Development of a multichannel far-infrared tangential interferometer/polarimeter for the National Spherical Torus Experiment

Completion of a multichannel far-infrared (FIR) tangential interferometer/polarimeter is under way for the National Spherical Torus Experiment, for the measurement of plasma electron density and toroidal magnetic-field distributions. The system is based on the three-wavelength configuration, which was first developed by Rommers and Howard in the RTP tokamak [Plasma Phys. Controlled Fusion 38, 1805 (1996)]. The probe beams enter the plasma through the windows on the midplane in Bay-K, and are reflected back to the detectors by retro-reflectors. The three FIR beams with a wavelength of 119 μm are produced by three Methanol lasers optically pumped by a CO2 laser tuned to 9.7 μm. The entire system is designed to have a full remote control capability using the LABVIEW program and fiber links. The technical details of the system development will be presented together with preliminary experimental result.