Optical Interferometric and Spectroscopic Measurements of Electron Density in a Plasma

A Mach-Zehnder interferometer is used to determine the electron density ({~}2×{}10¹⁷ cm^-3) behind the reflected shock wave in an electromagnetic, T-type shock tube filled with hydrogen to 2 Torr. The results are compared with simultaneous measurements of electron density from Stark broadening of the H_β line and the absolute continuum intensity. Using two-wavelength interferometry, agreement is found to be within the experimental accuracy of about 6%, thus showing the validity of the theory for these three methods of density measurement. Although spectroscopic methods indicate that the plasma is highly ionized ({~}90%) so that the refractivity is determined primarily by free electrons, single-wavelength interferometry yields results which are 10-15% too low. This is taken to indicate the presence of a boundary layer of cold, high-density neutral gas near the walls of the shock tube. In addition, it is concluded on the basis of this experimental data that the Griem asymptotic wing formulas for Stark-broadened hydrogen lines is more accurate than the earlier asymptotic wing formulas of Griem, Kolb, and Shen.