Building a Lyman-$\alpha$ detector for measurement of recycling rates in LTX

The measurement of the particle recycling rate is essential for understanding the performance improvements of lithium PFC devices like CDX-U and LTX. Recycling is usually measured by using atomic H-$\alpha$ (Balmer-$\alpha$) emission, but the signal can be difficult to interpret because H-$\alpha$ has a high reflectivity for many wall materials including lithium. In contrast, Lyman-$\alpha$ is known to have a low reflectivity at a lithium wall. To measure recycling rates for LTX, a Lyman- $\alpha$ detector with a photodiode and a directly deposited 117-131nm pass-band filter has been developed. The detector has been tested with collisional beam excitations with a background of hydrogen gas. Based on known inelastic collision cross sections of a hydrogen beam, the relative intensity of Lyman- $\alpha$ collisionally induced fluorescence (CIF) to H-$\alpha$ CIF has been calculated. By comparing the theoretical ratio with the measured one, the detector can be calibrated.