Scrape-Off Layer physics in limited plasmas in TCV

Controlled nuclear fusion is the most promising candidate for being an inexhaustible, clean and intrinsically safe energy source. Research in the last 60 years has focused on the fusion reactor concept called tokamak, in which a high temperature plasma is confined using magnetic fields, producing fusion reactions. Because of turbulence, the confined plasma diffuses to the outermost region of the tokamak, featuring open field lines, called the Scrape-Off Layer (SOL). In the SOL, the plasma is convected along the field lines and is deposited on the solid surfaces of the tokamak wall. The plasma-wall interaction through the SOL is believed to strongly affect the reactor performances, and the elevated heat loads on the solid surfaces are one of the most limiting factors for fusion. Even though a variety of magnetic configurations has been developed through the years to decouple the main plasma from the reactor wall and improve the reactor performances, the SOL physics is not completely understood, not even in the simpler, “limited” configuration. In this configuration, the main plasma touches the reactor wall, or a “limiter”, and the contact point defines the so called Last Closed Flux Surface (LCFS), separating the SOL region, featuring open field lines, from the core region, where the plasma is confined with closed field lines. In this thesis, we advance the understanding of SOL physics in limited plasmas, combining experiments and numerical simulations. In particular, two topics are addressed. First, the separation of the SOL into two different regions, the “near” and “far” SOL, is investigated. The near SOL extends typically a fewmm from the LCFS, features steep radial profiles of parallel heat flux and is responsible for the peak of heat deposition on the tokamak first wall. The far SOL, typically a few cmwide, features flatter heat flux profiles, and accounts for themajority of the heat deposited on the first wall. In a second instance, blob dynamics is investigated. Blobs are high density plasma filaments generated by turbulence, and are an ubiquitous feature of plasmas in openmagnetic field line configurations. The blobs are self-propelled and travel outwards to the reactor walls, substantially increasing the cross field transport. A series of dedicated experiments has been performed on the TCV tokamak in Lausanne, Switzerland. Several inboard-limited Deuterium (D) and Helium (He) plasma discharges are performed, varying the main plasma parameters (current, density and shaping). The heat fluxes deposited onto the wall are determined by means of infrared thermography. The cross field heat flux at the contact point is evaluated and discussed with respect to its dependence upon the plasma parameters. The radial profiles of the parallel heat flux are analyzed. The