Experimental and numerical investigation of X-point Radiator scenarios in WEST

Among the challenges on the path to fusion power plant (FPPs) construction, the control of heat flux and particle exhaust in a magnetic confinement device is one of the most critical ones. A significant fraction of the power produced by fusion reactions is convected from the core plasma region towards the edge plasma region. In the latter, the resulting steady-state heat flux is expected to be already very close to the technological limits. To manage larger FPPs, several strategies are under investigation, playing on power dissipation in the peripheral regions by impurity injection, a very efficient way to spread the energy over all the plasma-facing components. The understanding and control of plasma-wall interaction is crucial for the success of ITER and next-step fusion devices. Recently many experimental studies have been performed to investigate the so-called X-Point Radiator (XPR) regime consisting of a plasma detached from the tokamak wall and thus decreasing the heat load on the plasma-facing components. In WEST (Tungsten Environment in Steady-state Tokamak), located at the CEA Institute of Cadarache (France), such a plasma scenario has been developed with noteworthy results: an improved thermal confinement in the plasma core, for both electrons and ions, is observed. This thesis will focus on the investigation of the XPR regime analyzing experimental results and reproducing them numerically using the fluid code SOLEDGE3X-EIRENE. Pure deuterium plasma simulations and simulations with oxygen and nitrogen injection are conducted. Comparison with Langmuir probe data and visible spectroscopy are performed to reach a similar configuration from the numerical point of view. Numerical analysis allows a better interpretation of the experimental results and a deep insight into the physical mechanisms at play. Finally, a reduced model is applied to WEST to study the transition between the attached plasma and the XPR regime, also from an analytical point of view.