A Faraday-effect polarimeter for fast magnetic dynamics measurement on DIII-D

• Published in: Review of scientific instruments 2018-10, Vol.89 (10), p.10B101-10B101
• Main Authors: Chen, J., Ding, W. X., Brower, D. L., Finkenthal, D., Boivin, R.
• Format: Article
• Language: English
• Subjects: Broadband; Chords (geometry); Circular polarization; Collinearity; Cotton; Diagnostic systems; Electron beams; Electron density; Magnetic fields; Magnetic variations; Optical components; Optical feedback; Plasma; Plasmas (physics); Polarimeters; Polarimetry; Scientific apparatus & instruments; Systematic errors; Wave plates
• ISSN: 0034-6748; 1089-7623
• DOI: 10.1063/1.5035276

A Faraday-effect-based radial-interferometer-polarimeter diagnostic has been developed to explore fast magnetic dynamics in high-performance DIII-D plasmas. The instrument measures radial magnetic field perturbations using three chords positioned near the magnetic axis. Newly developed solid-state sources operating at 650 GHz provide phase noise down to 0.01°/kHz and tunable bandwidth up to 10 MHz. Various systematic errors which can contaminate the polarimetric measurement have been investigated in detail. Distortion of circular polarization due to non-ideal optical components is calibrated using a rotating quarter wave plate technique. The impact of perpendicular magnetic field, i.e., the Cotton-Mouton effect, is evaluated. The error due to non-collinearity of probe beams is minimized to less than 0.5° for electron density up to 7 × 1019 m−3 by alignment optimization. Optical feedback, due to multiple reflections induced by the double-pass configuration, is identified and reduced. Coherent and broadband high-frequency magnetic fluctuations for DIII-D H-mode plasmas are observed.