At JET, the distance from the plasma core (at hundreds of millions degrees Centigrade) to the vessel wall (at several hundreds of degrees) is about one metre. Most of the temperature drop occurs over the last few centimetres. This means that in this region the temperature may decrease by several tens of million degrees per centimetre. By comparison, the gradient within a candle flame, from wick (cool) to outer flame (hot), is of the order of one thousand degrees per centimetre. This region is called the plasma edge. The processes taking place at the plasma edge are crucial for both protecting the vessel’s inner wall (“first wall) from the plasma and for confining the plasma energy.
In a fusion reactor the plasma edge may be imagined as a protective skin: its properties control the power and particle exchange between the the plasma core and the vessel walls. There is a strong interplay between the plasma edge and the first wall.
Plasma particles and energy escape via open magnetic field lines
The plasma particles move along the magnetic field lines, which form closed loops in the plasma core. At the plasma edge, however, the field lines break open and intersect the vessel wall. Plasma particles that happen to be on such field lines are therefore guided into collisions with the first wall and deposit their energy onto the plasma-facing material.
In an idealised scenario, all plasma particles would stay along the closed field lines. However, there are processes that force them to leak out: Particles diffuse across the magnetic field or they may leave the confined volume simply due to the fact that their orbit around each field line has a finite radius. Furthermore they can “jump” from one guiding field line to another due to collisions with other plasma particles or due to fluctuating electric fields causing so called turbulent turbulent transport. Also instabilities at the plasma edge (ELMs) eject particles from the plasma.
Guiding escaping particles along defined field lines
It is not possible to design a tokamak without any open field lines. Much research therefore goes into the optimum design to guide the plasma exhaust along defined open field lines. JET and many other tokamaks use the divertor concept. The open magentic field lines are diverted into a dedicated region at the vessel wall, where the plasma exhaust collides with target plates or with gas. The divertor not only takes up the plasma exhaust, it also is the place where future fusion power plants remove the helium ash from the plasma.
The divertor was one of the most successful improvements and continues to be an important research topic. The material used for the divertor will not withstand the high heat loads produces by the next generation of fusion devices – DEMO and fusion power plants. Among the investigated solutions are modified magnetic field configurations, which either cool the plasma further down before hitting the wall (the Super-X divertor) or spread the heat over a larger wall area (the Snowflake Divertor).