A beat-modulated 118.8-μm CH₃OH laser interferometer system and its application to the electron density measurement of JIPP T-II plasma

A beat-modulated 118.8-μm CH₃OH laser interferometer system has been constructed and successfully operated for the electron density measurement of JIPP T-II plasma. A Michelson-type far-infrared (FIR) laser interferometer system with a corner-cube reflector has been adopted in order to compensate the mechanical vibration component of the system by means of a 632.8-nm He-Ne laser interferometer. The corner-cube reflector is directly attached to the JIPP T-II vacuum vessel because of simplicity of the interferometer system. It has been observed that the vibration component of the FIR laser interferometer is compensated completely and the electron density by the system agrees closely with the one by a 2-mm microwave interferometer system. The signal-to-noise ratios of the probe- and reference signals are 50 and 250, respectively. The minimum detectable line-averaged density of present system is 8 × 10¹¹ cm^-3 (that is, 1/40 fringes). (author)