Laser heterodyne measurement of electron densities in a hollow cathode discharge

Abstract

A laser heterodyne system has been developed to measure electron densities in a hollow-cathode discharge as used in metal vapour lasers. The beat frequency between two helium-neon lasers, one of which contains the discharge under investigation within its cavity, is monitored in real time as the discharge is rapidly switched off. The beat frequency changes because of refractive index changes in the plasma. By comparing the beat frequency changes at two wavelengths (0.6328 and 1.15 mu m), electron density and gas density changes have been determined. Electron densities have been measured in a cylindrical hollow cathode discharge (6 mm diameter, 10 cm long) in helium gas over a range of filling pressures (1-15 Torr) and discharge currents (10-150 mA). Electron density is found to behave linearly with discharge current, and to follow the cathode fall potential with pressure.