Posted on: 30th July 2012
Last week marks the end of the first period of JET operations with the all-metal ‘ITER-Like Wall’. The machine is now going into a period of several months of maintenance, ready to restart operations early in 2013.
The last 11 months of machine operations have been a busy and challenging time. The scientists at JET have gathered a lot of valuable data which is certainly going to be prominently featured at coming conferences. More data will be gathered over the next few months during which some components of the ITER-Like Wall will be removed from the inside of the torus for detailed inspection and analysis.
Although there have been a number of operational difficulties on some of the JET facilities a lot has been learnt and a few of the highlights will be mentioned briefly here. As you will remember, many aspects of the machine are new, and many challenges were associated with shaking down the new and highly modified systems. The majority of the difficulties were due to a total of only three single-point failures, which unfortunately affected critical facilities and resulted in lost operational time. Theses issues are now understood, and they are being addressed through procurement and installation of new components. A ‘Refurbishments Study Group’ has been set up by CCFE and is currently reviewing the JET facilities to identify where further refurbishments should be targeted.
JET has never been operated in divertor configuration with an all-metal wall before and this has required development of some different operational strategies. The metal wall was intended to reduce the amount of fuel retention, but that affects operations more than expected. The protection systems for the plasma facing components have been brought into operation and many of them are now fully commissioned. Heating and current drive systems have been operating at high power. In particular, the neutral beam injection systems have reached a record power of 25.8MW, even though 2 of the 16 beams have not been available due to technical issues.
High plasma current operation (at up to 3.5 MA) has also been demonstrated. This has required the operation of a device known as the ‘disruption mitigation valve’ which introduces a large puff of gas to spread the heat of the plasma more uniformly over the inner wall, preventing excessive heating of small areas. The DMV is now operating routinely and reliably.
Staff training has also been a big feature of operations, as key people learned the differences or were trained to implement the new systems. Covering 10 or 12 operational shifts per week has been quite a challenge.
Following on from the popular Shutdown Weekly, this series aims to give an insight into day-to-day activities at JET, from an engineering perspective. It aims to explain the technical aspects of operation of the world’s most successful tokamak. JET’s new ITER-Like Wall is being exploited for the benefit of its future successor, ITER.