Multichannel interferometer system for the helically symmetric experiment

A multichannel millimeter-wave interferometer system has been designed, fab ricated and installed on the helically symmetric experiment (HSX), located at the University of Wisconsin, Madison. The interferometer system will vie w the plasma cross section along nine adjacent chords with 1.5 cm spacing. With this arrangement, coverage will span from the low-field side plasma sc rape-off layer to well past the magnetic axis. For the plasma densities ant icipated on HSX, a solid-state source operating at 288 GHz will be utilized . At this frequency refraction will be manageable, being less than the chan nel spacing. The source will be bias-tuned and modulated with a sawtooth wa ve form at 750 kHz in order to generate the intermediate frequency necessar y for the heterodyne detection scheme. The signals will be measured using S chottky-diode corner-cube mixers. The interferometer will have sensitivity n(e)dl approximate to 8 x 10(11) cm(-2), being able to measure density chan ges <1%. Initially, the phase will be evaluated using analog electronics wi th bandwidth < 10 kHz providing real-time line-integrated output. A digital phase comparator scheme will also be implemented whereby the measured wave forms are directly digitized and the phase evaluated using a software-base d algorithm. This will increase the time response up to the modulation freq uency of 750 kHz. Improved time response will permit measurement of high-fr equency density fluctuations along with "fast changes in" the equilibrium p rofile. (C) 2001 American Institute of Physics.