Upgraded C-2W scrape off layer helium line ratio spectroscopy system

An upgraded helium line ratio (HLR) diagnostic aims to measure profiles of electron density and temperature in the scrape-off layer and halo regions of C-2W [1] which are not well diagnosed by Thomson scattering, interferometry, or Langmuir probes. These measurements will be used to constrain equilibrium reconstructions and classical fast ion power loss estimates. The upgraded polychromator system includes the addition of a fiber bundle with sixteen input fibers, imaged onto three avalanche photodiode arrays (APDs). The APDs are integrated into a subsystem that supplies high voltage bias, temperature control, and transimpedance amplification of the signals. Custom modifications to fiber-coupled lens assemblies on vessel-side optical mounts were added to provide 12+ lines of sight at the midplane.  Relative calibration of the optical system as well as subsystem measurements were taken prior to deploying the HLR diagnostic. Initial measurements are compared against survey spectroscopy data at the same lines of sight.

[1] H. Gota, et al., Nuclear Fusion 61, 106039 (2021).