A third dimension helps Tokamak fusion reactor avoid wall-destroying instability

ARS Technica Chris Lee – 9/13/2018

Currents and fields in the KSTAR Tokamak.

The success of Tokamaks for fusion is a story unto itself, with the toroidal magnetic containers setting records for keeping high-energy plasmas under control, a necessary step for sustaining fusion. The overriding narrative, at least on the scientific side, is that when you have an unstable plasma, it is really hard to build a control system that keeps the plasma hot and confined, even in a Tokamak.

Now, researchers have used the Korean KSTAR Tokamak to show that they can gain control of a particularly nasty plasma instability called an edge localized mode. The instability essentially exhausts the plasma onto the wall, ablating it away. If a plasma reactor the size of ITER were to have an edge localized mode instability, it would likely destroy the inner lining of the vacuum vessel.

Symmetries giveth, symmetries taketh

I don’t pretend to understand plasma instabilities in a Tokamak very well. But I do know that some of the problems are the result of the shape of the magnetic field. The shape of the Tokamak is a boon: it’s symmetric, which makes the device simple, it makes calculations possible, and it offers high confinement.

Unfortunately, that same symmetry is one of the reasons that there are many possible instabilities in the plasma, including edge localized modes.

One way to suppress these instabilities is to break the magnetic field’s symmetry by applying additional magnetic fields around the outside. In fact, because actual instruments can never be perfect, these magnets already exist to correct for any asymmetries in the field.

Early experimental results have suggested that carefully applied asymmetry has some benefits. The problem, it turns out, is not allowing some asymmetry—it’s figuring out which asymmetry to allow. Essentially, most asymmetries will make things worse: they reduce the confinement of the plasma, for instance. You want to pick out the ones that don’t.

Creating a map of treacherous and friendly locations

Instead of guessing which ones are helpful, the researchers built a simplified model of the magnetic fields associated with the plasma. The point (I think) is that the instabilities are magnetic field instabilities, so a model that can take a short cut to calculate the magnetic fields generated by the plasma would be very useful. The second idea is that the inner part of the plasma should (in the beginning) be unaffected by what is happening at the edge, so you can calculate the core and the edge separately, and then force the two calculated magnetic fields to agree at their boundary.

The final point is that the researchers don’t appear to be interested in the final geometry of the field or any spatial information at all. All they need to know is when the size of the magnetic field goes insane enough to set off an instability. Previous research has pointed to what these critical field values are, so it is basically a question of creating a geometry, calculating the field, and making sure that it falls beneath this cutoff for stability.

Then comes the grunt work. KSTAR is designed to have flexibility in the way the magnetic field is generated. That means you need to map out which magnetic fields will suppress instabilities, which fields will prevent plasma from being confined, and which fields will generate instabilities. Thanks to KSTAR’s flexibility, this is a 6D picture, where each pixel is color coded as good, bad, and really bad.

Stable operation

The researchers tested their findings against KSTAR’s operation and could successfully prevent the formation of edge localized modes. They also found that the unstable regions were indeed unstable. In fact, the model is remarkably accurate and shows quite a bit of promise for the future.

The researchers also investigated how KSTAR would fare if there were more magnets available. The additional magnets would allow KSTAR to operate in a wider variety of stable configurations. However, the new configurations require electrical currents that exceed KSTAR’s design limits.

I know that every time we write about fusion, someone somewhere says that fusion will always be 10 years away. But if we separate out the political and managerial problems associated with fusion, there has been remarkable progress in achieving high-confinement plasmas, control systems, and all the other engineering details.

In fact, since the last management shake up (in response to a scathing report), ITER seems to be hitting its construction targets regularly. This gives me some confidence that ITER will succeed in demonstrating fusion with a net energy win. Whether the fusion power is an economic win as well is a whole different story, of course.

Nature Physics, 2018: DOI: 10.1038/s41567-018-0268-8. (About DOIs)