Turbulence induced particle and heat load on the first wall in different confinement regimes

The turbulent dynamics taking place in the edge and scrape-off layer regions of tokamak devices determines the level of particle and heat fluxes impinging on plasma-facing components, both at the divertor targets and at the first wall. The purpose of the present doctoral thesis work is to contribute to the everlasting gap in the knowledge of the highly non-linear SOL turbulent nature, understanding the correlations between the main machine input parameters and the fundamental properties of the SOL profiles and fluctuations. Two extensive datasets have been acquired on the TCV tokamak, covering a great portion of the machine operational space by varying either the gas fuelling rate from different locations in the vessel or the plasma shaping, at constant plasma current, toroidal field and heating scheme. The transport level and the characteristics of the SOL profiles have been correlated with the αt turbulence control parameter defined in [Eich et al 2020 Nucl. Fusion 60 056016], acting as a proxy for the relative level of resistive ballooning transport against drift wave turbulence. Analysis of the plasma discharges within the gas fuelling scan database has revealed a transition from a type-I ELMy regime to the Quasi-continuous exhaust (QCE) scenario, respectively at low and high divertor dissipation or, equivalently, neutral pressure. Correspondingly, a continuous broadening of both the divertor heat flux and near SOL upstream density profiles have been registered, together with the progressive formation of a density shoulder in the far SOL. These changes have been correlated to an enhancement of the radial filamentary transport level, with blobs becoming bigger and travelling radially faster towards the first wall. The detected filaments have been found to belong mainly to the resistive X-point and resistive ballooning propagation regimes and, as such, they are likely to be disconnected from the target plates, reaching at most only partially into the divertor chamber. In the shaping scan database, the changes in edge safety factor induced by a scan in upper triangularity δup determine a modification of the αt parameter towards higher values and thus a more resistive ballooning unstable edge-SOL plasma. Once again, a transition from type-I ELMs to QCE takes place from low to high shaping, the divertor heat load and the near SOL density profile broaden, and a density shoulder forms in the far SOL. These behaviours have been correlated to an enhanced filamentary activity near the first wall, with the average value and standard deviation and the frequency of fast filamentary fluctuation measurements increasing at high plasma shaping. Finally, the work addresses also some indirect effects of δup on setting the parameters defining αt, in particular midplane neutral pressure measurements indicate that indirect effects of the shaping on local ionization sources do not play a significant role in setting the far SOL density profile. On the other hand, a significant increase of the total radiated power is observed at high δup, originating from a higher level of carbon impurity intake from the first wall tiles due to the changes in magnetic topology from low to high shaping.