July marked the end of the first period of JET operation with the all-metal ITER-Like Wall. The machine is now going into a period of maintenance and will be ready to restart experiments in 2013. The eleven months of operation have been a busy and challenging time. The scientists at JET have gathered a lot of valuable data. Training has also been a big feature of the operation since control room staff were learning how to run the machine under the new conditions. Covering ten or twelve operational shifts per week has been something of a challenge.

Operating JET with an all-metal wall (as opposed to the previous carbon wall) required the development of different recipes to create, maintain and ramp down the plasma. By the end of the campaign, the heating and current drive systems had been routinely operating at high power. The neutral beam injection systems, in particular, had reached a record power of 25.8 megawatt. High plasma current operation (at up to 3.5 mega ampere) had also been demonstrated successfully. The high plasma energy required mitigating disruptions – plasma events that cause large heat loads at the vessel wall. One solution is to introduce a large puff of gas at precisely the correct moment to spread the heat of the plasma more uniformly over the inner wall, thus preventing excessive heating of small areas. This ‘disruption mitigation valve’ is now operating routinely and reliably to do that.

The main reason for interrupting JET experiments is the removal of some of the 4500 new wall tiles for analysis. These tiles had been marked with special layers of beryllium, molybdenum and tungsten. Careful laboratory examination of the marker layers will reveal which areas have been eroded by interaction with the plasma, and where that eroded material is deposited. One might think of this as being similar to erosion of part of a coastline by the action of the sea. The material that is removed from one place is washed along the coast and deposited somewhere else. A good understanding of this process in a tokamak can help us to make predictions regarding the lifetime of the plasma-facing components, and to estimate the amount of tritium that would be retained and buried under deposited layers.

While the machine is out of action, there is an opportunity for other equipment to be maintained and for a parallel programme of work to be carried out which aims to improve the performance of the machine.  Altogether, this work will ensure that JET keeps its leading position in magnetic confinement fusion research for years to come.

Nick Balshaw, CCFE