Estimating plasma size by visible emission patterns in the Wendelstein 7-X stellarator

Determining the plasma size in the Wendelstein 7-X stellarator is complicated by the complex 3D structure and lack of axial symmetry. The effective radius ($r_{eff}$) of the visible radiation belt around the plasma gives a suitable estimate. This study presents a novel method to determine $r_{eff}$ using the W7-X fast camera system. We correlate preprocessed EDICAM images with synthetic images based on the expected emissions of infinitesimally thin radiating layers from individual flux surfaces. Images are preprocessed to make the method reliable against wall reflections and increased divertor radiation. The width of the radiation belt is also estimated, which is relevant in the study of turbulent structures. We demonstrate the applications of this method in the case of detached and “breathing plasmas”.