Use of a double-Wollaston prism laser differential interferometer in a plasma focus

Abstract

A double-Wollaston prism laser differential interferometer (LDI) has been developed to observe, for the first time, the evolution of the imploding current sheath (CS) and the high temperature and high density plasma pinch in the plasma focus. The light source is a Q-switched and frequency-doubled YAG laser operated in a single-pulse mode: λ=532 nm, the pulse width (FWHM) about 10 nm (also the resolution time of the LDI). The LDI measures density gradients and has a line sensitivity of about ne′=(2.58±0.46)×1025 m−4 for a typical sheath thickness of 1.6 mm and a minimum distinguishable fringe shift of 5%, and a spatial resolution of 1.26 mm. The density gradient and equivalent density of the CS are measured to be ne′≈(2.58±0.46)×1026 m−4 and ne≈(3.25±0.59)×1023 m−3, respectively. More importantly in this article, the double-Wollaston prism LDI, with a simple optical arrangement, gives direct physical pictures of the high density gradient plasma configuration and structure which are of interest in studying the status of plasma motion.