Experimental Study on the Performances of Second-Harmonic Dispersion Interferometers at 10.6 Μm and 1064 Nm for Plasma Density Measurements

Two common-path interferometers based on C O 2 and N d : Y 3 A l 5 O 12 (Nd:YAG) lasers are benchmarked with a two-arm microwave interferometer on a hydrogen plasma produced by an RF discharge and injected into a large magnetic-confinement vessel. The ∼ 10 19 m − 2 line-integrated electron density is clearly measured in agreement by the interferometers. The frequency spectrum of the measured data is analyzed on the 20 kHz range for all interferometers and up to 600 kHz for the Nd:YAG laser-based interferometer. Mechanical vibration measurements performed on the components of the two common-path interferometers result in a peak-to-peak displacement up to about one and twenty wavelengths for the C O 2 and Nd:YAG laser-based interferometers, respectively. Such results set for the first time, to the best of our knowledge, a quantitative limit for the displacement that the two second-harmonic dispersion interferometers can sustain while still providing a high sensitivity for accurate plasma density measurements.