New insight into how plasma heats up could help optimise fusion reactions

A new theory about how plasma works could move scientists closer to the goal of emission-free fusion energy.

Plasma is the fourth state of matter, consisting of atoms that have been broken apart so that what remains is a cloud of ions and free electrons, all electrically charged. The extreme density and heat needed to create plasmas are found both in space, for example in stars and giant planets, and in laboratories on Earth.

Producing energy from fusion would have such a profound effect on humankind that anything we can do to improve our understanding of even a small part of the process is important. Dr Stuart Mangles

One of the reasons why scientists create plasmas is to fuel fusion reactions, which are a potentially huge source of carbon-free energy. However, fusion reactions are difficult to control, meaning more energy is used to create and sustain them than is produced.

Now, a team of physicists from Imperial College London have discovered a new way plasma exchanges energy between the ions and electrons in the early stages of heating up. The mechanism, published today in Physical Review Letters, could help improve the control of fusion reactions.

Co-author Dr Stuart Mangles, from the Department of Physics at Imperial, said: “Producing energy from fusion would have such a profound effect on humankind that anything we can do to improve our understanding of even a small part of the process is important.

“Our new model shows that when the charge of an ion in a plasma fluctuates this can drive energy from the electrons to the ions or vice versa, and could help explain how plasmas heat up in the early stages of fusion experiments.”

Exchanging energy

During fusion reactions driven by lasers, the plasma goes through a phase called ‘warm dense matter’, where it is very dense but not yet very hot (in relative terms – it’s still around 10,000°C). This phase, as it is heating up, is poorly understood.

The main question surrounds how energy is exchanged between the ions and the electrons. The energy supplied to the plasma is not shared equally between the two, but both need to achieve the same temperature in order to heat up effectively. Therefore, they must exchange energy, but previous attempts to explain how this happens have relied on mechanisms that would be too slow to account for the heating observed.

The Imperial team have suggested an alternative model of energy exchange that works much faster. In a plasma, because ions are charged particles, they are repelled by each other. However, when an electron and ion collide, they can briefly re-form an atom that has no charge, allowing any nearby ions to approach the atom.

Then, when a further collision breaks apart the neutral atom, the recreation of a charged ion pushes the other ions away again, providing them with energy. In this way, energy can be transferred from electrons to ions, and the process can also run in reverse, transferring energy from ions to electrons.

Co-author Dr Rory Baggott, from the Department of Physics at Imperial, said: “Previous experiments have measured vastly different heating rates for plasmas where existing mechanisms predict the rate should be the same. The new mechanism behaves differently because it depends on atoms re-forming, so it can explain a diversity of measurements in a way that wasn't previously possible.”

The team are now planning to test the mechanism using the Gemini laser system at the Science and Technology Facilities Council’s Central Laser Facility, where extremely short flashes of x-rays will be able to measure the temperature of the ions and electrons as they exchange energy. 

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‘Temperature Equilibration Due to Charge State Fluctuations in Dense Plasmas’ by R. A. Baggott, S. J. Rose, and S. P. D. Mangles is published in Physical Review Letters.