Effect of graphite and molybdenum wall tile reflections on visible light diagnostics in tokamak experiments

Visible light reflectivities are measured for graphite and molybdenum tokamak wall tiles as a function of incident angle and wavelength. Using the measured reflectivities together with a ray-tracing routine, estimates of the effect of wall reflections on Dα light measurements in the DIII-D and Alcator C-Mod tokamaks are made. It is found that the contribution of reflections is small (typically <10%) for divertor view channels, but can be significant (>50%) for other view chords. Of the nondivertor channels, near-midplane view chords are found to be least susceptible to reflections from the divertor and are therefore best suited for main chamber recycling measurements.