A snowflake might not be the first thing you associate with a hot fusion plasma. But it is a concept designed to handle the heat where the plasma touches the vessel wall. It is not surprising that this task presents technical difficulties. However, the scale of the problem is remarkable: the predicted heat load on the ITER targets is greater than that on the soil beneath a launching rocket!

PROBLEMS AT THE EDGE

Two of the biggest scientific and technological challenges facing ITER and DEMO are associated with plasma-wall interaction. Firstly, how to minimise the heat load on the target plates. Secondly, how to prevent impurity particles from entering the core plasma and causing heat loss. For this reason, such troublesome particles should be kept to a finite, well-defined area.

BEYOND THE LIMITED CONFIGURATION

Early tokamaks achieved this aim using a limiter – typically a rail extending a short way inwards from the inner wall of the tokamak. In this configuration, it is relatively easy for the impurity particles to re-enter the core plasma. A solution was proposed as early as in the 1950s, split the flux surfaces at a certain point. The field lines will cross each other and end at two divertor plates, some distance away from the last closed flux surface, keeping the core plasma “safe and clean”. However, it wasn’t until the 1980s that this more complex configuration started to be used in fusion devices.

Limited vs diverted configuration. The limited configuration is shown on the left, with the limiter itself in blue, last closed flux surface in bold red and flux surfaces in red. On the right is the diverted configuration, with the target in blue and magnetic surfaces as before. Snowflake flux surfaces on TCV.

Limited vs diverted configuration. The limited configuration is shown on the left, with the limiter itself in blue, last closed flux surface in bold red and flux surfaces in red. On the right is the diverted configuration, with the target in blue and magnetic surfaces as before. Snowflake flux surfaces on TCV.

DETACHING THE PROBLEM

The diverted configuration also tackles the tremendous heat load problem. No material would be able to withstand such harsh conditions. The greater distance between the target and the flux surface allows density and temperature gradients to form along the magnetic field lines, thus reducing the temperature at the target to well below that in the core. The heat load can also be reduced by either spreading the same total heating power over a greater plate area, or by radiating more heat before it reaches the target. This requires a strongly radiating “cushion” of dense neutral gas between the target and X point. It remains very difficult to simultaneously reach the detached regime for the targets and maintain the high-confinement mode, which optimises the core plasma performance.

ADVANCED DIVERTORS

ITER must operate in both high confinement mode and the detached regime. The challenge is to maintain the detachment front in a stable way. It is here that the shape of the flux surfaces close to the target becomes important. So, we need a new concept: advanced divertors. These are magnetic configurations in which there is not one but two magnetic X points. The second X point modifies the angle at which the field lines arrive at the target as well as the change in separation between the flux surfaces as they approach the target. Current experiments are trying to find the magnetic field which provides the most effective heat load reduction.

IT’S JUST THE BEGINNING

One such advanced configuration is called the “snowflake”. It is named after its 6-fold symmetry, achieved by a secondary X point close to the primary X point. Researchers have discovered that this reduces the heat drastically. The area that receives the heat becomes much larger. Also, the distance along a field line from the X-point to the target is now longer, allowing a much greater drop in temperature along the line. It is clear that we need to understand the effect of divertor geometry on the heat load. Theoretical modelling of diverted geometries is just beginning, but already there are hints that first principles models can recover results of experiments such as those carried out on Tokamak à configuration variable (TCV). The Swiss Tokamak à configuration variable (TCV) from above. So, watch this space!

DIVERTOR

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A divertor is the in-built vacuum cleaner of a fusion reactor and is situated along the chamber floor. Build-ups of helium ash and impurities in the plasma must be removed during operation. These heavier particles are pushed to the edge of the plasma by centrifugal forces, where they escape through a specially designed magnetic “gap” at the bottom of the plasma and fall into the divertor. The divertor shape and materials are also constructed to bear the brunt of the heat load from the plasma, thus protecting the surrounding walls.

Clear

authorbox_Carrie-BeadleI am a PhD student studying plasma turbulence in the outermost region of the tokamak via numerical simulation. I find fusion plasma physics exciting because it has so many different aspects, challenges and problems to be solved! My article is about the problem of overheating materials where they interact with the plasma and how we can change the magnetic field configuration to keep vessel walls from melting.

Carrie Beadle (23) from U.K. is currently based at: Swiss Plasma Center, Lausanne. (Picture: private)