• ‘Heat barrier’ prevents tungsten from contaminating the plasma.
Scientists from EUROfusion including some from the United Kingdom Atomic Energy Authority (UKAEA) working at UKAEA’s record-breaking Joint European Torus (JET) have discovered a way to boost fusion performance.
JET uses magnetic fields to confine its plasma, a superheated gas of hydrogen isotopes, in the tokamak. Under intense heat – 150 million degrees celsius, ten times hotter than the core of the sun – and pressure, the isotopes fuse into helium, releasing energy as neutrons.
To withstand the intense heat of the fusion process, metals with a high melting point, such as tungsten, are used in the tokamak’s inner walls.
However, tungsten comes with its own Achilles heel – it can contaminate and dilute the plasma. This happens when the hot plasma interacts with the machine’s inner metallic walls.
If tungsten contamination is not controlled, the heavy tungsten impurities can excessively cool the plasma by absorbing heat, which is then lost from the plasma in the form of light. This reduces power from fusion reactions within the plasma.
They have found that by creating a particular type of ‘heat barrier’, like a thin skin on the outside of the plasma, in the form of a large temperature drop of 20 million degrees celsius, the tungsten contaminants are prevented from entering the core of plasma.
This method of retaining a ‘clean’ plasma was hailed as a hypothesis before recently demonstrated by the EUROfusion and UKAEA scientists. The method was trialled as part of a series of experiments contributing to JET breaking the world-record in sustained fusion energy announced last year.
The demonstration is a major boost for international fusion project ITER, the larger and more advanced version of JET, and for potential future fusion powerplants that use magnetic confinement to produce fusion.
Dr Anthony Field, senior fusion researcher at UKAEA (UK):
“Our measurements showed that we are one step closer to solving one of the greatest scientific quests of our time. The plasma can keep itself clear of tungsten contaminants that would cool it, by maintaining a temperature drop of twenty million degrees celsius at the edge of our plasma. This prevents the tungsten ions from stopping us reaching fusion conditions.”
Dr Joëlle Mailloux, JET task force leader at UKAEA (UK):
“This is a key result that was predicted by theory, which we’ve never observed before: under the right conditions, we can have our plasma expel the metallic impurities to its very edge, so they don’t cool the confined plasma where fusion takes place. This is a crucial ingredient for sustained high fusion performance in ITER.”
Dr Alberto Loarte, head of the science division at ITER (France):
“The JET results confirm a long-standing prediction that high-performance plasmas required for fusion power production can shield themselves very effectively from tungsten impurities coming from the wall. This was very challenging to demonstrate experimentally, because you need to approach ITER-like conditions in smaller, present-day tokamaks. The JET scientists have gotten close enough to ITER conditions to see this new effect at play. This provides a robust basis for our predictions for how ITER plasmas will behave.”
Dr Athina Kappatou, scientific co-ordinator of JET experiments from the Max-Planck Institute for Plasma Physics IPP (Germany):
“A key challenge we face when trying to sustain high fusion performance in JET is keeping out metallic impurities that originate from the metal walls of the device. We were aware that theory predicted how ITER plasmas could use a steep temperature drop to keep these impurities out, but this had not yet been demonstrated in a current device. It was very exciting to see that we could approach ITER conditions close enough to demonstrate this effect in JET plasmas and benefit from it in our quest for high fusion performance.”
The research has been published by IOPScience. They are conclusions of deuterium-deuterium experiments conducted by JET in 2020 and in preparation for the deuterium-tritium experiments in 2021.
The landmark JET results announced in February 2022, demonstrated a record-breaking 59 megajoules of sustained fusion energy and was achieved by the EUROfusion consortium of experts, students and staff from across Europe, co-funded by the European Commission.
In-depth analyses of the results from the series of experiments conducted when JET broke the world-record in sustained fusion will be released at the IAEA’s Fusion Energy Conference later this year.
Fusion is the process that powers the Sun and promises to provide a near-limitless source of clean energy.
Fusion could be transformative for energy security and is important in the fight against climate change. It has the potential to provide ‘baseload’ power, complementing renewable and other low carbon energy sources as a share of many countries’ energy portfolios.
