Prototype C-2W scrape-off layer helium line ratio spectroscopy system and preliminary measurement results

A measurement gap exists in the scrape-off layer of C-2W [1] that core Thomson scattering and interferometry and edge Langmuir probe diagnostics cannot access. Motivated by previous success using the helium line ratio method to measure electron density and temperature at the edge region of fusion plasmas [2, 3], a new helium line ratio (HLR) diagnostic has been designed and tested to fill this gap. Compared to the HLR diagnostic in the C-2W divertor [4], the new diagnostic trades spatial resolution for higher temporal resolution. Discrete optical fibers couple sightlines to the polychromator which splits a single input into three wavelength channels. Avalanche photodiodes (APDs) are used as detectors. Preliminary measurements used a single line of sight intersecting a pulsed neutral helium target injected by a valve with a supersonic converging-diverging nozzle. The measurements were compared for consistency with those from other diagnostics to determine how well the helium line ratio diagnostic captures electron temperature and electron density. The full instrument will provide 12+ simultaneous measurement points and routine relative calibration reference is in the process of being deployed.