Optical observations of the plasma motion in a fast plasma focus operating at 50 J

Abstract

Plasma images with 5 ns exposure time were obtained in a very small plasma focus. The plasma focus operates at low energy (160 nF capacitor bank, 65 nH, 25 kV, 50 J). A single frame ICCD camera was used to obtain plasma images in the visible range. The umbrella-like current sheath running over the end of the coaxial electrodes and the pinch after the radial collapse can be clearly observed in the photographs. The velocity of the radial collapse is of the order of 10 5 m s −1 . The observations are similar to the results obtained with devices operating at energies several orders of magnitude higher.

Research on pinch plasma focus devices of hundred of kilojoules to tens of joules

“…The umbrella-like current sheath running over the end of the coaxial electrodes and the pinch after the radial collapse can be clearly observed in the photographs. Radial collapses of the order of 10 5 m/s were observed [10]. The first frame (37ns) suggests that there is not a good current sheath formation over the insulator.…”

“…As first stage of a program to design a repetitive pulsed radiation generator for industrial applications we constructed two very small plasma focus operating at an energy level of the order of a) tens of joules (PF-50J, 160nF capacitor bank,20 − 35kV , 32 − 100J, ∼ 150ns time to peak current) [8][9][10] and b) hundred of joules (PF-400J, 880nF , 20 − 35kV , 176 − 539J, ∼ 300ns time to peak current) [11]. 2.1 Design criteria.…”

“…The anode and cathode were separated by an alumina tube of 21mm length. In the case of the PF-50J after some improvements from the original design [9,10,26], the electrodes configuration consists of a 29mm long, 6mm diameter copper tube anode, and an outer cathode of eight, 5mm diameter cooper rods uniformly spaced on a circumference with diameter of 27mm. Anode and cathode were separated by an alumina tube of 24mm length.…”

“…2.3 Images from the visible plasma light. An intensified CCD camera (ICCD) gated at 5ns exposure time, and synchronized with the discharge has been used in order to obtain side view images of the visible light emitted from the plasma [10]. For imaging the plasma over the microchannel plate in the ICCD camera, a regular bi-convex lens with a focal length of 12.5cm and 5cm of diameter was used.…”

“…In fact, we can question if good focussing can be achieved below 1 kJ, and if so which are the appropriate design criteria and scaling laws in this energy region. Experimental research with a plasma focus driven by a capacitor bank of tens to hundred of joules would allow to extend the theoretical models to the region of low energy [8][9][10]. Moreover, a capacitor bank under the kilojoule has a small size in comparison with banks in the kilojoules range, thus it would be easier to operate in a repetitive regime from Hz to kHz, since the power requirements and the spark-gap erosion are consequently lower.…”