Development of a vibration-compensated CO2 interferometer for the RFX experiment

The accuracy of a vibration-compensated CO_2 interferometer is often worse than expected in the design phase. This paper describes a laboratory study performed during the development of a multichord two-color CO_2 interferometer for the RFX experiment with the aim of assessing the performance of this diagnostic. The phase demodulation system (quadrature phase comparators and AGC linear IF amplifiers) has been carefully tested and a set of optimum conditions has been found for which its overall accuracy is better than 1°. Other sources of error such as noncompensated vibrations or standing waves in the acousto-optic cells have been sought for and studied on a test bench interferometer. A scheme is proposed by which density measurements with an accuracy better than 10^18 m^-2 can be achieved.