Development of a dispersion interferometer for magnetic confinement plasma and its application to atmospheric pressure plasmas

A CO2 laser dispersion interferometer (DI) has been developed for both magnetically fusion plasmas and atmospheric pressure plasmas. The DI measures the phase shift caused by dispersion in a medium. Therefore, it is insensitive to mechanical vibrations and changes in the neutral gas density, which degrade the resolution of the plasma measurement. We installed the DI on the Large Helical Device (LHD) and demonstrated a high density resolution of 2×10 m without a vibration-free bench. This shows good agreement with results of another interferometer. The DI system is applied to the electron density measurement of the atmospheric pressure plasmas, which is smaller than 1 mm. The significant suppression of the phase shift caused by the neutral gas is proven. The achieved density resolution was 1.5×10 m with a response time of 100 s.