How special forces combine to catch plasma killers

THE FACTS

It had all started well. We managed to heatup the reactant in order to make the expected fusion reaction happen: in this case, deuterium, an hydrogen isotope. We initiated a current into the tokamak, that strange doughnut-shaped reactor. Very strong magnetic fields are used to ensure that hot particles don’t touch the inner wall of the tokamak. Finally, we were very pleased when we witnessed that the deuterium started to warm up, became ionized and converted into a gas called plasma. Ion and electron temperatures were increased successfully in the centre of the plasma, up to 150 million Kelvin. Fusion is happening right now. So far so good.

THE DEAD OF THE PLASMA

There are about twenty people glued closely to the screens in the control room. One of us, the pilot, makes the decisions. He or she watches the live footage from inside the tokamak. All of the plasma parameters had been finely tuned before the experiment started. Now the plasma control system manages everything automatically. The pilot is simply in charge of the emergency button to immediately stop the operation in the event the machine may be damaged. Meanwhile, another part of our team is responsible for monitoring the experimental signals from previous experiments. But suddenly, the core temperature starts to drop. This is exactly what we don’t want: a decline in the plasma temperature! This surely would kill the highly desired fusion reaction. Immediately, the system responds and increases the central heating function. But the temperature keeps dropping. Within a couple of seconds, the plasma dies, releasing enough energy to damage the walls. The camera shows a sudden light, like a flash, then it all goes dark. In the control room, nobody speaks. Wordless question marks hang in the air. Something went wrong. What happened?

THE EVIDENCE

Fusion experts like us measure several plasma parameters: current, temperature, density, radiation. In this instance, the radiation emitted by the plasma had increased over time. This is the signature of one suspect: Tungsten, or W. We know where this tungsten comes from, a special area called the divertor. It is designed to receive a lot of energy. is called integrated modelling. It is a very complex and sensitive tool. Simulating just a few seconds of plasma can take days, or weeks, depending on the level of complexity.

More on divertors: see also the article „A snowflake for fusion?“ by Carrie Beadle.

More on Tungsten: see also the article „Making ths reactor wall smarter“ by Janina Schmitz.

HOW A THREE SECONDS SABOTAGE REQUIRES MONTHS OF WORK

As a result, we have to gather our Special Forces, a team made up of experts with various scientific backgrounds in order to finally catch the saboteur and to figure out its modus operandi. This is what fusion and the realisation of fusion energy is about. We, as scientists, are dealing with phenomena that have not yet been investigated. We are pioneers … and, more often, even detectives.

Tungsten is a popular fusion material because of its high temperature resistance and its low erosion rate. Unfortunately, some erosion is still caused by the energy that ends at the divertor. W enters the plasma, but is so heavy it does not get fully ionised. This means that not all the electrons are torn away from the nucleus, and that causes W to radiate. And W is the only species in the tokamak that has this property: it has to be the material used.

The W investigation, allegory, Cartoons: B. Simony

The W investigation, allegory, Cartoons: B. Simony

THE SUSPECT AND THE TIMELINE

So now, we have found the saboteur. And we have to stop it. To prevent this situation from occuring again, we simply must understand how tungsten managed to radiate so much that it made the plasma collapse. We decide to reconstruct the timeline of W impurities, just like in a police investigation. First, we gather together the information we have about the temperature, density and rotation profiles, radiation levels, and especially the radiation distribution in the plasma.

Aha, here is our first important clue: the radiation first appeared at the edge of the plasma. But after a couple of seconds, all of the emitted radiation is derived from the centre of the plasma. This means that W travelled from the edge to the centre. And this is what has caused the core temperature to drop and the plasma to collapse. But how did W travel through the plasma? W was transported, W had accomplices.

PERSONS OF INTEREST AND MODUS OPERANDI

The measurements are necessary to reconstruct W’s time evolution, but not sufficient to figure out the transportation of W. We need to use another tool: simulation. The W transport obeys known mechanisms and equations. Informatic codes have implemented these. The inputs are W‘s accomplices, the Persons of Interest: temperature, density and rotation profiles. The outputs are the coefficients that quantify how far and how fast W is transported. But W also impacts the evolution of temperature and density,and radiation as we have seen before. There are many feedback loops to be simulated if we wish to reconstruct the modus operandi. The combination of several codes is called integrated modelling. It is a very complex and sensitive tool. Simulating just a few seconds of plasma can take days, or weeks, depending on the level of complexity.

HOW A THREE SECONDS SABOTAGE REQUIRES MONTHS OF WORK

As a result, we have to gather our Special Forces, a team made up of experts with various scientific backgrounds in order to finally catch the saboteur and to figure out its modus operandi. This is what fusion and the realisation of fusion energy is about. We, as scientists, are dealing with phenomena that have not yet been investigated. We are pioneers … and, more often, even detectives.

TUNGSTEN

info iconMaterials inside a fusion reactor must be able to operate for a long time under neutron bombardment and hot plasma attack. Tungsten is the most promising material for use as plasma facing components. Tungsten is a robust, rare, metal chosen for its very high melting point (3422 °C), low tritium retention, and low erosion rate. However, even relatively small amounts of eroded tungsten dust are able to poison the plasma, cool it down and cause a disruption, which may result in serious damage to the machine.

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AUTHOR:

authorbox_Sarah-BretonIn four months I will finish my PhD in nuclear fusion (hurrah!). I believe that fusion is capable of solving the energy crisis and I want everyone to know about it. A teacher once told me “If you can’t explain your job in a way that anyone can understand, then you don’t understand it yourself”. Writing about fusion in an accessible and illustrated way is challenging but so fun and fulfilling!

Sarah Breton (26) from France is currently based at: Aix-en-Provence, France. (Picture: private)

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CARTOONIST

authorbox_Benoit-SimonyI just finished my PhD in passive neutron coincidence counting on radioactive waste drums. Although it is not applied to fusion, I am also very interested in it, and more generally, in wider topics of physics. Furthermore, I like to use my passion for drawing in order to illustrate physical phenomena and the life of researchers.

Benoît Simony (26) from France is currently based at: Aix en Provence,
France. (Picture: private)