Prototype tests of a combined CO2 laser interferometer/polarimeter for ITER (abstract)

Abstract

Line density measurements on ITER are expected to be provided by a conventional vibration-compensated CO2 laser interferometer. This type of interferometer relies on maintaining an accurate measurement of hundreds of vibration fringes during the entire time history of the plasma. Because of the very long pulse lengths (1000 s) and the possibility of fringe counting errors, a second method that measures the Faraday rotation of a laser beam with a tangential laser beam path through the plasma will be used to provide a history-independent measurement of the line density with somewhat reduced accuracy. These two measurements can be accomplished in a combined interferometer/polarimeter system that uses a single CO2 laser and detector. We have constructed a prototype of this type of system, which uses two acousto-optic modulators to generate two beams, shifted from the laser frequency by 40 and 45 MHz. These probe beams are converted to left- and right-hand circular polarization and combined into a single probe beam. Vibrations are simulated by moving a mirror in the reference beam path, and the Faraday rotation is simulated by rotating the linear polarized probe beam. Frequency analysis of the interference signal from a single detector is used to separate the phase of the interferometer and the phase of the polarization rotation. Limits on the phase resolution of the two methods and possible sources of error will be presented.