Optical and Mechanical Design of C-Mod Motional Stark Effect Diagnostic
D.I. Simon, N.L. Bretz, E. Marmar, R. Bravenec, and R.F. Parsells
Date of PPPL Report: November 1999
Presented at: The 18th IEEE/NPSS (Institute for Electrical and Electronics Engineers/Nuclear and Plasma Science Society) Symposium on Fusion Engineering (SOFE) which was held in Albuquerque, NM,on October 25-29, 1999.
A Motional Stark Effect (MSE) instrument is being installed on the Alcator C-Mod tokamak at MIT. This MSE diagnostic will provide measurements of the spatial profile of the internal poloidal magnetic field. The MSE has its primary collection optics inside the vacuum vessel. The light collected by the internal optics passes through a vacuum window and is relayed to a fiber optic array. The MSE optics are shared by a Beam Emission Spectroscopy (BES) diagnostic which measures electron density fluctuations and their spatial correlations. This optical system requires high throughput and spatial resolution of less than 1 cm at the focal plane in the plasma. The design requirements for the internal optics also include the effects associated with plasma impingement, plasma disruptions, and thermal excursions. The parameters that affect polarization measurement include the location and orientation of optical elements, the choice of substrates and optical materials. These unique design requirements led to a number of interesting optical and mechanical design features which are presented here.