Mastering plasma instabilities for future nuclear fusion reactors

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The optimization of plasma instabilities in a nuclear fusion reactor will pave the way for its use as a clean source of perpetual energy.

Nuclear fusion is often seen as a promising future source of clean energy that could one day eliminate the need for fossil fuels to generate electricity. Although this method of power generation can bring enormous economic and environmental benefits, scientists and engineers have not yet been able to create a single fully functioning fusion reactor despite tireless efforts.

The problem is that for the fusion of two hydrogen nuclei into a single helium nucleus, which releases substantial amounts of energy, a temperature of several million degrees is needed at which matter exists as a plasma. No material can withstand contact with a substance as hot as this plasma, and therefore in a fusion reactor, a magnetic field must be used as a “container”.

This field interacts with charged particles, but can be used to confine a neutral plasma, since the latter is a mixture of positively charged nuclei and negatively charged electrons, which cannot bond together to form neutral atoms because at such extreme temperatures, the particle collisions are so energetic that the atoms decompose immediately.

Ensuring a sufficiently long confinement of the plasma in a limited volume turns out to be a very difficult problem. To solve it, scientists have proposed different types of nuclear fusion reactors, the most popular and studied of them being the tokamak reactor, in which the plasma is donut-shaped.

Plasma instabilities in a tokamak

The complicated dynamics of plasma at extreme temperatures maintained inside a tokamak results in the development of instabilities – in some regions, disturbances to its continuous flow develop rapidly, resulting in particles and bursts of energy. One of the most problematic instabilities are ‘edge-localized modes’, which, as one might guess from their name, form at the confined plasma boundary.

If these instabilities are too large, then they are dangerous for the stable operation of the reactor since the escape of a large quantity of super hot material can destroy its walls and take too much energy from the plasma, disturbing the normal course of the reaction.

To better study and potentially tame these instabilities, a team led by Georg Harrer of the Technical University of Vienna conducted research on the ASDEX Upgrade experimental tokamak at the Max Planck Institute for Plasma Physics in Germany, the results of which were published in the journal Physical examination letters.

Physicists investigated how plasma density and magnetic field configuration affect the size and frequency of localized edge modes. They found that increasing the density and appropriately adjusting the shape of magnetic field lines suppressed large instabilities and increased the frequency of small localized modes.

This result, according to the team, is encouraging because the formation of small modes is useful for the nuclear fusion reaction. Harmless to the walls of the tokamak, they can eliminate helium nuclei from the plasma, thus preventing its contamination by infusible particles.

The properties of the plasma were measured using a filterscope, which captures the visible light generated when the plasma interacts with a divertor – a device in the walls of the tokamak that allows real-time removal of waste from the plasma during the operation of the reactor.

To confirm their conclusions, the scientists performed computer simulations of plasma dynamics in the tokamak, which fully confirmed the accuracy of the results obtained.

Although the ASDEX upgrade is only an experimental facility, the mode of operation of the tokamak developed by the physicists could eventually be used in future reactors. However, as the researchers admit, none of the currently existing devices can fully replicate all the conditions inside a much larger real reactor.

On the other hand, there are many other experimental reactors around the world with different parameters on which the results obtained by physicists can be tested, which can hopefully bring the time of obtaining energy closer to the using a fusion reaction.

Reference: Georg Harrer et al., Quasi-Continuous Escape Scenario for a Fusion Reactor: The Revival of Small Edge Lumped Modes, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.165001

Image credit: John Doyle on Unsplash

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