Absolute Calibration Of The Lyman-Alpha Measurement Apparatus At Diii-D

The LLAMA (Lyman-Alpha Measurement Apparatus) diagnostic was recently installed on the DIII-D tokamak [Rosenthal et al., Rev. Sci. Instrum. (submitted) (2020)]. LLAMA is a pinhole camera system with a narrow band Bragg mirror, a bandpass interference filter, and an absolute extreme ultraviolet photodiode detector array, which measures the Ly-alpha brightness in the toroidal direction on the inboard, high field side (HFS) and outboard, low field side (LFS). This contribution presents a setup and a procedure for an absolute calibration near the Ly-alpha line at 121.6 nm. The LLAMA in-vacuum components are designed as a compact, transferable setup that can be mounted in an ex situ vacuum enclosure that is equipped with an absolutely calibrated Ly-alpha source. The spectral purity and stability of the Ly-alpha source are characterized using a vacuum ultraviolet spectrometer, while the Ly-alpha source brightness is measured by a NIST-calibrated photodiode. The non-uniform nature of the Ly-alpha source emission was overcome by performing a calibration procedure that scans the Ly-alpha source position and employs a numerical optimization to determine the emission pattern. Nominal and measured calibration factors are determined and compared, showing agreement within their uncertainties. A first conversion of the measured signal obtained from DIII-D indicates that the Ly-alpha brightness on the HFS and LFS is on the order of 10(20) Ph sr(-1) m(-2) s(-1). The established calibration setup and procedure will be regularly used to re-calibrate the LLAMA during DIII-D vents to monitor possible degradation of optical components and detectors.