LLAMA - A Diagnostic to Measure Edge Neutral Density Profiles in DIII-D

The LLAMA (LLAMA is the Lyman-Alpha Measurement Apparatus) diagnostic was recently installed on the DIII-D tokamak. LLAMA is an absolutely calibrated pinhole camera system, with narrow band Bragg mirror, bandpass interference filter and an AXUV photodiode detector array. It measures the Ly-α brightness profiles in the toroidal direction on the inboard and outboard side with twenty lines of sight on each view. The cameras views are centered around 77 cm below the midplane and provide a radial resolution of approximately 8 mm, covering in total 214 mm. The combined mirror and filter transmission has a full width half maximum of 5 nm, centered around the Ly-α wavelength of 121.6 nm and is capable of rejecting significant, parasitic carbon-III emission. Absolutely calibrated, spatially resolved Ly-α measurements combine the advantages of a bright, isolated line and low parasitic surface reflections. Ly-α emissivity profiles are combined with electron density and temperature profiles as well as standard rate coefficients to determine neutral particle densities on DIII-D, enabling quantitative comparisons to modeling studying divertor leakage, main chamber fueling and radial particle transport.