An account of JET’s experiments, which helped ITER make a design decision that offers substantial cost savings.

There were a few frayed nerves in the JET control room in August, as experiments to deliberately melt tungsten tiles in the divertor got underway. These apparent acts of scientific vandalism were actually courageous experiments which have helped ITER make a decision that substantially reduces investment costs.

Physicist Gilles Arnoux keeps a close eye on the JET fusion energy experiment (Picture: EFDA)

Physicist Gilles Arnoux keeps a close eye on the JET fusion energy experiment (Picture: EFDA)

The tests were requested by ITER to support its assessment of what material should be used for its plasma-facing wall. Although tungsten is a front-runner, there is a concern about the effect that the melting of a tungsten tile might have on the plasma. Therefore, the original ITER design plan foresaw a more forgiving carbon wall for the run-in phase of the machine with a change to tungsten at a later stage.

“In earlier melt experiments in ASDEX Upgrade, molten metal was sprayed around, and it became hard to operate because the plasma disrupted a lot,” said Guy Matthews, who leads the JET ITER-Like Wall project. “But here nothing catastrophic happened, it’s quite well behaved!” Gilles Arnoux, one of the scientific coordinators of the experiment agreed. “It was a smooth melt; the plasma didn’t seem to notice. I was surprised at how little impact it had.”

Melting under ITER conditions

The difference between these JET experiments and previous ones at ADEX Upgrade is that the melting was achieved by the transient bursts of turbulence rather than by bulk melting. Transient bursts are caused by certain plasma instabilities called Edge Localised Modes or ELMs. They raise the temperature above the melting point for milliseconds only, and so the melting behaviour is harder to predict. JET is the only tokamak in the world which has enough energy in its plasma to melt tungsten with ELMs, thereby modelling conditions in ITER, in which ELMs could potentially carry a fearsome amount of energy.

The tests at JET involved subjecting a small area of a deliberately misaligned tungsten wall tile to regulated bursts of turbulent events. The peak temperature of the tile during the transient bursts was slowly increased until it exceeded tungsten’s melting point, 3422 degrees Celsius. The aim was to assess what effect molten tungsten might have on the operation of the plasma. In particular, it was feared that a melt event might contaminate the hydrogen-based plasma with tungsten and lead to a disruption – an uncontrolled energy dump from the plasma – which could cause further surface melting in a fusion experiment as large as ITER.
Instead, as shown in the picture, the molten tungsten moved smoothly to one end of the tile and formed a droplet that grew with each additional plasma pulse. Curiously the molten metal did not run downwards – a result of the magnetic forces inside the tokamak – and, to the scientists’ relief, moved away from the hottest part of the plasma rather than being swept back into the exposed area. Subsequent experiments were performed without any interruption in the proceedings.

Please click the picture to see the molten tungsten move and form a droplet

Please click the picture to see the molten tungsten move and form a droplet

Joining the JET team in the control room was the leader of ITER’s Divertor and Plasma Wall Interactions section, Dr Richard Pitts, who has been involved throughout the planning of the experiment. He said: “It has been a great success and has achieved what it set out to do: to demonstrate that repetitive, fast transient heat pulses pushing tungsten over the melt threshold for just one or two milliseconds each time.” He continued: “ they do not drive melt splashing nor do they appear to have any observable effect on the core plasma. 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, which use the same computer codes.”

Two months after these first experiments on JET, the ITER Council Science and Technology Advisory Committee proposed to the ITER Council, the highest committee in the ITER Organization, to equip the ITER divertor with tungsten targets from the very start of operations. In November, this proposal was approved and the ITER Council decided to commence ITER operation a full tungsten divertor.

Phil Dooley, EFDA