Effect of insulator surface conditioning on the pinch dynamics and x-ray production of a Ne-filled dense plasma focus
Abstract
The dense plasma focus (DPF) can be an intense source of x rays, wherein the insulator sleeve strongly dictates the electrical breakdown, which subsequently affects the formation of a plasma sheath and a collapse phase. Experiments on a 25 kJ DPF (operated at 4.4 kJ) are carried out to demonstrate the influence of insulator surface morphology on the pinch structure, dynamics, and x-ray yield using a Ne fill. Two borosilicate insulators are directly compared, one with a smooth finish and the other machined with four circumferential grooves traversing the perimeter of the exterior insulator surface. Comparisons are made through same-shot imaging diagnostics of the evolving plasma sheath during breakdown, rundown, and at the pinch in addition to the time-resolved measurements of emitted x rays via filtered photodiodes. The presence of structures on the insulator sleeve reduces x-ray production across all fill pressures by a factor of 2.8 ± 2.4 on average and reduces the highest x ray producing shots by a factor of 5.5 ± 1.8. Observations of sheath asymmetry and inhomogeneity at lift-off are observed and correlated with subsequent observations of off-axis radial collapse. Taken together, this suggests that local variations in the insulator surface decrease the spatial uniformity of the sheath, leading to an azimuthally asymmetric focus, reduced electron densities, and, ultimately, degraded x-ray production.
Document Details
- Document Type
- Pub Defense Publication
- Publication Date
- Jun 09, 2021
- Source ID
- 10.1063/5.0050203
Entities
People
- A. Link
- David Housley
- Eric Hahn
- Fabio Conti
- Farhat Nadeem Beg
- Jeff Narkis
- Justin R Angus
Organizations
- Air Force Office of Scientific Research
- Lawrence Livermore National Laboratory
- United States Department of Energy
- University of California, San Diego