title: Design of a millimeter-wave polarimeter for NSTX-Upgrade and initial test on DIII-D.
abstract: Polarimetry is a powerful diagnostic technique to probe plasma equilibria and magnetic fluctuations in fusion plasmas. In a high beta plasma such as the National Spherical Torus eXperiment (NSTX), these measurements are important to understand plasma stability and anomalous transport. A 288 GHz polarimeter operating along a major radial chord in retroreflection geometry has been developed and is being tested on the DIII-D tokamak to prepare for future implementation on NSTX-Upgrade. The system launches a rotating linearly polarized beam and detects the phase shift directly related to the polarization change caused by the plasma. To accomplish this, a pair of orthogonal linearly polarized beams with a stable difference frequency is generated using a single sideband modulation technique, then combined and transformed to be counter-rotating circularly polarized. To improve phase resolution, quasi-optical isolation, using Faraday rotators and polarizers, is utilized to eliminate a multi-path feedback effect, which is found to be the primary source of phase error. The bench tests in the laboratory and DIII-D power supply test discharges indicate ≤1° phase resolution.
authors.affiliation: Department of Physics and Astronomy, UCLA, Los Angeles, California 90095-1547, USA. xyzhangj@physics.ucla.edu
authors.forename: J
authors.initials: J
authors.lastname: Zhang
authors.forename: W A
authors.initials: WA
authors.lastname: Peebles
authors.forename: T A
authors.initials: TA
authors.lastname: Carter
authors.forename: N A
authors.initials: NA
authors.lastname: Crocker
authors.forename: E J
authors.initials: EJ
authors.lastname: Doyle
authors.forename: S
authors.initials: S
authors.lastname: Kubota
authors.forename: X
authors.initials: X
authors.lastname: Nguyen
authors.forename: T L
authors.initials: TL
authors.lastname: Rhodes
authors.forename: C
authors.initials: C
authors.lastname: Wannberg
authors.forename: L
authors.initials: L
authors.lastname: Zeng