How to contain a plasma of 150 million degrees in a rolled donut

Physicists are cool, but that cool!? That’s what this reporter thought when the young nuclear physicist’s Instagram account had almost 27,000 followers on Instagram. The explanation? The nuclear physicist became famous in 2009 when he won the Junior Eurovision Song Contest with the song Click clack.

As a child, Ralf Mackenbach watched while his parents thought he was sleeping Cosmos, a TV docuseries about the universe with American astronomer Carl Sagan. Then, between musical performances, his fire for physics was sparked.

At the end of November, Mackenbach (28) defended his dissertation on nuclear fusion at TU Eindhoven. Cum laude. In a small, white, fluorescent-lit room at the university, he talks about his research over the past four years while leafing through his thick dissertation. What Mackenbach likes about nuclear fusion is that it is “diehard physics”. Nuclear fusion is about plasmas: flowing ‘liquids’ that respond to electromagnetic fields. And he also likes that his research has a clear social purpose, a sustainable energy source for the future.

The sun shines thanks to nuclear fusion. At the extreme pressure and temperature in the interior of this star, nuclei of lighter atoms fuse, creating new, heavier atoms: for example, four hydrogen atoms create helium. This releases an enormous amount of energy. On Earth, physicists try to imitate this energy source in reactors. Promisingly, a nuclear fusion reaction does not produce greenhouse gases. Nuclear fusion also produces less radioactive waste than nuclear fission, which gets its energy from splitting heavier atomic nuclei into lighter elements.

We heat the center of the donut to 150 million degrees Celsius. That can be done with microwaves

US climate envoy John Kerry called nuclear fusion a “revolution for the world” at the climate summit in Dubai. Is that right?

“We have to tackle the climate problem in 2050 net zero CO₂emissions. I don’t think nuclear fusion will mean much in such a short period of time. There is currently no reactor anywhere that generates more energy than you have to put into it. We also need to build an infrastructure for nuclear fusion. But in the long term, nuclear fusion is very interesting. The world population is growing and in more and more countries people are demanding more and more energy. Nuclear fusion can play a role in this growing energy demand.”

Also read
‘Some nuclear fusion companies sell flying carpets’

The ITER site in an aerial photo from September. This European fusion reactor, on a site northeast of Marseille, is still under construction.” class=”dmt-article-suggestion__image” src=”https://images.nrc.nl/dUXBul8ECzWyMAVsNtAsPrEwOeg=/160×96/smart/filters:no_upscale()/s3/static.nrc.nl/images/gn4/stripped/data109231149-aa3c99.jpg”/>

How does it work, fusing atomic nuclei on Earth?

“We do that in a fusion reactor: a kind of large, metal donut with cables around it. You could squat in the donut I visited in Germany; it is about half a meter high. We heat the center of the donut to 150 million degrees Celsius. This can be done with microwaves that a microwave also uses to heat food. We put the light elements deuterium and tritium in the donut, and by heating a glowing hot plasma is formed. The temperature is high enough for the atoms to collide hard and ultimately fuse.”

It takes a lot of computing power to calculate exactly which vortices will arise in a particular plasma

Mackenbach is investigating how heat leaks from the donut to the outer walls. That is one of the major, complex problems that must be solved to make nuclear fusion an efficient energy source.

“You don’t want that heat to travel quickly to the walls of the donut; then they would melt. We try to keep the plasma on the inside with magnetic fields – to which the plasma sticks like a string of beads. By the way, those magnetic lines explain the donut shape. If the magnetic line were straight, it would collide with a wall somewhere, and that’s where you lose the hot plasma. We generate the magnetic fields with coils that are on the outside of the donut. The temperature of the coils is just above absolute zero (-273°C). This gives you a huge temperature difference in the donut. And a temperature difference wants to equalize itself. Just like when you put cold milk in hot tea. Then you will see vortices forming with which the cold milk and hot tea mix. This also happens with the plasma in the reactor. And you want to get rid of that. These vortices cause the heat to leak away, making it more difficult to keep the fusion reactions going.”

How did you investigate those swirls?

“Swirls are chaotic. It takes a lot of computing power to calculate exactly which vortices will arise in a particular plasma. Our idea is: we’re not even going to try to calculate that. For my research I wondered: can you calculate the maximum amount of energy that can be contained in all those vortices in a given plasma? And the answer is yes, and I set up the formula. It was reassuring that, if according to the formula there was a lot of energy in the vortices, and therefore a lot of heat could leak away, according to computer simulations a lot of heat actually leaked into that plasma setup.”

How does that knowledge help nuclear fusion on Earth?

“You can use the formula to make a guesswork calculation to predict, for example, in which plasma donut shape the least heat energy is lost through vortices. This is not about the shell of the fusion reactor, but the shape of the path that the plasma takes along the magnetic lines. You control that shape with the coils.”

It’s great fun to find a balance between telling the truth and explaining something in an understandable way

Around his neck, Mackenbach wears a silver chain with a charm that looks like a twisted donut the size of a euro coin. It is the shape of the plasma in a fusion reactor in Germany.

And? Which form is the most efficient?

“We don’t know exactly yet. It depends on several factors: exactly how big do you want to make the reactor and how exactly do you heat the plasma? However, the optimal shapes we have found so far resemble real reactors: a twisted donut like in Germany seems to do well.”

Where does your future lie?

“I never plan far ahead. But I am sure that I enjoy doing research the most. In the new year I will go to Switzerland for my postdoc to continue with donut shapes.”

You gave a presentation for the University of the Netherlands about nuclear fusion. Would you like to talk about science on stage as Carl Sagan?

“Yes! It’s great fun to find a balance between telling the truth and explaining something understandably to a wide audience. I also know – through my experience in music – how to hold attention. You don’t want to send too much information per minute; you also just want to make it fun. During the performance you don’t always talk for twenty minutes between two songs.”

To share





Email the editor