Wendelstein 7-X (W7-X), the world’s most advanced superconducting stellarator-type fusion experiment, is equipped with 10 EDICAMs (Event Detection Intelligent Camera), monitoring the entire inner wall. The unique features of the EDICAM allow us to operate the cameras simultaneously in surveillance and scientific mode: We are able to monitor (100 Hz) the plasma discharge evolution, the integrity of the first wall and look for unexpected events, as well as to produce high-speed recordings (up to 20 kHz) of physical processes such as pellet injection or plasma filaments.

Understanding the sources of visible radiation, both bulk and localized, has key importance for a fusion reactor, as radiation is a major energy loss channel of the plasma.

The visible radiation of the bulk plasma usually forms a thin belt at the edge, where temperature is low while density is high enough to produce visible line emission. In W7-X, visible radiation is dominated by Hydrogen and Carbon lines, the level of Bremsstrahlung is at least an order of magnitude lower (but still detectable between stronger lines). The characteristics of the visible radiation belt (size, thickness, profile) are therefore strongly dependent on the composition of the plasma, as well as on the distribution of the impurities within the plasma.

This can be complemented by a deeper understanding of the processes leading to (visible) radiation emission in the plasma, including plasma-wall interaction (PWI). PWI often emerges localized to a small region or even into a point-like spot, resulting irregular flashes of light emission often referred to as hot-spots (or bright spots). Hot-spots can be produced by e.g. the misaligned plasma touching an inner structure or by fast particles hitting a surface, and are generally observed by visible cameras earlier than the actual increase in the surface temperature because the enhanced PWI usually comes with immediate emission of light. The understanding of these phenomena has key importance in machine safety, and could serve as a broadened basis for this study.

The task of the applicant will be to analyze in detail and compare EDICAM measurements to numerical simulations (EMC3-EIRENE code) modelling the impurity distribution and radiation emission of the plasma. Additionally, the task involves processing available literature on PWI processes, which can be complemented with dedicated experiments.