A LASER-INDUCED FLUORESCENCE MEASUREMENT TECHNIQUE FOR OBTAINING NEUTRAL HYDROGEN DENSITIES IN PLASMAS

The resonance fluorescence of neutral hydrogen illuminated by H-alpha radiation has been used as a technique for the spatially and temporall y resolved density measurements of neutral hydrogen in high temperatur e plasmas, such as in the tokamak and magnetic mirror plasma fusion de vices. The fluorescence signal, usually very weak and buried in the ba ckground of stray laser light and H-alpha emission, is very difficult to extract and ifs measurements are inaccurate. This paper discusses t he improvement of the signal extraction using two optical path laser-i nduced fluorescence (LIF) methods. One optical path carries the fluore scence signal and the background (the stray laser light and H-alpha em ission), whereas the other path carries only the background signal. By combining these two signals, a clean fluorescence signal can be isola ted by subtracting out the background using a differential amplifier. The measurement is obtained instantaneously from these two signals whi ch are taken simultaneously in one pulse rather than being extracted f rom two separate spectra taken in two sequential pulses (double pulses ). This method, therefore, makes a significant improvement on the doub le pulse technique in terms of the accuracy of the measurement and the time resolution. Using this LIF technique the measurement of the neut ral density profile in the exhaust of a tandem mirror plasma propulsio n device is obtained and presented. (C) 1995 American Institute of Phy sics.