ELECTRON DENSITY MEASUREMENT OF A TOKAMAK PLASMA BY AN HCN LASER INTERFEROMETER

An HCN-laser interferometer has been constructed for the electron density measurement of plasmas. The output power of the HCN laser has been adjusted for long-term operation. The interferometer is of the Michelson-type with a concave mirror as the reflecting mirror so as to reduce the refractive effect due to plasmas. The mean electron density of SPAC-II Tokamak plasma (of the order of 3 to 7.5 times 10 to the 13 power/cu cm) was measured by this interferometer. The signal-to-noise ratio was 20, which was mainly determined by the fluctuation of the laser output power. Changes of the phase shift corresponding voltage spikes in the one-turn-loop voltage signal were also observed.