Heterodyne interferometer of coaxial CO₂ and He-Ne lasers for plasma density measurements

An interferometer using coaxial CO₂ and He-Ne lasers has been developed for electron density measurements in the reversed field pinch plasma. The interferometer uses the phase modulating heterodyne detection of 60 MHz. The CO₂ laser interferometer is free from the mechanical vibrations due to the reduction of the vibrations by the air springs and the compensation of the vibrations by the He-Ne laser interferometer. The electron density measurement during a long-time discharge of 50 ms without the disturbance by the vibrations is achievable. The resolution is less than 1/100 fringe of the CO₂ laser interferometer, therefore the electron line density as small as 1.1×10¹⁴ electrons/cm² is detectable.