pulse Switch With Water As Dielectric

Water, with its dielectric constant ofε= 80,allows the design and construction of more compact pulse forming lines(PFLs) compared to PFLs where'classical'dielectrics such as PTFE or other common insulators are used. With a dielectric strength exceeding 1MV/cm gap distances can be reduced to several hundreds of micrometers, and still allow the switching of tens of kV. The small gaps allow us also to reduce the switch inductance and ,consequently, to obtain high values of voltage and current rate of rise. The recovery time, defined as the time required to reach the full voltage across the gap after a previous switch event. Flowing the water through the switch at high speed allows us to remove debris and vapor bubbles left from previous shots, and to operate the switch at a high repetition rate.This work focuses on the design and operation of a compact pulsed power system with a water-filled Blumlein pulse forming line and a water switch of coaxial columns configuration. The pulse length of 40 ns is determined by the length of the Blumlein line, and the 40Ωimpedance by the PFL geometry and the dielectric (water).The holdoff voltage is 5kV,and the voltage rise time is in the ns range. The pressurized water flow system used to clear the switch gap is expected to allow kHz repetition rate operation in a burst mode for several minutes. 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. Flowing the water through the coaxial columns configuration with a flow rate of 5m/s allow us to reach recovery rates of 500Hz.