Application of a submillimeter laser as an interferometer for studying a decaying plasma

The paper reports on the use of a submillimeter HCN laser as an interferometer for the investigation of a decaying plasma. The HCN laser operates with a methane-air mixture in the regime of continuous generation (lambda = 377 mcm, radiation power approximately 30 mW). The laser radiation has been detected by a detector on the base of point contact of metal - InSb at T = 300 t K. A three-mirror interferometer has been used to measure electron density in the range of 1013 to 1016 cm-3 and to investigate the time dependence of plasma density of an electrodeless induction discharge. Oscillograms are given of discharge current and interference signal at a discharge initial pressure p = 4x10-2 mm Hg and Usub(ch) = 15 kV, the period is 9 mks. It should be noted that there is no interference at the acive stage of the discharge and that the interference amplitude damps completely at the initial stage of a plasma decay. The lower limit is 1013 cm-3, that is in a good agreement with microwave measurements of electron density at a wavelength of lambda = 8.2 mm. The maximum density is limited by the value of 6x1014 cm-3. Taking into account the results obtained in measuring the density and temperature of plasma electrons at the discharge active stage (Nsub(e) approximately equal to 9x1015 cm-3, Tsub(e) = 1.5-2.105 K), the authors come to the conclusion that the strong attenuation of submillimetre radiation in plasma which causes disappearing interference is due in the main to the Coulomb absorption. It has been shown that the interferometer may be used to determine the frequency of electron-ion collisions and the plasma temperature in the range of 103 to 105 K