title: Collinearity alignment of probe beams in a laser-based Faraday effect diagnostic.
abstract: Two counter-rotating circularly polarized beams are used in a laser-based polarimetry diagnostic providing a phase measurement of the Faraday effect. Collinearity of these beams is a key issue that affects measurement accuracy. Spatial offset from even small misalignment induces systematic error due to density gradient and path length difference. Here, we report an alignment technique using a rotating dielectric wedge, which is capable of reducing spatial offset of two probe beams below 0.1 mm for beams with 40 mm diameter. With optimized alignment, 0.05° Faraday effect fluctuations associated with global tearing modes are resolved with an uncertainty below 0.01°.
authors.affiliation: Department of Physics and Astronomy, University of California-Los Angeles, Los Angeles, California 90095, USA. lianglin@ucla.edu
authors.forename: L
authors.initials: L
authors.lastname: Lin
authors.forename: W X
authors.initials: WX
authors.lastname: Ding
authors.forename: D L
authors.initials: DL
authors.lastname: Brower