Influence of neutral particles on edge dynamics of magnetically confined plasmas

Thermonuclear fusion of light atomic nuclei in future reactors promises a source of electric energy which is clean, safe, and sustainable. A propitious design for such reactor is the tokamak, in which hot plasma is suspended in a twisted toroidal magnetic field. The geometry of toroidal connement results in turbulent transport of particles and heat from the conned bulk region radially outwards towards the first wall of the vacuum chamber.

The plasma passes through the edge layer of the conned region, for which the conditions have been found to be defining for the overall plasma connement. This makes the plasma edge a region of uttermost importance to understand, if energy from reactor based fusion is to be realized. In the scrape-off layer (SOL) region, between the edge region and the first wall, plasma is transported outwards in field-aligned laments.

The SOL is characterized by open magnetic field lines which terminate on material surfaces, and it is signicantly colder and less dense than the conned plasma. This allows for maintaining a considerable population of neutral particles. It has been experimentally observed that the presence of neutrals in influences the conditions in the edge region.

This makes the understanding of transport of neutrals and their interactions with the edge and SOL plasma equally important. In this work a novel numerical model for describing the transport of neutral particles, in particular the interactions with the electrons and ions, is formulated. Both molecular and atomic neutral species are included in the model to account for various neutral-plasma interactions.

The model for the neutrals is coupled to an existing drift-fluid model for edge and SOL plasma, and the combined model is applied in a series of studies on the effect of neutral interactions on the plasma and vice versa. Particularly the relative roles of molecules and atoms in the plasma fuelling process, as well as the local effect that the field-aligned plasma laments have on the neutral densities, are elucidated.

The effect of the molecular puffing rate on the plasma turbulence characteristics is likewise investigated.