Alistair Arnold, Max Planck Institute for Plasma Physics, Greifswald, Germany

"Parallel expansion of a pellet plasmoid"

Abstract: An effective method of refuelling MCF devices is the injection of cryogenic hydrogen pellets. Shortly after injection, heating by the ambient plasma begins to ablate the pellet surface and ionise the resulting gas. The gas cloud follows the pellet, leaving behind newly-formed plasma on magnetic field lines. On a given field line that intersected the gas cloud, there is a localised density of plasma – a plasmoid. The plasmoid induces a potential well, which serves to trap plasmoid electrons and rapidly accelerate plasmoid ions parallel to magnetic field lines, away from the centre of the plasmoid. The finite well height naturally splits the electron distribution into trapped and passing parts, which interact both collisionally and collisionlessly. We exploit the separation of scales between the trapped electron bounce time, trapped electron collision time, and plasmoid expansion time to simplify the electron kinetic problem. We find that an electron quasi-equilibrium state is established, characterised by a vanishing bounce-averaged collision operator, sharing properties with an equilibrium distribution. It follows that we can reduce electron dynamics during expansion to a fluid problem. Approximate solutions to the quasi-equilibrium problem are provided, suggesting that plasmoid electrons are heated more slowly than previously predicted, implying an increased channelling of energy to ions during parallel expansion.