UC San Diego

The Dense Plasma Focus (DPF) is a Z-pinch configuration that can produce intense bursts of energetic particles (electrons, ions and neutrons) and X-rays. This makes DPF attractive for applications where intense X-rays and energetic particles are needed. However, significant variation between shots is a challenge. Here we detail experiments on a 4.4 kJ DPF (250 kA current, 2.5 μs rise time) using neon fill gas that demonstrate the critical role of insulator surface conditioning on pinch structure, dynamics, and X-ray yield. This is accomplished through same-shot imaging of the evolving plasma sheath during breakdown, rundown, and at the pinch with X-ray sensitive photodiodes. Smooth surfaces on the borosilicate glass insulators demonstrate the highest X-ray production whereas conditioning the insulator surface with micro-scratches or machined grooves reduces X-ray signals by a factor of 4 on average. Specifically, horizontal exterior surface structures were shown to decrease the yield by a factor of six, whereas vertical structures on the exterior surface and interior horizontal structures both decreased the X-ray yield two fold. Small differences in sheath liftoff timing are measured on the order of ~100 ns. Sheath asymmetry and inhomogeneity at liftoff are observed, suggesting that local variations in surface structuring decrease the uniformity of the sheath, ultimately leading to reduced performance. This is likely due to the disruption of electron flow over the modified surface as the insulator flashes over and the inherently higher azimuthal variability in the local structure of modified surfaces.

*Research supported by Air Force Office of Scientific Research grant FA9550-18-S0003.