A SENSITIVE HE-NE INTERFEROMETER WITH PASSIVE BEAM POWER STABILIZATION FOR LOW-DENSITY PULSED-PLASMA MEASUREMENTS

A sensitive interferometer based on a Michelson scheme was constructed and applied to low-density pulsed-plasma measurements. Using a modera te power He-Ne laser and common photodetectors, a sensitivity as high as 1/1000 FWHM of an interference fringe was reached with a minimum li ne-integrated plasma density as low as [N(e)L]min=8 X 10(13) CM-2. Thi S sensitivity was obtained in a real plasma experiment with space and time resolution of 1-2 and 120 ns, respectively. The time resolution f or this technique is restricted mainly by the oscilloscope finite inpu t capacitance and could be made higher but with a loss of sensitivity. In order to obtain such high a sensitivity, we used loads for the pho tocurrent registration higher than the 50-OMEGA matched load. The init ial phase is not assumed to be optimized and is recorded separately du ring the operation of the interferometer. Suppression of electrical no ise and of instabilities associated with the laser power requires a su pplementary detector for background subtraction to improve the signal- to-noise ratio. A description of the high sensitivity interferometer w ith the passive beam power stabilization, analysis of the data treatme nt procedure, as well as measurements for an electron density distribu tion, are presented.