An accelerator-based in-situ surface diagnostic for plasma-wall interactions science on Alcator C-Mod

Boundary science in magnetic fusion devices is severely hindered by a dearth of in-situ diagnosis of plasma facing component (PFC) surfaces. The ideal in-situ PFC diagnostic would perform surface composition measurements on a plasma shot-to-shot time scale with 1~$\mu$m depth and 1~cm spatial resolution over large PFC areas. To this end, the customary laboratory surface diagnostic - nuclear scattering of MeV ions - is being adapted to the Alcator C-Mod tokamak. A compact ($\sim$1~m), high-current ($\sim$mA) radio-frequency quadrupole accelerator injects 0.9 MeV deuterons into the vacuum vessel. The deuterons are steered to PFC surfaces with tokamak magnetic fields in between plasma shots, where they induce high-Q nuclear reactions with low-Z isotopes in the first $\sim$10~$\mu$m of material. The induced gammas and neutrons are detected with scintillators, where energy spectroscopy provides quantitative surface analyses. Techniques to measure the thickness of low-Z PFC film coatings and profiles of retained hydrogenic fuel are presented along with simulated measurements by ACRONYM, a comprehensive Geant4 synthetic diagnostic.