The use of gas discharge lasers in plasma diagnostics

Abstract

Two (non-resonant) methods using a gas discharge laser as a light source in connection with plasma measurements are discussed: A Michelson interferometer setup to determine the integral index of refraction (f n(l) dl) as a function of time and space; and a Faraday rotation experiment determining the integral ~ = u f B(1) n(l) dl. Several experimental arrangements and recording methods are described and their problems and limitations considered. Results indicate in both cases that gas discharge lasers will provide very useful instrumentation for plasma measurements. 1. The use of pulsed or c.w. lasers - modulated or not - W, ith many possible transitions between 6000 and 30000 A in an interferometer experiment eliminates several problems and permits a number of experiments in an electron density range which has not been easily accessible so far to interferometric techniques. It also allows or necessitates slightly different methods from those used with extended light sources. In the following slides we discuss briefly the theoretical basis of the measurement; then study two different methods of displaying the phase shift produced in a Michelson interferometer illuminated by a He-Ne laser, and determine the accuracy of the method: Figure 1 Basic equations; Figure 2, The product n_ 1 as a function of )t with Ar as parameter; Figure 3, Experimental setup ; Figure 4, Phase shift measurements by means of a photomultiplier; Figure 5, Streak camera representation of the fringe pattern. 2. Another method to measure plasma properties uses the rotation of a linear polarized electromagnetic wave. If (by another measurement) the electron density is known, we may obtain from the rotation the actual magnetic field trapped inside of the plasma. Using a laser as light source in the visible or infrared offers again several advantages, in particular high spatial resolution and high sensitivity. The theoretical background of the measurement and the experimental setup are shown.