Snapshot-imaging motional Stark effect polarimetry

Measurement of the motional Stark effect (MSE) for Balmer alpha light emitted from heating or diagnostic neutral beams is a standard technique for estimating plasma toroidal current density in tokamaks. Most techniques typically rely on a determination of the polarization angle or relative intensities of the multiplet components which are spectrally resolved using either an array of interference filters or a high-throughput grating instrument. Neither of these approaches is amenable to two-dimensional MSE imaging. This paper proposes an alternative measurement scheme that is suitable for single snapshot twodimensional imaging of the current distribution. This is achieved using a spectro-polarimeter that encodes the polarization and spectral information upon spatially orthogonal linear interference fringe patterns illuminated by the image of the Stark-shifted H-alpha emission from the neutral beam. The new technique opens the possibility to use synchronous detection methods and gated cameras to resolve the magnetic structure of periodic tokamak phenomena including sawteeth and magnetohydrodynamic activity. (Some figures in this article are in colour only in the electronic version)