Boundary Plasma Physics: An Accessible Guide to Transport

Book Preface

Generating electrical energy from nuclear fusion on Earth is challenging, potentially very rewarding and an incredibly interesting research subject. In this Chapter, we introduce the fundamental concepts associated with it, but we focus in particular on one specific aspect: plasma exhaust. When fusion reactions occur, part of the energy generated remains in the plasma and heats it up. This is obviously good because it means we need less power to keep the plasma at the temperature required for fusion to occur. On the other hand, this energy will eventually leak out and reach the solid surfaces surrounding the confined plasma, which can be irremediably damaged by this interaction, unless we put in place adequate countermeasures. In this Chapter we give a bird’s-eye view of the problems and phenomena associated with the boundary plasma, where the exhaust of the plasma energy occurs.
The book that you are reading is about the physics of the plasma boundary in tokamaks. A good start would therefore be to give an operative and general definition of this fascinating and important region of the plasma. However, before we can do that, we need to briefly explore the geography of a magnetic fusion device.
While the scientific community has imagined a number of possible designs for future fusion reactors, our discussion will deal with the specific problems associated with tokamaks, although some concepts can be applied more generally. In these devices, the plasma, which fuels the fusion reactions if sufficiently hot, is contained in a vessel of toroidal shape—substantially a big scorching doughnut. It is not the walls of the vessel that really hold in place the hot nuclear fuel, as it needs to be brought to temperatures that are 10 times higher than our Sun’s core, and there is no solid material that can withstand that. At these temperatures, the fuel is ionized and constituted of charged particles. Luckily, magnetic fields can be used to constrain the motion of charged particles by forming a sort of immaterial backbone for the plasma, which aims at keeping it sufficiently contained and separated from the walls. It is for this reason that the tokamak confinement scheme is called magnetic fusion, and it because of the nature of the magnetic field that the device needs to be toroidal, as we will shortly see.
A fundamental concept for the boundary plasma physicists is that, inside the vacuum vessel containing the plasma, the magnetic field lines have two basic topologies: open, when they intersect material surfaces, or closed when they do not (see Fig. 1.1). In axisymmetric tokamaks, which are configurations that do not vary in the toroidal direction,1 the two topologies are neatly separated by a surface called separatrix (reality is more complicated, but we will discuss this later on). Typically, the region inside the separatrix is called the core plasma, while the region outside the Scrape Off Layer (SOL), on account of the fact that the plasma peels off the separatrix and is swept towards the material surfaces.
We are now ready to define the boundary as the part of the plasma that goes from the material surfaces—the plasma facing components—up to the well-confined plasma. The more rigorous among the readers will already be frowning at this statement because it is vague in establishing the inner frontier of the boundary plasma. However, this vagueness is intrinsic in the nature of our problem. Indeed, the external part of the core and the Scrape-Off Layer are strictly integrated and communicating bi-directionally through fluxes and profiles. How deep the tendrils of this interaction penetrate into the core is still subject of speculation, but we can reasonably estimate that the interested region is of the order of 10% of the minor radius, although it might make more physical sense to express this in terms of Larmor radii. The radial correlation length of the turbulent eddies just inside the separatrix or the ionization mean free path of the neutral particles provide reasonable order of magnitude estimates.
Armed with our definition, in the rest of this Chapter we will explore the basic issues associated with the boundary plasma and the reasons why it has become of central importance to the realisation of fusion reactors.