JET’s new ITER-Like-Wall has delivered a factor of ten reduction in fuel retention, much to the delight of the team that designed and installed it. “Although the calculations and lab tests suggested it might perform this well, often in the real machine things are different,” said Task Force Leader Dr Guy Matthews, who led the project to replace JET’s carbon tiles with beryllium and tungsten components. “But it has behaved exactly as predicted – we are very happy!”

Very low fuel loss

The amount of fuel retained in the vessel after a plasma pulse is a key parameter for JET’s performance. Although carbon withstands extreme heat well, it has been replaced as wall material because it binds all too readily with the hydrogen isotopes used as fusion fuel. This loss of fuel is a particular problem in the case of the tritium, because its short lifetime makes it scarce and therefore expensive. Also, handling of the vessel is greatly complicated if it still contains a significant amount radioactive tritium.

As both these issues would be even more significant in the scaled up context of ITER, scientists throughout the seven ITER nations have been following these experiments closely. Fortunately, following the move to metal tiles, the fuel retention has dropped by an order of magnitude.

In fact, the levels of fuel lost in each pulse are so low now that the measurement system had to be improved significantly to detect them. The fuel retention is determined by comparing careful measurements of the volume of gas injected into the chamber with the amount of exhaust. These results were initially not conclusive, because the retention quantities were now as small as the uncertainty in the gas temperature measurements. However, an overhaul of the temperature sensors increased the precision and gave the team confidence that the fuel retention really was as good as they’d hoped.

Cleaner plasma

As well as reducing fuel retention the metal wall has led to a cleaner plasma overall. “Since the first plasma, we have had to do no conditioning at all,” says Dr Matthews.
With the carbon wall, weekly cleaning procedures were required, such as overnight glow discharges or beryllium evaporation. Also, after a disruption a recovery pulse was often required to remove contamination before the machine would operate at high power again. However with the new wall, says Guy Matthews, “the need for cleaning has currently all but disappeared, and we have had no false starts at all. I don’t think anyone would have expected that!”

Phil Dooley, EFDA