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.

Development of a portable pulsed fast ⩾106 neutron generator based on a flexible miniature plasma focus tube

“…The plasma focus tube was filled initially with low D 2 gas filling pressure (1-2 mbar) during the initial insulator conditioning shots [40,44]. After conditioning of the insulator surface during the initial few plasma focus discharges, a large dip was observed in the current-derivative waveform near the quarter time-period and, correspondingly, in the current waveform, as shown in figures 4(a) and (b).…”

“…The insulator plays a crucial role in the plasma sheath formation during the initial gas breakdown phase. An insulator of appropriate dimension and with a smooth surface (free of surface defects, such as micro-cracks) results in a uniform plasma sheath in the initial breakdown phase, which subsequently results in strong plasma focus formation [40,41]. Out of the total length of 5 cm of alumina tube, the exposed length of around 1.5 cm was polished via a special technique using diamond paste to make it compatible for smooth and efficient plasma sheath formation [28].…”