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A CO2-laser interferometer with heterodyne detection has been built to measure electron line densities of plasmas in the range of 10(18) to 5 x 10(22) m(-2). Heterodyne operation is accomplished by using two counteracting acousto-optic modulators at a difference frequency of 500 kHz. A digital phase comparator is used to obtain direct read-out of the phase shift. The time resolution is 3.5 micros.
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Hugenholtz et al. (1979) studied this question.
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