Nuclear fusion is a potential green energy source that could meet the growing energy needs of humanity. One of the leading fusion concepts is the tokamak, in which a hot plasma is confined by magnetic fields. To efficiently support fusion reactions, temperatures 10 times hotter than the center of the sun must be maintained in the core of such devices, while the edge must be sparse and cold so as to not damage the wall and contaminate the plasma with wall-materials.
This requires large temperature and density gradients; modern tokamak operation typically involves the creation of a transport barrier near the edge. This region – known as the pedestal – features the largest known steady-state temperature and density gradients, going from room- to fusion-temperatures in only a few centimeters.
While the pedestal is crucial to achieve fusion in a reasonable sized device, it challenges the conventional theory of how particles and heat are transported in fusion plasmas. In the pedestal, conditions vary significantly over the radial extent of a typical particle orbit, which implies that particle and heat transport cannot be studied in isolation at individual magnetic surfaces, but a coupling between those becomes important; the transport is said to be radially global.
For the last few years, we have been the main developers and users of the radially-global code PERFECT [M. Landreman et al, Plasma Phys. Control. Fusion 56 (2014) 045005. On GitHub: https://github.com/landreman/perfect]. Understanding the pedestal region is crucial to the international fusion program, and interest in our work has been large. During the past year, we have become involved in analyzing experimental data. Among other things, we have found that collisional transport of energy in JET (the largest tokamak in the world) is negligibly small, which is in contrast with findings from the smaller tokamak ASDEX Upgrade, where collisional heat transport dominates in the pedestal. It is important to understand this difference in order to extrapolate to large, reactor-sized experiments, and we plan to analyze more experimental pulses from both JET and ASDEX this year. Kebnekaise's large memory nodes are uniquely suited for the PERFECT code, and has become our primary computational resource in the previous year. Thus, we apply for a continuation of our project for another year.