Improving the signal to noise of FIR polarimetry on constrained-access plasma experiments

Experimental results are presented that demonstrate the pronounced effects produced on the polarization state of an FIR (far infrared) laser beam when a portion of the beam intersects a metallic tubulation at a glancing angle. The effects produced by this interaction are sufficient to totally obscure the desired measurement of Faraday rotation. A model is developed which qualitatively predicts the observations. A method of suppressing this effect, which is viable for the heterodyne method of measuring Faraday rotation, is presented. It is further shown that reflections off the mechanically vibrating tubulations are the source of a variable effective birefringence, which is perceived as an anomalous Faraday rotation. Minimizing these reflections by threading the inside of the tubulations overcame this problem and allowed measurements to be obtained.>