High-resolution laser interferometer for measurement of electron density in transient plasma

A laser interferometer of the Mach-Zehnder type based on the use of a single-mode frequency-stabilised He-Ne gas laser, 6329 Å in wavelength and with 100 ¿V output, has been used to measure accurately electron densities in the range 1014¿1015cm¿3 in a transient mercury-vapour plasma. An `amplitude¿ (intensity) modulation system is employed in order to obtain a wide dynamic range. The interferometer system is mounted on a massive movable table in order to obtain the spatial distribution of electron density by means of sequential discharges. The experimental results show clearly that there are definite time delays between the peak of current in the discharge and the maximum value of the (average) electron density. The results are in good agreement with the values of electron densities obtained by the extrapolation from microwave measurements.