Precise density profile measurements by using a two color YAG/CO2 laser imaging Interferometer on LHD

Abstract

A multichannel imaging heterodyne interferometer is installed on Large Helical Device. Two branches of the interferometer: one with CO2 laser (wavelength λi=10.6 μm) and other with diode pumped yttritium–aluminium–garnet (YAG) laser (wavelength λi=1.06 μm) are used for electron density measurements and vibration compensation. Two slab beams and one circular beam are injected vertically. Liquid nitrogen cooled linear detector arrays are used for detection of slab beams and room temperature detector is employed for single circular beam for CO2 beams. Avalanche photodiodes record signals for YAG beams. Chordal resolution is from 15 to 22.5 mm, phase resolution is 10−3 of CO2 fringe, which is determined by the electrical noise of phase counter. About 5×10−3∼10−2 of CO2 fringe uncompensated signal remains after vibration compensation. Combining existing 13-channels far infrared (wavelength λi=119 μm) laser interferometer, profile measurements at high density (>1020 m−3) become possible.