Development of a New Far Infrared Laser Interferometer in Heliotron J and First Results

A new far infrared (FIR) laser interferometer with high time resolution has been developed in Heliotron J for measuring high performance plasmas. The FIR laser interferometer is a heterodyne-type Michelson interferometer with a 1 MHz intermediate frequency. The interferometer uses a super rotating grating and the viewing chord passes through an off-axis position. Refraction in high density plasma is estimated using the TRAVIS ray-tracing code. The results suggest that it is possible to extend the range of the interferometer up to 1.5×1020 m−3. The first line-averaged plasma density measurements using the FIR laser interferometer have been made in ECH and NBI heated plasmas. The relative change in the density profile shape is evaluated from the ratio of the line-averaged density obtained by the FIR laser and microwave interferometer with a different viewing chord. The difference in the measurements suggests that a more peaked density profile is formed in NBI plasma than in ECH + NBI plasma, which is in agreement with Nd:YAG Thomson scattering measurements.