N. J. Conway

A motional Stark effect (MSE) diagnostic is now installed and operating routinely on the MAST spherical tokamak, with 35 radial channels, spatial resolution of $\sim 2.5\text{cm}$ , and time resolution of $\sim 1\text{ms}$ at angular noise levels of $\sim 0.5°$ . Conventional (albeit very narrow) interference filters isolate $\pi$ or $\sigma$ 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 $\pi$ component is observed to be significantly stronger than $\sigma$ , in reasonably good agreement with atomic physics calculations, and as a result, almost all channels are now operated on $\pi$ . 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.

One of the clearest signatures of the transitions from L- to H-mode plasmas is the steepening of the edge density gradient. A concomitant narrowing of the visible radiation, usually dominated by Balmer $D_{\alpha }$ emission, is generally observed. Typically, the $D_{\alpha }$ emissivity width is 4–6 cm for the L-mode, similar for ELMs, and 1–3 cm for H-mode MAST plasmas, comparable with typical edge $n_e$ scale lengths. A means of extracting electron densities from this narrowing of the radial $D_{\alpha }$ emissivity profile was pioneered on the START device and developed further on MAST. It takes advantage of the near insensitivity to electron temperature of both the ionization and emission rates of hydrogen at temperatures well in excess of the ionization potential (13.6 eV). The $D_{\alpha }$ emissivity profile is obtained from spatial inversion of line-of-sight integrated intensities recorded by a linear camera $\text{(Δt=125\hspace{0.17em}μ}\mathrm{s},$ 256 pixels). The 300 point Thomson scattering diagnostic has been used as a yardstick in comparing experimental data from a wide range of edge density gradients and a range of edge temperatures in checking the models validity and assumptions.