ITER researchers requested their colleagues operating UK’s tokamak JET to expose the machine to extreme loads to find the best materials for ITER’s plasma facing wall.
“There was a slight worry we would see uncontrolled, firework-like splashes of molten metal and that might affect subsequent experiments,” said Dr Gilles Arnoux, one of the scientific coordinators of the experiment. “But it was a smooth melt; the plasma didn’t seem to notice. I was surprised at how little impact it had.”
The tests at JET involved subjecting a small area of a deliberately misaligned tungsten wall tile to regulated bursts of turbulent plasma. During the experiment, the exposed tile was heated up to peak temperatures of 3422° C, to allow the researchers assessing what affect molten tungsten might have on the plasma.
The team’s biggest concern was the melt event could result in contamination of the hydrogen-based plasma with tungsten, which would cause disruption and uncontrolled energy dump of the plasma. Such situation would lead to further melting of the surface, which might become detrimental to the equipment's safety.
However, the molten material moved smoothly to one end of the tile during the experiment, forming a droplet that grew with each additional plasma pulse. Surprisingly, the droplet did not run downwards, but moved away from the hottest part of the plasma instead, due to magnetic forces inside the tokamak.
“It has been a great success and has achieved what it set out to do,” said Dr Richard Pitts, the leader of ITER’s Divertor and Plasma Wall Interactions section, who joined the JET team during the experiment.
“It seems that we can broadly understand what we have seen on the basis of complex computer simulations describing the melt dynamics and thus our confidence is increased in the extrapolations we make for the behaviour to expect on ITER,” he said, adding the results are extremely significant for the ITER team, currently considering the use of tungsten for the construction of ITER.
To achieve fusion inside a tokamak, gas needs to be heated to more than one hundred million ° C, at which temperature it turns into sun-like plasma. Of particular concern to researchers are bursts of turbulence on the edge of the plasma similar to solar flares, which can momentarily inflict heat loads far greater than a blow torch on small areas of the tokamak wall.
The experiments at JET, which is currently the world’s biggest operating tokamak, will continue until mid-2014.