title: Design of a new optical system for Alcator C-Mod motional Stark effect diagnostic.
abstract: The motional Stark effect (MSE) diagnostic on Alcator C-Mod uses an in-vessel optical system (five lenses and three mirrors) to relay polarized light to an external polarimeter because port access limitations on Alcator C-Mod preclude a direct view of the diagnostic beam. The system experiences unacceptable, spurious drifts of order several degrees in measured pitch angle over the course of a run day. Recent experiments illuminated the MSE diagnostic with polarized light of fixed orientation as heat was applied to various optical elements. A large change in measured angle was observed as two particular lenses were heated, indicating that thermal-stress-induced birefringence is a likely cause of the spurious variability. Several new optical designs have been evaluated to eliminate the affected in-vessel lenses and to replace the focusing they provide with curved mirrors; however, ray tracing calculations imply that this method is not feasible. A new approach is under consideration that utilizes in situ calibrations with in-vessel reference polarized light sources.
authors.affiliation: MIT Plasma Science and Fusion Center, Cambridge, Massachusetts 02139, USA.
authors.forename: Jinseok
authors.initials: J
authors.lastname: Ko
authors.forename: Steve
authors.initials: S
authors.lastname: Scott
authors.forename: Manfred
authors.initials: M
authors.lastname: Bitter
authors.forename: Scott
authors.initials: S
authors.lastname: Lerner