Observation of the metallic vapor from a plasma focus with a laser differential interferometer

Abstract

Summary form only given. The experiments were performed using DPF-16 (16 kJ, 20 kV, 380 kA). The electrode geometry represents an accelerator of Mather type. Both anode (66 mm diam.) and cathode are 265 mm in length and were machined from oxygen-free copper. The anode is a solid cylinder with a tungsten stud, 10 mm in diameter and 6 mm high, set on the top as a target for electron bombardment. The working gas is hydrogen with a pressure of 70/spl sim/650 Pa. The evolution of the plasma sheath above the anode was recorded with a laser differential interferometer. From the interferograms taken at the times (t>200 ns) well after the focus was over, we saw a small volume of high density emerging from the target of anode and developing rapidly. We believe the high density volume to be the metallic vapor produced on the target surface by bombardment of an intense electron beam which originated from the collapsing pinch. This view is supported by the facts given below. Firstly, the erosion of the target after many shots of discharge confirms the evaporation of the target material. Secondly, this high density volume can not be seen when the solid anode with a target is replaced by a hollow anode. Thirdly, since this high density volume appears well after the focus is over, it must be something new other than the plasma sheath. Finally, the delay time from the pinched plasma column to the appearance of the high density volume was about 280 ns, which may be the time required to hold up a sufficient concentration of metallic vapor to be seen clearly in the interferogram.