Interpretation of spatially resolved helium line ratios on MAST

The tokamak boundary plasma is inherently 2D/3D, which impedes the detailed validation of transport models due to the limited spatial coverage of most diagnostics. A potential method for determining the 2D profiles of ne and Te in the plasma volume is to resolve spatially the emission ratios of atomic helium, principally the lines at 667, 706 and 728 nm. Unfortunately, there are several challenges associated with this approach: the traditional use of line-of-sight spectrometer data; crowding by other impurity lines; low signal levels; reliance on accurate atomic physics; and He I meta-stables. Several of these issues are explored in the present study, which utilises tomographic reconstruction of filtered CCD camera images to measure He emission throughout the divertor. Plasma gradients are resolved and compared with results from the OSM–EIRENE code. The near-target ne and Te values agree with Langmuir probe measurements to within a factor ∼2.