Dual CO2 laser interferometer with a wavelength combination of 10.6 and 9.27 μm for electron density measurement on large tokamaks

A new CO2 laser interferometer has been developed to measure the electron density of tokamak plasmas. Two different wavelength oscillators of 10.6 and 9.27 μm are utilized for simultaneous measurement of the density component and the optical path length change. A new technique using a common frequency shifter for two color lasers improves the stability of the system by a complete matching of both beat frequencies. The system provides advantages for practical problems of large tokmaks related to window darkening and large mechanical vibrations of reflection mirrors and offers improved laser beam monitoring and a simplified optical layout by using closer wavelengths. The electron density of JT-60U is successfully measured for a plasma current of up to 3 MA. The density behavior during a fast major disruption is also diagnosed without a fringe loss. An effective density resolution is observed to be 2×1019 m−2, which corresponds to about 1/10 of a fringe. A very high resolution (1/104 fringe) phase comparator has been developed to improve the density resolution of the dual CO2 system. The performance achieved shows the feasibility of the dual CO2 laser interferometer for future large devices such as the International Thermonuclear Experimental Reactor.