Optical and pressure diagnostics of 4-MV water switches in the Z-20 test Facility

We are studying the behavior of self-breaking, high-voltage water switches for the Z refurbishment project. In Z-20, three or four water switches in parallel are charged to 4 MV in ∼220 ns. The water gap between switch electrodes is 13-15 cm, and the enhancement of the positive and negative electrodes is varied to study time-evolution of the breakdown arcs, current sharing, and switch simultaneity. In addition to the standard electrical diagnostics (V,I), we are looking at one or more of the switches during the breakdown phase with two optical diagnostics: a streak camera and a fast framing camera. The streak camera has ∼1-ns resolution, and the framing camera provides seven frames with >5 ns exposure times. For identical electric fields, the streamers originating on the positive electrode form earlier and move more rapidly than the streamers originating on the negative electrode. We observe four distinct phases in the closure of the water switches that depend on the macroscopic electric fields in the water: 1) No streamers propagate at E-fields below ∼100 kV/cm from positive electrodes or voltages below ∼140 kV/cm for negative electrodes; 2) streamers propagate with constant velocity between 100 and ∼300 kV/cm; 3) above 300 kV/cm, the streamer velocities become linearly proportional to the electric field; 4) above 600 kV/cm, the velocity of streamers from the negative electrodes appears to saturate at ∼100 cm/μs. The velocity of the streamers from the positive electrode continues to increase with E-field, reaching ∼1% of the speed of light when the switch reaches closure.