Fusion scientists gathered in Aachen, Germany, from 21 to 25 May discuss JET’s behaviour with its new ITER-Like-Wall. Their verdict: A vessel wall made of beryllium and tungsten will work for ITER.

The 20th International Conference on Plasma Surface Interactions, hosted by Forschungszentrum Jülich, was the first occasion to examine in depth the much-awaited results from JET’s new ITER-Like-Wall (ILW). Project leader Guy Matthews from CCFE was the conference’s first speaker and provided an overview of the many encouraging insights gained since operations started in August 2011.


Plasma in JET with the ITER-Like-Wall (Picture: EFDA)

Smooth plasma formation

Scientists were surprised how easy it was to generate a plasma with the new metal wall. While the carbon vessel needed several attempts to create the first plasma after a long shutdown, the ILW-machine instantly produced a 15 seconds long plasma pulse with a respectable current of one megaampere. Even after disruptions, which are rapid events during which the plasma loses all its energy, the following JET pulse would start without hesitation. This had often not been possible with the carbon wall where a number of pulses could be wasted and so it is good news for ITER, where precious experimental time will be shared among many more scientists than at JET.

Low fuel loss

The tendency of carbon to absorb tritium is the main argument for choosing metal walls for the second, tritium-deuterium phase of ITER and for future fusion power plants. Tritium is both valuable and radioactive and must not be allowed to accumulate in the vessel wall for reasons of safety and economics. Comparisons of matched plasma scenarios carried out with the previous JET carbon wall and the ILW show that about ten times less tritium is retained in the ILW. The results so far meet the predictions from studies carried out in advance but we will only know the full story when dedicated wall tiles are removed for analysis later this year.

Clean plasmas

Analysis of these wall tiles will also give insights into the erosion of tungsten and beryllium. These materials contaminate the plasma when the vessel wall gets eroded by the heat. Tungsten atoms absorb much more plasma energy than carbon and are therefore a concern. The observed tungsten influxes behave just as expected but fortunately the experiments also show that techniques developed at ASDEX Upgrade can keep the tungsten levels sufficiently low in JET as well.

Disruptions under control

With the ILW, disruptions impose much higher heat loads on the wall. The reason lies in the fact that carbon impurities in the plasma strongly radiate its heat away, whereas beryllium radiates much less leaving much more to heat the wall during a disruption. Although the heat loads risk melting the new tiles, a disruption mitigation system based on fast injection of argon and deuterium gas at high pressure has proven an effective way to quench a disruption – rather like a bucket of water on a fire. The JET results are important for ITER, which will need such a system.

Successful wall protection

The JET upgrade was quite daring in the sense that it installed a metal wall, which does not resist heat as well as carbon, and at the same time upgraded the plasma heating systems. In other words the new machine is operating with a more sensitive wall in more severe conditions. Therefore considerable effort has gone into designing tiles to maximise their power handling capabilities. Also a wall protection scheme has been devised in which an automated CCD-Camera system surveys the wall temperature and adjusts the plasma position or even terminates the plasma pulse, if necessary. The system works reliably – so far, the wall is undamaged – and allows the scientists to take the machine closer to its limits so that the plasma conditions can be pushed towards those in ITER.

ITER scenarios achieved

Good progress has been made in establishing the plasma scenarios ITER will need with the new JET wall but there have been some surprises and there is more work to do. The power required to obtain the high energy confinement plasmas that ITER needs is lower than expected which is good news. There has however been a need to learn how to access the highest plasma performance whilst keeping the tungsten impurities low in the core of the plasma. Good progress has been made in both the ITER baseline scenario – a high confinement plasma largely relying on induced current – and the so called hybrid scenario which promises longer pulse operation in ITER. One surprise in the inductive scenario is that injecting nitrogen to reduce the power load to the wall can actually increase the plasma performance. This may be good news for ITER which will need to inject impurities to limit the power flow to the wall.

Encouraging news for ITER

The first JET results with a beryllium wall and tungsten divertor are very encouraging for ITER. It is also good to see that techniques developed at ASDEX Upgrade with its tungsten wall have turned out to be applicable at JET. This gives confidence that they will also scale up from JET to ITER.