Description of complex viewing geometries of fusion tomography diagnostics by ray-tracing

Ray-tracing techniques are applied to bolometry, a diagnostic where the finite collection volume is particularly sensitive to the machine and detector configuration. A technique is presented that can handle arbitrarily complex aperture and collimator geometries, neglecting reflection effects. Sight lines from the ASDEX Upgrade bolometer foils were ray-traced with a path tracing algorithm, where the optical path is represented by a statistical bundle of ray paths connecting the foil surface with the slit geometry. By using the full 3D machine model for the detector box and first wall, effects such as occlusion and vignetting were included in the calculation of the bolometer's étendue. Inversion matrices calculated with the ray-tracing technique were compared with the more conventional single-ray approach and shown to be naturally more constrained, requiring less regularisation. The two models were tested on a sample radiation scenario, and the common single-ray approximation is shown to be insufficient. These results are particularly relevant for the divertor where strong emission gradients may be present. The technique developed generalises well to arbitrarily complex viewing geometries and collimators, opening up a new design space for bolometer configurations that might not normally have been considered.