Kinetic simulations of collisional transport in tokamak transport barriers
The tokamak is currently the most promising magnetic confinement concept for future fusion power plants. In a tokamak plasma, particles, energy and momentum are confined by a magnetic field, and the cross-field transport of these quantities play a crucial role in reaching an efficient, self-sustaining, fusion plasma. The confined region of the fusion plasma is made up of two qualitatively different regions: The plasma core is characterized by lower pressure gradients due to efficient heat and particle transport driven by plasma turbulence. In contrast, turbulence is almost absent in the so-called edge transport barrier, which, although it only composes a small fraction of the total plasma volume, is responsible for a significant fraction of the total energy confinement. Due to the reduced transport level in the transport barrier region, it supports much larger pressure gradients than the core, a feature which complicates modeling efforts. Specifically, particle orbits in the transport barrier typically move across plasma regions with widely different temperatures and densities, which means that the transport can no longer be described simply by local values of density, temperature, etc., but needs to be modeled in a spatially global fashion. However, such global treatments are in general complicated, and has been avoided in the past, even when the use of the simpler local models is unjustified. Our group is currently the lead developer and user of a radially global Eulerian collisional transport solver called PERFECT [M. Landreman et al, Plasma Phys. Control. Fusion 56 (2014) 045005. On GitHub: https://github.com/landreman/perfect] that uses a novel approach to retain the spatially non-local feature of the problem, while remaining reasonably fast and flexible by separating the collisional processes from the turbulent ones (the former being more important in the transport barrier). In this project we will investigate collisional transport phenomena in tokamak transport barriers using our unique radially global solver. In particular fluxes of heat, particle and momentum – together with the experimentally more accessible flows – will be calculated to gain understanding on the effect of isotopic plasma composition and various impurity species. These aspects are of great importance to prepare the currently largest tokamak experiment, JET, to its upcoming deuterium-tritium experimental campaign. In the long term, these results will be used to establish the physics basis of ITER (an international fusion reactor under construction in France), which will operate with various isotopic compositions during its different experimental periods. Although we have started to use and develop the PERFECT code relatively recently, our results have already sparked interest from the community. Our latest study on the effect of non-trace impurities earned oral presentations at the two largest plasma physics conferences (those of the European- and the American Physical Society) and two invited presentations in more specialized meetings.