Nuclear fusion with a twist

Ralf Mackenbach’s eyes light up when he talks about the sun. For the physicist, the sun represents more than just vacations, freedom, and the good life - which are all great things, of course - but how does the big yellow ball get its energy? He patiently explains the basic principles of nuclear fusion.

The sun has a gigantic mass and because of the immense gravity in its core, it can compress and heat everything to around 15 million degrees Celsius. Under these conditions, hydrogen atoms, among other things, can fuse together to create new elements; a process that releases enormous amounts of energy and heat. The sun has been performing nuclear fusion for billions of years. But when it comes to replicating this energy process on Earth, we run into one challenge after another.”

Twisty donut

Because three main criteria are required to create a ‘sun in a box’. Mackenbach lists them: you need many particles close together, it needs to be hot enough, and it needs to be sufficiently insulating. Several types of nuclear fusion reactors are currently being developed - two of the important ones are the so-called tokamak and stellarator - in which temperatures can reach up to 150 million degrees Celsius.

But what can withstand such temperatures in the long term? We work with immensely strong magnetic fields, Mackenbach explains. “These magnetic fields are capable of holding the particles in the hot plasma together. To do so, they need to have a special shape, namely a kind of donut, as you can clearly see with the tokamak. The stellarator’s magnetic fields have a more complex shape, it’s more like a twisty donut.”

Mackenbach shows the ring he wears on a chain around his neck. “Wendelstein 7-X to scale, the largest stellarator-type nuclear fusion reactor located at the Max Planck Institute for Plasma Physics in Greifswald, Germany. Throughout my doctoral program, I spent about three months there each year. This way, I always carry my research with me,” he adds with a wink.

“And it’s very useful for explaining my work to friends outside the field. Visualizing it makes a term like “twisty donut” much more understandable.” 

Tea with milk

So with magnetic fields in the right shape, the fusing fuel can stay confined in the reactor. However, plasma is rather unpredictable, which poses quite a few challenges. For example, there is a huge temperature difference.  Whereas it is 150 million degrees Celsius at the center of the donut, the superconducting coils surrounding it operate at a few degrees above zero.

This results in swirls that try to equalize the temperatures. Mackenbach compares it to a hot cup of tea that you pour cold milk into. “Physics in practice. But whereas the swirls in your tea result in a desirable mixture, we actually want to minimize this turbulence in a nuclear fusion reactor to avoid cooling.”

Powerful model

To know which buttons to push to minimize turbulence, you must first be able to describe those swirls for each and every situation. For this purpose, Mackenbach and his colleagues developed a mathematical model with an unconventional approach. Interestingly, he does not calculate the plasma’s behavior at a specific point in time and what the turbulence looks like at that moment, but only looks at packets of energy.

“What we do, essentially, is we determine how much energy we can initially extract from the plasma under a set of minimal conditions. This amount of “available energy” could then tell us something about the amount of energy that could be in the turbulence.”

The seemingly simple theory was initially worked out by Mackenbach with little more than pen and paper, and in a later stage on a simple laptop. In Greifswald, he used supercomputers to examine whether his model holds up under different conditions.

“There was already a model that could calculate this available energy in very specific reactors. Our first step was to expand the model to any reactor. Especially for stellarators, there’s a lot more freedom in terms of shape: where is the twist, at what angle, how many bending points? We found that the available energy in both the tokamak and stellarator can be a good indicator for predicting turbulence. For me, such a simple model aligns very closely with the laws of physics, in which you try to explain as much as possible with as few assumptions as possible.”

Open new doors

In order to minimize swirls in fusion materials, Mackenbach says lots of further research is required. As a theorist, he will not be the one pushing the buttons and turning the knobs; he emphasizes the importance of teamwork.

“Mutual cooperation is essential; the best ideas are born through cross-pollination. I’ve learned that science can be very social, even for physicists.” But the ability to predict swirls opens new doors in this field of research, according to Mackenbach.

Will he stay involved? He says he prefers not to look ahead for more than a year or two. His personal life has also seen turbulent times in the past, for example when he tap-danced his way to victory for the Netherlands in the Junior Eurovision Song Contest.

“Music will always be a part of my life, but for now, I’m happy where I am as a physicist.” He reaches for his necklace. “Soon I will move to Switzerland to further refine my model, a great challenge to bring us one small step closer to fusion energy.”