Transition from ion- to electron-repelling wall in tokamaks

In magnetised plasmas for fusion energy, the design of the region of the plasma in contact with the wall determines the heat and particle load to the chamber that contains the plasma. Simply scaling current machines to reactor size would give heat loads to the wall that are more than one hundred times the maximum load that any known material withstands. We need clever solutions to enable the wall to survive. A key aspect of this challenge to consider is the character of the wall-plasma interaction (the physics of the plasma "edge"). If the wall charges negatively, it will repel electrons and accelerate ions. The accelerated ions then hit the wall and sputter part of the material away. In contrast, positively charged walls repel ions and prevent sputtering, but they attract a large heat load from hot electrons. Due to the different dynamics of the two species and also different collective dynamics of the plasma along and across the magnetic field lines, the angle between the wall and the magnetic field determines whether the wall repels electrons or ions. Current machines seem to be in the electron-repelling regime, but designs of future fusion reactors are considering angles that are within approximately one degree of the transition between the electron-repelling and the ion-repelling cases (these designs are based on other optimisation considerations, which ignore this transition). This DPhil project focuses on a full theoretical characterisation of the transition between the electron- and ion-repelling regimes using novel analytical techniques. The idea is to determine whether angles of the order of one degree are indeed beneficial by considering that an ion-repelling wall would lead to a completely different balance between the ion and electron particle and energy losses to the wall, profoundly changing the plasma upstream (inside the device).
This project falls within EPSRC Plasma and Lasers research area.