A laser interferometer, which uses the coupling effect between the laser cavity and an external one, is used here for measuring the time variation of the electron density simultaneously with its radial profile. Expansion rate of the laser beam while passing through the external cavity containing the plasma, whose radial electron density profile is approximated to be parabolic, is calculated by the ray transfer matrix. The relation between the amplitude decrement of the fringe pattern and the electron density profile is then deduced.
The electron density on the axis and its radial profile are determined simultaneously from the observed fringe patterns for the afterglow plasmas in He and Ar at pressures of 4. 6 torr and 1.7 torr respectively, using a He-Ne laser at 3. 39 μm. The principle of the measurement is confirmed by measuring the electron density at 0. 5 cm from the axis.
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