Plasma takes on donut shape - PRL article by Casimir PhD student Chris Berg Smiet, Leiden PI Dirk Bouwmeester and others


Leiden physicists have discovered that a free, magnetised plasma reconfigures into a stable shape in certain conditions. They demonstrate through computer simulations that initially helical magnetic field lines in a plasma eventually take the form of a donut.

The sun holds a great promise for solving the global energy problem. Aside from developing efficient solar panels, scientists are trying hard to fully understand the processes that happen inside our host star. They are eager to learn about one of the sun’s special tricks – nuclear fusion. With a fusion reactor on Earth we would be able to generate nearly unlimited amounts of energy. But to start up an efficient fusion process, we have to peek very carefully, with special emphasis on plasma’s behaviour within the complex magnetic fields inside stars.

Up until now, no description existed of the reconfiguration of helical magnetic field lines within a free plasma. Now scientists at the Leiden Institute of Physics (LION) have created a computer simulation which shows that plasma, starting from different helical configurations, end up in a toroidal shape. They take the parameters from this simulation to formulate a mathematical description of the field lines. The fact that a donut shape is the end product of different initial configurations, shows that this form is a fundamental structure of a free plasma.

Another indication towards that conclusion is that a torus is not just the minimum energy state, into which structures usually degrade, but a delicate balance between the present magnetic forces and the internal pressure within the plasma. Normally it is assumed that a magnetic configuration degrades into a form without forces acting on it, like a weak pudding collapsing without a mould. The Leiden finding contradicts the so-called Taylor’s theorem.

In the peaceful state of equilibrium, magnetic islands appear on the torus’ surface. ‘One of the issues with nuclear fusion is the emergence of small islands in the plasma,’ says lead author Christopher Berg Smiet, PhD student of prof. Dirk Bouwmeester at LION. ‘They fly outwards and damage the reactor wall. In our simulation we see stable islands that don’t do this. We’re talking about a different situation, because our plasma is not captured inside a reactor, but it does give us more insight into plasma’s behaviour.’

This insight is also important for our fundamental understanding of astrophysical processes. Smiet: ‘There are countless of helical magnetic plasma structures in the sun, which sometimes fly off into space. Now we know that these turn into donuts after a while. This is something we had never expected beforehand.’

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