Development of CO2 laser interferometer/polarimeter system in JT-60SA

Abstract

Development of the CO2 interferometer/polarimeter system for line averaged electron density measurement in JT-60SA was progressed significantly. For the measurement, two viewing lines were selected, i.e. a tangential line and a vertical line. Two beam convertors were designed to optimize the beam size for long-distance transmission up to ∼ 240 m. A CO2 laser spectral beam axis/profile monitoring system and a beam axis stabilizing system were newly developed. By use of blue light for fluorescence of targets of the monitoring system, instead of conventionally used ultra-violet light, monitoring stability was significantly improved. Practical characteristics of a high power RF discharge laser for JT-60SA such as wavelength stability and beam quality were investigated by the monitoring system. Beam axis deviation was controlled by the stabilizing system within 1% of the beam diameter. By a performance test of the RF discharge laser using a single-color table-top interferometer, a good linearity of phase difference is confirmed in whole of the fringe range by a plasma (∼ 6 fringes) in JT-60SA. Obtained resolution of the phase difference (statistical error/5 μ s) is within 10°, which is promising to satisfy the target resolution of 1% for the line averaged electron density measurement.