Far ultraviolet polychromator for spectroscopic characterization of the tokamak divertor and plasma scrape-off layer
Review of Scientific Instruments
The Plasma Spectroscopy Group of The Johns Hopkins University is developing diagnostics for spectroscopic characterization of the tokamak plasma scrape-off layer and divertor regions. A far ultraviolet polychromator has been designed for radiative divertor studies at the Alcator C-Mod and D-IIID tokamaks. Local measurements of resonant transitions of lithium-to boron-like ions of intrinsic or seeded low Z impurity elements will be performed along multiple chords around the X point. Planar
... ction gratings and stacked grids will be used for dispersion and angular collimation of radiation. Phosphor wavelength converters coupled to a photomultiplier tube by an optical fiber will be used as detectors. The design provides a wavelength resolution of Ӎ10 Å, a spatial resolution of р2 cm, and an adequate photometric sensitivity. The in-vessel instrument, proposed for the Alcator C-Mod tokamak, will measure intensities of the lines at 1240 ͑N V͒, 765, 923 ͑N IV͒, and 990 Å ͑N III͒. The port-mounted polychromator at DIII-D will be able to monitor intensities of 1550 ͑C IV͒, 977, 1176 ͑C III͒, and 1335 Å ͑C II͒ lines. This, together with visible and bolometric diagnostics, should enable estimates of power losses, charge state distribution and local transport of the impurity ions in the divertor. A one-channel prototype of the C-Mod and D-IIID instruments is being built for the CDX-U spherical tokamak. Line-integrated brightnesses of the 2s -2p transition at 1550 Å will be measured and inverted to obtain C IV emissivity distribution.