Fusion operations not only need stable and hot plasma, they also need an ashtray. An ashtray? – Right. A fusion operations produces heat but also ashes, in the form of helium. But the ‘ashes’ of a fusion operation are tricky to handle. The ashtray of a tokamak or stellarator is called a divertor. As the name says, it diverts waste particles out of the plasma directly into the trash with the help of magnetic field lines. What sounds quite easy on paper, is actually hard to achieve since we are dealing with temperatures ten times hotter than the Sun. This process therefore requires materials that are able to resist such harsh conditions.

What is a divertor?

The ‘fusion ashtray’ at the bottom or the top of a tokamak is a specially armoured structure containing the so called divertor plates, where the diverted particles come into contact with the material. The divertor is designed to handle the heat and particle loads thrown at it, but still requires frequent maintenance. Hence, in future devices such as ITER, it is built as a cassette allowing easy removal and replacement.

Why is the divertor so important?

Firstly, it acts as a pump for the helium ash to prevent the dilution of the plasma. Secondly, it provides a specially armoured region to handle the power that escapes from the plasma. The energy can be thus moved away from the core plasma. This reduces the amount of impurities, as released through plasma-surface interactions, that are able to enter the core plasma. Improving divertor designs is one of the main targets of EUROfusion’s research. Mission 2 of the European fusion roadmap prioritises the so called ‘Heat-exhaust systems’. The outcome should result in magnetic configurations that will reduce the heat loads and ensure divertor materials capable of handling them. At the moment, EUROfusion is evaluating even more research projects which deal with heat exhaust and resisting materials.

Divertor – new for tokamaks?

After the first fusion operations were carried out in the 1970s, researchers realised that plasma leaked out of the main confinement area. Although strong magnetic fields keep the plasma in place, some particles are able to escape and impact the wall of the vacuum chamber. They are so energetic that they can cause damage to the wall and, due to the resulting release of impurities, pollute the fusion plasma. To stop this, researchers incorporated a limiter. This was a protruding piece of material made of steel which protected the original vessel wall from the outer edge of the plasma. However, the limiter still produced impurities close to the plasma which flowed back in and polluted it.

Chart of a divertor. Picture: © Copyright protected by United Kingdom Atomic Energy Authority

Chart of a divertor. Picture: © Copyright protected by
United Kingdom Atomic Energy Authority

X marks the spot

Instead of adding just another wall, the fusion experts decided to try working with magnetic confinement again. By adding magnetic coils they were able to create a zone where the field lines crossed, known as an X-point. The field lines are ‘diverted’ to form a so called scrape-off layer (SOL). This appears then at both outer edges of the D-shaped plasma. Inside this layer, which is separated from the core plasma, charged particles follow magnetic field lines into the divertor where they hit the above-mentioned divertor plates. Here, the diverted particles transfer their energy.

Different divertor concepts

The team of the newly set up MAST Upgrade is very proud of its flexible divertor concepts. Indeed, the tokamak will be able to transition smoothly from one configuration to the other by changing the magnetic fields appropriately. Those configurations have lovely names such as the Super-X divertor or the Snowflake divertor approach. The names are assigned according to the characterisation of the ‘diverted’ field lines, which form what are known as legs below the X-point. The magnetic fields can be adjusted so that the length of these legs varies. In a conventional divertor, these legs are typically quite short and hence the particles do not have far to go before reaching their target and so do not lose much energy on the way.

One or two X-points?

The divertor can be located on top of the fusion device or at the bottom or even at both ends. ITER will have the divertor at the bottom of the tokamak just like JET (Joint European Torus) which is the only divertor device that has operated a deuterium-tritium campaign so far. As a result, ITER features a conventional „one X-point and one divertor“ structure. However, there is interest in plasma systems with two X-points and two divertors. ASDEX Upgrade and MAST Upgrade, for instance, have this capability. Two X-points are known scientifically as “Double null divertors”.

The Super-X divertor

The Super-X configuration extends the legs and thus the particles travel a longer distance before reaching the target plates and accordingly interact more often with other particles along the way. In fusion terms, this essentially means that the power which hits the divertor plate is reduced greatly. It requires a set of divertor coils that extends and controls a long plume of exhaust plasma. The length of the plume allows high radio-active cooling before the plasma reaches the target.

The Snowflake divertor

The Snowflake variation creates more than the two usual legs. The power is separated into many snowflake-like branches which spread the particles out and reduce the load so that the existing materials are able to tolerate it. EUROfusion’s researchers have been investigating the Snowflake divertor already. The recently enhanced TCV (Tokamak à configuration variable) in Switzerland is able to employ this special divertor approach.

Background

A fusion reaction occurs when two light nuclei fuse and kinetic energy is exchanged between the products. This energy results in heat. The temperatures created in a fusion device are, on average, ten times hotter than the Sun. Future fusion plants will fuse deuterium and tritium and generate alpha-articles and neutrons. The neutrons carry 80 percent of the total energy. This energy is harvested for electricity production via the vessel blanket. The alpha-particles or the produced helium ash, still carry 20 percent of the fusion energy and will be diverted into the ashtray, the divertor, of fusion experiments.