Radially global neoclassical transport in tokamak pedestals

Licentiatavhandling, 2017

Nuclear fusion has the potential to become a sustainable energy source in the foreseeable future. The most developed system for fusion power production is the tokamak, which magnetically confines a plasma at high enough temperature for fusion reactions to take place. Tokamaks operating in the H-mode feature the largest known steady state density and temperature gradients, located in a region at the edge of the plasma known as the pedestal. These steep gradients result from a spontaneous reduction in turbulence, and as a result of these steep gradients, the plasma behavior couples between nearby radial locations, and can no longer be evaluated in terms of plasma parameters at a single radius. The plasma behavior is said to be radially global. This makes it challenging to model the transport of particles, heat, etc., which is needed to design and evaluate future reactors. In this thesis, we study collisional, radially-global transport in toka- mak pedestals, using numerical methods to solve a drift-kinetic equation for the distribution of particles in both velocity and configuration space. Particular focus is put on the influence of non-trace impurities, and the effects of changing the mass and charge of the bulk ions. Order unity deviations from radially-local results are observed in plasma flows and cross-field fluxes, both in the pedestal and the near-pedestal core. In ad- dition, a significant radial transport of angular momentum arises in the radially-global description, which may have implications for the plasma rotation, which is understood as a crucial component for the transition to H-mode.