15 CHORD FIR POLARIMETRY SYSTEM ON MTX

A far-infrared polarimeter diagnostic has been added to an existing fifteen chord interferometer on the Microwave Tokamak Experiment (MTX). The polarimeter utilizes a new technique for determination of the Faraday rotation angle based on phase measurements of a rotating polarization ellipse. This technique allows the rotation angle to be determined even in the presence of signal amplitude variations caused by refraction. The implementation of this instrument requires no new detectors and minimal optics, making it quite inexpensive to add on to existing multichord interferometers. The MTX polarimeter has been operating for about a year and has achieved a resolution of less-than-or-equal-to 0.2-degrees with a bandwidth of congruent-to 1 kHz and a chord spacing of 1.5 cm. Typical Faraday rotation angles on MTX are in the range of 5-degrees-15-degrees. To obtain the poloidal field, the line-integrated density and Faraday rotation profiles are inverted in a manner consistent with the Grad-Shafranov equilibrium to first order in the inverse aspect-ratio expansion. Profile measurements during normal ohmic operation are presented.