Plasma tomographic reconstruction from tangentially viewing camera with background subtraction

Abstract

Light reflections are one of the main and often underestimated issues of plasma emissivity reconstruction in visible light spectral range. Metallic and other specular components of tokamak generate systematic errors in the optical measurements that could lead to wrong interpretation of data. Our analysis is performed at data from the tokamak COMPASS. It is a D-shaped tokamak with specular metallic vessel and possibility of the H-mode plasma. Data from fast visible light camera were used for tomographic reconstruction with background reflections subtraction to study plasma boundary. In this article, we show that despite highly specular tokamak wall, it is possible to obtain a realistic reconstruction. The developed algorithm shows robust results despite of systematic errors in the optical measurements and calibration. The motivation is to obtain an independent estimate of the plasma boundary shape.

Physically principled reflection models applied to filtered camera imaging inversions in metal walled fusion machines

“…A common approximation used in many of the previous studies is to model the BRDF function as a linear combination of the two ideal limits [5][6][7]15,18 , Real physical materials exhibit a complex combination of both specular and diffuse behaviours in addition to transmission and absorption. For this work, the BRDFs of fusion relevant materials were modelled with the Cook-Torrance BRDF 9,29 , which was parameterised in terms of the Fresnel equations and the GGX micro-facet surface model 30 ,…”

“…Filtered cameras can give the required spatial resolution and have been deployed with some success in carbon walled machines 3 . However utilising these for routine physics analysis in metal walled machines has proven more challenging due to reflection effects [4][5][6] . The reflected light of bright plasma regions, such as the strike-points or x-point, by wall features can lead to artifacts in the measured images.…”

“…Previous studies that address reflection effects in filtered imaging have used two main techniques. The first uses optical ray-tracing methods combined with a simplified first wall model that is often an axisymmetric surface [5][6][7] . The reflection properties of the wall in these models are often approximated as linear combinations of ideal specular and diffuse reflections.…”

“…The most advanced previous ray-tracing study that attempted to include non-axisymmetric wall features was undertaken at the COMPASS tokamak 6 . An image of the vessel under diffuse background light captured all the asymmetric reflectivity patterns (e.g.…”

Effect of reflections on 2D tomographic reconstructions of filtered cameras and on interpreting spectroscopic measurements in the JET ITER-like wall divertor

“…Tomographic reconstructions of tangential camera views are an established method for resolving the 2D distribution of divertor emission in various tokamak devices, [15][16][17][18][19][20][21][22][23] including the spectroscopic divertor camera system in JET. 24,25 The reconstructed 2D emission distributions assist in localizing the line-integrated divertor spectroscopy measurements, allow inferring, e.g., the spatial evolution of ionization and recombination fronts, and provide valuable reference data for divertor plasma modeling.…”