Conway, N. J.De Bock, M. F. M.Michael, C. A.Walsh, M. J.Carolan, P. G.Hawkes, N. C.Rachlew, E.McCone, J. F. G.Shibaev, S.Wearing, G.2015-12-072015-12-070034-6748http://hdl.handle.net/1885/17059A motional Stark effect (MSE) diagnostic is now installed and operating routinely on the MAST spherical tokamak, with 35 radial channels, spatial resolution of ∼2.5 cm, and time resolution of ∼1 ms at angular noise levels of ∼0.5°. Conventional (albeit very narrow) interference filters isolate π or σ polarized emission. Avalanche photodiode detectors with digital phase-sensitive detection measure the harmonics of a pair of photoelastic modulators operating at 20 and 23 kHz, and thus the polarization state. The π component is observed to be significantly stronger than σ, in reasonably good agreement with atomic physics calculations, and as a result, almost all channels are now operated on π. Trials with a wide filter that admits the entire Stark pattern (relying on the net polarization of the emission) have demonstrated performance almost as good as the conventional channels. MSE-constrained equilibrium reconstructions can readily be produced between pulses.This work was funded partly by the United Kingdom Engineering and Physical Sciences Research Council under Grant No. P/G003955 and by the European Communities under the contract of association between Euratom and CCFE.7 pages© 2010 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Review of Scientific Instruments and may be found at http://doi.org/10.1063/1.3494254 http://publishing.aip.org/authors/copyright-reuse http://www.sherpa.ac.uk/romeo/issn/0034-6748 Author can archive publisher's version/PDF (Sherpa/Romeo as of 7/12/2015).motional Stark effect (MSE)MAST spherical tokamak35 radial channelsspatial resolution∼2.5 cmtime resolution∼1 msangular noise∼0.5°π componentσatomic physics201010.1063/1.34942542015-12-08