Confirmation of abnormally high electron density in a transient mercury-vapour plasma by laser-interferometer measurements

An optical interferometer which employs a low-power (100 ¿W) beam from a frequency stabilised He-Ne laser has been used to find the effective change in path length due to the transient plasma in terms of fractional fringe shift. The results confirm the magnitude of electron density previously deduced from the extrapolated microwave results and indicate a distinct time lag in the buildup of maximum electron density with respect to the transient current maximum. This implies a fall in drift velocity of electrons from the current peak to the electron-density peak which is a function of the degree of ionisation achieved.