Study of plasma-wall interaction by fast cameras and numerical models

The present thesis is the product of three years of research in the field on magnetically confined nuclear fusion, that has the ultimate goal of demonstrating the possibility of obtaining clean energy from fusion of light nuclei. One of the current problems in this field deals with the interaction between the plasma and the wall of the containing device. Heat and particle fluxes impinging on the first wall can damage the machine structure and, on the other hand, particles expelled by plasma facing materials can degradate the performance and threat the very goal of reaching controlled fusion. The activity carried out during the Ph.D. concerned the study of plasma-wall interactions, focusing on three main topics. The first was a numerical study of advanced magnetic configurations to mitigate heat load and power density deposited on the divertor plates of FAST, a tokamak proposed as a satellite experiment for ITER. The following activity has ben done at RFX-mod, a Reversed-Field Pinch magnetic confinement experiment located in Padova, Italy. The plasma interaction with tungsten covered samples has been studied to investigate the properties of the material for its possible employment in a machine upgrade. As last activity, heat fluxes of the plasma edge have been measured through the instertion of graphite samples in limiter configuration, measuring the heat decay length for the first time in an RFP. The work is organized as follows. Chapter 1 gives a general introduction to fusion plasmas, magnetic confinement and main toroidal configurations studied. A short presentation of RFX-mod is given along with a quick glance to ITER. Chapter 2 gives a more detailed introduction to the topic of plasma edge, introducing concepts that are at the basis of the work presented later, as the divertor and the limiter configuration. Physical phenomena as recycling and superthermal electrons are also introduced here. Chapter 3 describes numerical simulations in the framework of preliminary studies for the proposed FAST tokamak. A quasi-snowflake divertor configuration has been studied and compared against a standard divertor using the EDGE2D/EIRENE code. Chapter 4 presents experimental measurements of surface properties of tungsten covered and full graphite samples. The activity was carried out at RFX-mod with a fast visible camera that measured the samples interactions with the plasma and a comparative analysis has been performed. Chapter 5 describes the investigation of heat flux properties of the plasma edge of the RFP through graphite samples inserted as limiters in the RFX-mod plasma. The measured temperature has been converted to heat flux with two software tools, that have been benchmarked. Heat fluxes and the measured heat decay length have been analyzed and correlated with plasma parameters. Conclusions are presented in the last chapter with an overview of possible future developments