Collisional and turbulent transport in fusion devices
Controlled thermonuclear fusion is a clean and safe, large scale energy option for the future with abundant fuel resources. The proposed project concerns one of the most important unsolved problems in magnetic confinement fusion; the transport and stability in transport barrier regions. The performance of a fusion device, and ultimately its viability as a reactor, largely depends on the transport of heat, particles and momentum through a region that makes up only a small fraction of the fusion plasma: the transport barrier. The transport in these formations is still not well understood, and a reliable predictive capability is lacking. Plasma parameters can vary on very short spatial scales, comparable to the width of ion orbits, which makes the transport problem challenging. Currently two approaches are pursued. For exploratory studies and to provide input for other codes, radially local Eulerian drift-kinetic codes are used, since they are light-weight, however they are formally outside their validity in a transport barrier, as they cannot handle finite orbit width effects. For more detailed studies, a possible alternative is to use full distribution function (“full-f”) gyrokinetic particle in cell codes which, while capturing the physics correctly, is unreasonably expensive for the problem (millions of CPU hours/simulation), thus their use for routine calculations - such as an input for magnetohydrodynamic stability calculations - is not realistic. Our group is currently the main developer of the 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], which captures finite orbit width physics and features a full linearized Fokker-Planck collisional operator. This code brings some of the advantages of the two mentioned approaches together, in being computationally cheap while still being suited to study transport barrier physics. The code uses efficient parallel routines of the PETSC library. The injection of certain impurities is observed to have beneficial effects on the achieved performance of the edge transport barrier, but this is not yet understood theoretically. Our theoretical studies show a sizable effect of non-trace impurities on flows and momentum transport. Encouraged by these initial findings, we will perform experimental modeling of experiments at the largest currently operating tokamak device JET. We will also study the effect of the isotopic composition of the main ions on collisional phenomena, in preparation the next deuterium-tritium (DT) campaign of JET. These studies are also important for improving our physics knowledge basis for the non-activated and the DT operation phases of ITER, the first reactor-scale experiment that is under construction in France. Large variations of the density and temperature of impurities on a flux surface are also observed and can be used to diagnose the transport barrier. While some of these poloidal variations are already reproducible with the code, we plant to extend it to allow for stronger variations and eventually for sonic flow speeds of various species. We will also use computational resources to support this development.