A stellar stellarator —

Stellarator’s plasma results show a triumph of engineering and modeling

A possible route to fusion makes a very impressive start.

Max Planck Institute for Plasma Physics

Many moons ago, Ars was introduced to the Wendelstein 7-X stellarator (W7-X), an experimental fusion concept. At the time, W7-X—the world's largest stellarator—had just completed some warm-up tests and had been shut down to install more bits and pieces. That installation is not yet complete, but the results from some of those early runs are being analyzed, and they look good. The scientists may not be cracking champagne bottles, but they are certainly drinking boutique beer in celebration of the agreement between theory and experiment.

Banging rocks together to create bigger rocks

All of the elements heavier than hydrogen are the result of fusion. To create a heavier element through fusion, you first strip all the electrons away from two lighter atoms and then force the two nuclei together. That is difficult, because they are both positively charged and repel each other vigorously. But if you succeed in getting the nuclei to bang together, they may stick, creating a heavier nuclei.

In doing so they release energy. That energy powers the Sun, and we hope that local, slightly smaller versions might someday supply electricity.

To get fusion on Earth, we have to heat nuclei to the point where they fly together so fast that they cannot avoid a collision. Then, to increase the chance of these collisions, we have to confine the nuclei in proximity to each other. The high energies provided by the heat and the need for confinement will typically work against one another, making this a serious engineering challenge.

Physicists manage that challenge by confining plasmas in magnetic fields. A plasma is a gas that consists of a mixture of electrons and positively charged ions (atoms that have at least one electron removed). 

Closing a magnetic bottle

A stellarator attempts to provide this high confinement by having a carefully designed set of magnetic fields. The idea is that the ions spiral along magnetic field lines. As long as the field lines have a loop shape, the ions will follow the loop. Unfortunately, the charged ions can shift from field line to field line (through collisions, for instance). On average, the ions travel from areas with high pressure (where there's more of them) to areas with low pressure. The points of low pressure, unfortunately, are also where the magnetic field is weakest; hence, the ions escape from their magnetic prison by being bumped over the lowest point in the wall, as it were.

To fix this, the magnetic field is twisted so that the charged particles spiral inward to high-pressure, high-field locations. This compensates for the drift outward.

The stellarator does this by having the most beautiful set of superconducting magnets that you will ever encounter. That beauty hides an astounding precision: if the magnetic field is not shaped as designed, the plasma will rapidly leak out.

That precision comes about because we have models of how plasmas respond to these magnetic fields, allowing us to design the fields and, ultimately, the magnets that create them. But the only way to fully test whether the models we've made are right is to compare them with observations. And that requires building and installing the magnets.

On our last visit to this topic, the magnets of the W7-X were all in place and the field had been mapped: it was accurate to about one part in 100,000. At present, the plasma hits uncooled graphite tiles when it is exhausted. That means that the total energy in any given pulse should be such that carbon is not evaporated—about four million Joules.  

So, what’s new?

The researchers tested their models of plasma confinement by comparing temperatures and densities for two different magnetic field configurations. For our purposes, the difference in the two configurations is unimportant. Instead, the important factor is that the models do a pretty good job in predicting plasma densities, electron temperature, and ion temperature. 

They also looked at something called the bootstrap current. Essentially, the plasma generates a magnetic field that causes the charged particles to follow a kind of banana-shaped path. Electrons move in the opposite direction to ions, so there should be no net current. However, the plasma also has a pressure difference across the orbit, so the number of particles on the inside track is larger than on the outside track, resulting in an imbalance and a net current. This current degrades the plasma confinement.

The W7-X is optimized to minimize the bootstrap current. In the researchers' experiments, their two magnetic field configurations were shown to suppress the bootstrap current to differing degrees, right in line with their model predictions. And the best results were 3.5 times better than for an equivalent tokamak device.

These results are an important step toward one critical missing component in the stellarator: the diverter. The diverter is the one place in the vacuum chamber where the plasma should hit the wall, rather than studiously avoiding it. It is the exhaust, and it has to cope with extremely high temperatures and particle fluxes. The W7-X will be the first stellarator with a diverter, but no one was (or is) sure if the concept will really work.

The basic principle is that you must have a bunch of islands where the magnetic field terminates or passes through. For a tokamak, that is fine because the field has to terminate somewhere, and the device operates in pulses. The stellarator is supposed to operate continuously, so maybe you only want the diverter in the plasma at certain times, and then you have to walk the plasma onto the wall at the right location. That last is a bit of a guess on my part, though. 

In any case, the important point is that the two different configurations of magnetic field changed where the heat load was placed on the carbon tiles. This indicates that there is every chance of designing an operating procedure where the plasma hits the diverter in a controlled way.

Although not discussed as much, the researchers also showed that the W7-X is on track in a number of other ways: in a six-second run, the number of ions and their temperature quickly stabilized at a constant value. This indicates that the plasma confinement is good, and the goal of continuous operation should be achievable. And, when you use a standard metric of energy and confinement time, the W7-X performs exactly in the middle of the best-performing, similarly sized tokamak devices. The researchers didn’t trumpet this result, but I think the brilliant purple markers on the graph above made their feelings clear.

What next?

Soon, the researchers will start playing with the W7-X with fully lined walls. They’ll change magnetic field configurations, test instruments, and all the other things you need to do to understand a custom instrument. Then comes the sphincter-puckering moment: water. To go to full power, full water cooling has to be incorporated. All the pipes and heat exchangers are in place but not hooked up. There are a lot of pipes and joints, and they are all in a vacuum—if any of them leak, they are looking at a long period of playing a game of find to weld inaccessible pipes. Better they than I.

Nature Physics, 2018, DOI: 10.1038/s41567-018-0141-9

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