Enhanced edge Thomson scattering on MAST

Enhanced edge Thomson scattering on MAST

Enhanced edge Thomson scattering on MAST 150 150 Mathew

A new edge Thomson scattering diagnostic has been implemented at MAST to complement an existing high spatial resolution ruby laser system and the high time sampling core Nd:YAG system. The Nd:YAG system comprises of four independently controllable lasers. Scattered light from these lasers is viewed at large scattering angle 153° by a special optical arrangement in the new edge system. The Nd:YAG lasers are viewed at 16 contiguous spatial locations separated by 1 cm each, located at the plasma outboard pedestal and scrape-off layer region. Here the use of a low f -number lens for the collection of a large solid angle of scattered light is particularly beneficial due to low plasma density n e . The spectrum of scattered light is significantly broader at large scattering angles, allowing diagnosis of lower plasma temperatures T e while using the same spectrometer design as the core system. The four Nd:YAG lasers follow two separate slightly offset 1/3 of a spatial channel optical paths through the vessel. This is useful when the lasers are used in burst mode for detailed edge studies of fast events such as ELMs. Polychromators have been designed to allow for both Raman and Rayleigh calibration. First results from this diagnostic are presented showing H -mode pedestal behavior. A novel spectral fitting technique has been devised and is applied to edge pedestal fitting.

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03/10/2006