Runaway electrons are the most dangerous particles in fusion plasma. What is it that forces them to speed up like world record sprinters and how can they be stopped? The fusion scientists of the EUROfusion Medium Size Tokamak (MST) Task Force are working hard on this during the current experimental campaign.

The most dangerous event

A gas with a temperature of several tens of millions degrees is used to fuel fusion experiments. At these temperatures, negatively charged electrons in the atoms are able to obtain sufficient energy to be freed from their bindings with their parenting nuclei – leaving them behind as positively charged ions. In fusion plasmas, hot electrons and ions move in random directions frequently colliding against each other. The process seems as chaotic as a bunch of flies in a box.

Scientists want these motions and collisions to take place since nuclear fusion reaction occurs only when two light nuclei (ions) come close to one another at a speed fast enough to overcome their Coulomb (charge) repulsion, allowing the nuclei to fuse. But what happens to the lonely electrons? Usually they move around in the plasma, collide with other particles and cause no harm. But one of the scariest moments during fusion experiments occurs when electrons are able to change their motion and beginning to rush unstoppably on their race course. When this happens, they reach a velocity almost as fast as the speed of light and carry a significant amount of plasma electrical current. Under these conditions they are referred to as runaway electrons and may cause major damage to the tokamak.

Beams as a result of disruptions

Piero Martin, together with his fellow MST1 Task Force leaders, is in charge of an international experimental campaign at EUROfusion’s MSTs ASDEX-Upgrade and Tokamak à Configuration Variable, where strong emphasis is devoted to the study of runaway electrons. A MST1 team led by scientific coordinators Joan Decker, Basilio Esposito, Marco Gobbin, Gergely Papp and Gabriella Pautasso is currently examining obstacles for dashing electrons which would effectively stop them from creating dangerous electron beams. The particles are able to literally run away when disruptions happen during a fusion experiment. A disruption is a very sudden termination of the plasma which can be caused by plasma instabilities.
“To make it clear: we know how to run tokamaks without disruptions – they won’t have the highest performance though as required for ITER target regime for Q=10. So disruptions are associated to the search for high performance”, says Martin.
However, the physics of disruption is not yet fully understood. Disruptions and runaways are the riskiest events in a fusion experiment. Talking about risk in fusion means the risk of damaging the tokamak, which costs money and time. “Since we want to learn how to run a fusion reactor in an efficient and cost-effective way, the EUROfusion programme undertakes high priority experimental and theoretical efforts to understand how to avoid them or mitigate their effects. We – in the MST task force – are taking this commitment very seriously”,
states Martin.

Magnetic fields as race tracks

Chasing the evil electrons

Cartoon of a runaway electron. (Image: EUROfusion)

Image: EUROfusion

“It can occur, that the drag force of the collisions becomes smaller than the driving force due to the intense electric field present when plasma collapses”, says Piero Martin. Being finally unstoppable, a large fraction of plasma electrons feels a need to reach the finish line as  soon as possible. Just like Usain Bolt, regarded as the fastest man in history, at the beginning of a very straight and empty track. The runaway electrons will accelerate along the magnetic field lines which act as tracks for them, to a velocity close to the speed of light. The resulting high power beam then circulates like a strong ring through the doughnut-shaped vessel of the fusion experiment. If it stays focused and hits a plasma facing component it may cause major damage, such as melting or even destroying materials.

Flooding the race course with people

As Piero Martin explains, there are two ways to stop the electrons from speeding. One way is the injection of a noble gas such as argon or neon when the beam materialises. The large atoms of the additional gas in the plasma become great obstacles for the little sprinting electrons. “It is like flooding the race course of a speeding Usain Bolt with hundreds of people. He will bump into them and unavoidably stop. Just like what happens to us when we are late and try to run across a station trying to catch a train … the crowd will slow us down”, explains Piero Martin. “This is what we call the Massive Gas Injection mitigation experiment, and the preliminary results of the experiments in ASDEX-Upgrade are very encouraging”.
Another solution designed to put the brakes on the evil runaways would be to bend their path to increase their unfocused losses. With the help of an additional magnetic field – produced with the same coils used for ELM mitigation (see the article ‘Keeping it on the boil: Three tokamaks and one stellarator’) – the electrons are forced to zigzag through the plasma, and are no longer able to create their focused beam. As a result, the beam is defocused and no longer dangerous.

The crash test for the tokamak

Like a crash test on a newly developed car, the experts have provoked such dangerous events in order to examine them properly. As of last year, they are able to trigger those electron beams
under safe conditions in ASDEX-Upgrade. So, the scientists study mitigation techniques without damaging the machine. Besides ASDEX-Upgrade, TCV and JET, also devices such as COMPASS,
FTU and RFX are used for studying the problem and a significant modeling effort is undergoing in several labs. The Task Force Leader reveals what is behind all the sprinting research:
“What we are in fact working on is an airbag for disruptions and run-away beams in ITER, that protects our investment in the device.We don’t ever want these things to happen, and we plan carefully to avoid them. But if they do, then we will know how to deal with them.”