Posted on: 21st August 2012
It is now three weeks since the last plasma pulse of 2012, and JET is now in a state called ‘shutdown’.
At the end of operations a lot of work has to be done before access is gained to the inside of the torus, which is planned to happen in a few weeks. When the machine is running, the torus is kept at 200 degrees celsius and inside the torus there are some cryo panels which are cooled to nearly minus 270 celsius. The cryopanels continuously collect (freeze) ‘condensable’ vapours such as water and practically all common gases. By keeping these two components at such a large temperature difference the exceptional cleanliness of the plasma-facing components is maintained.
After the end of operations, first the cryopanels were warmed to ambient temperature and the gas was collected. Then the torus was cooled. As it cools it contracts and the outer diameter of the torus decreases by about 30 mm. Essentially it shrinks onto some supporting features and locks itself into a fixed position until the end of the shutdown.
Now the task of removing some of the larger ancillary items has begun, with several big diagnostic systems being lifted out of the torus hall for storage. Some of them weigh tens of tonnes, but this is light work for the main crane. Once they are out of the way, engineers can get into the areas that they need to use to gain access to the inside of the torus, which brings us to the reason for this shutdown.
As you will know if you have followed the recent history of JET in the Shutdown Weekly series, the main purpose of 2009 shutdown was to allow a complete metal inner wall to be installed, replacing the previous carbon wall. Some of the 4,500 new tiles had been marked with thin layers of beryllium, molybdenum and tungsten. The layers are typically only 10 microns thick and these are the tiles that we plan to remove and replace during this shutdown. Careful examination of the marker layers will reveal which areas have been eroded by interaction with the plasma, and where that eroded material is deposited. You 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.
Of course this is only part of the work planned for the next few months. While the machine is out of action there is an opportunity for other equipment to be maintained. As the shutdown progresses you will see regular updates on the progress of some of this work.
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.