Multichannel interferometer system for the helically symmetric experiment

Abstract

A multichannel millimeter-wave interferometer system has been designed, fabricated and installed on the helically symmetric experiment ͑HSX͒, located at the University of Wisconsin, Madison. The interferometer system will view 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 scrape-off layer to well past the magnetic axis. For the plasma densities anticipated on HSX, a solid-state source operating at 288 GHz will be utilized. At this frequency refraction will be manageable, being less than the channel spacing. The source will be bias-tuned and modulated with a sawtooth wave form at 750 kHz in order to generate the intermediate frequency necessary for the heterodyne detection scheme. The signals will be measured using Schottky-diode corner-cube mixers. The interferometer will have sensitivity n e dlϷ8ϫ10 11 cm Ϫ2 , being able to measure density changes Ͻ1%. Initially, the phase will be evaluated using analog electronics with bandwidth Ͻ10 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-based algorithm. This will increase the time response up to the modulation frequency of 750 kHz. Improved time response will permit measurement of high-frequency density fluctuations along with ''fast changes in'' the equilibrium profile.

Impact of magnetic islands in the plasma edge on particle fueling and exhaust in the HSX and W7-X stellarators

“…Discussions and conclusions from these data will be presented in section VII along with results from the W7-X study. The main measurements used in this work are line averaged densityn e measurements from an interferometer 25 , density n e (r) and temperature T e (r) profiles from Thomson scattering 26 and a filtered photodiode array system 27 to measure line emission of Balmer-α (λ α =656.3 nm) as line integrated signal at nine radial intersection points. The system also has been upgraded with additional helium channels to measure He-I and He-II line emission profiles for τ * p,He measurements.…”

Far-forward collective scattering measurements of density fluctuations in the helically symmetric experiment stellarator

“…1. 3,4 While interferometer systems have been successfully employed to measure density fluctuations ͑typically coherent modes͒ as well as the equilibrium density profile, in an effort to improve sensitivity we have recently operated the diagnostic in a mode whereby far-forward collective scattering from the electromagnetic probe wave is measured. It can be frequency-modulated so that heterodyne detection at the modulation frequency of ϳ1 MHz is possible.…”

First results from the multichannel interferometer system on HSX

“…1. Details of the optical system, detection scheme, and system calibration have been previously described elsewhere 2 and will only be briefly reviewed here.…”

“…2 This system has nine evenly distributed spatial channels with 1.5 cm spacing and width. The interferometer is designed to measure the plasma density distribution at one of the four box ports where the closed flux surfaces are highly elongated but symmetric about the midplane.…”