A new solution to one of the major problems of fusion research
Left: Cross section of the separatrix of the high (solid) and low (dashed) lHFS→LFS time points. lHFS→LFS is the length of a field line spanning from the LFS to the HFS midplane. A poloidal projection of lHFS→LFS is depicted with the red arrow. Right: Temporal evolution of lHFS→LFS and the safety factor at 95% flux throughout the three discharges. Credit: Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.165001

The walls of fusion devices can be melted. Researchers from the IPP and the Vienna University of Technology came up with a way to get them under control. Their work is in a journal.

Nuclear fusion power plants could be used to solve our energy problems. This method of energy generation, which mimics processes on the sun, is being researched around the world. The principle to work on Earth requires the plasmas to be heated to 100 million degrees Celsius. The wall of the reactor is protected from melting by the magnetic fields. The plasma edge is very well insulated, so this only works. In this region, there are often edge local modes. The wall of the reactor could be damaged by energetic particles from the plasma.

There is an operating mode for fusion reactor that avoids this problem, according to researchers from the IPP in Garching. There are many small instabilities that do not pose a problem for the reactor's wall.

The research group leader at IPP in Garching, Germany, says that their work represents a breakthrough in understanding the occurrence and prevention of largeType IELMs. The most promising scenario is the operation regime we propose. The results have been published.

There was a renaissance of a disregarded mode of operations.

Ultra-hot particles move quickly in a fusion reactor. The particles are not able to hit the reactor wall with destructive force because of the powerful magnetic coil. Friedrich Aumayr is a professor of Ion & Plasma Physics at the Institute of Applied Physics.

The motion of the particles is dependent on a number of factors. Different regimes of operation are possible depending on how one chooses to use these parameters. A long-standing collaboration between the TU Vienna group of Friedrich Aumayr and the IPP Garching group led to an operating regime that can prevent the destructive type-I ELMs.

Experiments have shown a recipe against the dangerous Type-I ELMs, in which the plasma is slightly deformed by the magnetic coil, so that its cross-section is no longer elliptical. The density of theplasma is increased at the edge.

"At first, this was thought to be a scenario that only occurs in currently running smaller machines such as ASDEX Upgrade at IPP in Garching and is irrelevant for a reactor," says Lidija radovanovic, who is currently working on her PhD. New experiments and simulations have shown that the regime can prevent dangerous instabilities.

It was like a pot with a lid.

Many small instabilities occur due to the triangular shape of the plasma and the injection of additional particles at the edge. The small particle bursts hit the wall of the reactor faster than it can heat up and cool down again. Individual instabilities don't play a major role for the reactor wall. The mini-instabilities prevent the large instabilities from causing damage.

The water starts to boil, like a cooking pot with a lid. The lid will lift and rattle if the pressure builds up. If you tilt the lid slightly, steam can escape and the lid can remain stable. The ASDEX upgrade reactor in Garching is one of a number of fusion plants that can be implemented with this regime.

The research was carried out within the EU project EUROfusion.

More information: G. F. Harrer, et al. A quasi-continuous exhaust scenario for a fusion reactor: the renaissance of small edge localized modes, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.165001 Journal information: Physical Review Letters