The MAST motional Stark effect diagnostic

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.
October 2010
Review of Scientific Instruments;Oct2010, Vol. 81 Issue 10, p10D738
Academic Journal
A 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.


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