A STARK-TUNED LASER APPLICATION FOR INTERFEROMETRY AND POLARIMETRY ON THE NATIONAL SPHERICAL TORUS EXPERIMENT

Abstract

Interferometer/polarimeter systems based on far infrared lasers have suffered from lack of frequency modulation techniques that can allow higher intermediate frequency (IF) frequency for a fast time resolution together with stable operation including low frequency drift. This is mainly due to the intrinsic narrow gain profile of the lasers. It is known that the Stark, effect can broaden the laser gain profile as has been demonstrated in the laboratory. A Stark-tuned optically pumped far infrared (FIR) CH3OH laser together with two additional FIR lasers at 119 mum have been successfully implemented in the far infrared tangential interferometer/polarimeter system which will provide temporally and radially resolved two-dimensional electron density profile [n(e)(r,t)] and toroidal field profile [B-T(r,t)] data for the National Spherical Tokamak Experiment. The characteristic frequencies of the IF system are similar to3, similar to4, and similar to7 MHz and a phase lock system was utilized for tracking the drift of the IF frequencies. In this article, the characteristics and operation of the Stark-tuned laser are described in detail. The measured electron density and Faraday rotation for various physics operation regimes are compared to the line integral of Thomson scattering measurement and EFIT results, respectively. (C) 2003 American Institute of Physics.