Successfully mitigating plasma instabilities

A new technique should protect future fusion devices from damage caused by fast plasma electrons

April 30, 2021

A new method to slow down electrons escaping from the magnetic cage has been developed by a team from the European fusion research consortium EUROfusion, the European JET device, the international ITER experimental reactor and the DIII-D fusion device in the US. The new technique may end up protecting the inner vessel walls of future fusion power plants, the team writes in the physics journal Physical Review Letters. The numerical simulation of the processes and the theoretical explanation of the underlying plasma instabilities were provided by a team around plasma theorists at IPP.

Simulation of the JET plasma at different stages: Shown is the formation of magnetic islands, the momentary dissolution of the magnetic field structure, which disperses and removes the fast electrons over a large area, and after 194 microseconds, the reconstitution of the field.

In order to ignite the fusion fire in a future power plant, the fuel – a low-density, ionised hydrogen plasma – must succeed in being confined in magnetic fields almost without contact and heated to a high temperature of over 100 million degrees. Unfortunately, a whole series of instabilities can develop in the interaction of the charged plasma particles with the confining magnetic field – including, in tokamak-type fusion plants, so-called current disruptions: the electric current flowing in the plasma, which builds up part of the magnetic cage, is then lost within a few milliseconds. Electrons in the plasma can be accelerated and sweep away other, slower electrons like an avalanche, until finally a concentrated high-energy electron beam strikes the inner wall of the plasma vessel. The larger the facility, the stronger the effect. At the international experimental reactor ITER, it would be powerful enough to cause significant damage to the vessel surface.

To calm down this instability and radiate the released energy evenly, heavy atoms such as argon were previously shot into the plasma. In smaller tokamak devices, a second dose of heavy atoms was then sufficient to get rid of the electron runaways. In the large European joint experiment JET in Culham, Great Britain, however, this technique proved less effective; this left an unsolved problem for the even larger ITER.

Based on observations at the US fusion device DIII-D in San Diego, a team of European and US fusion researchers led by Cédric Reux of the French CEA has now been able to show at JET that deuterium atoms injected into the plasma as a second dose can effectively suppress the unwanted fast electrons. The study, in which also scientists from IPP were involved, has now been published in the journal “Physical Review Letters”.

The theoretical explanation for the physical processes in this JET experiment was already published in February in “Plasma Physics and Controlled Fusion”. The international team around IPP scientists Vinodh Kumar Bandaru and Matthias Hölzl succeeded in computationally modelling the termination of the electron beam triggered by the second deuterium injection. The non-linear magnetohydrodynamic simulation agrees well with the observed phenomena and their temporal development. It can also reproduce the spread of the electron beam observed along the circumference of the vessel and thus explain why the dreaded hotspots are avoided. The work therefore supports the expectation of being able to develop an effective beam termination scenario for ITER.

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