Published in the scientific journal Nuclear Fusion, the work could herald a breakthrough for magnetic confinement fusion. Tungsten, with its high melting point, is used in the inner walls of tokamaks. But when hot plasma interacts with those metallic walls, the tungsten can contaminate and dilute the plasma, taking space in the plasma that could be occupied by the fusion fuels, resulting in fewer fusion reactions.
The EUROfusion team found that by creating a particular type of ‘heat barrier’ at the edge of the plasma just a few centimetres thick - in the form of a large temperature drop of 20 million degrees Celsius - the tungsten contaminants are prevented from entering the core of plasma.
"Our measurements showed that we are one step closer to solving one of the greatest scientific quests of our time,” said Dr Anthony Field, senior fusion researcher at UK Atomic Energy Authority (UKAEA).
“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."
According to the researchers, this method of retaining a ‘clean’ plasma was hypothesised before recently being demonstrated by scientists at JET. The technique was trialled as part of a series of experiments contributing to JET breaking the world-record in sustained fusion energy last year.
"This is a key result that was predicted by theory, which we’ve never observed before,” said Dr Joëlle Mailloux, JET task force leader at UKAEA. “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."