Posted on: 3rd February 2012
During one busy weekend in January, while plasma operations had paused, another upgrade was installed on JET. The item in question is called a ‘collimator’. It weighs about 2 tonnes and it is made of layers of lead and polythene, contained in a stainless steel envelope.
To understand what a collimator is, consider when you were a child and your mother gave you a cardboard tube to play with. You probably put it to your eye and looked through it, pretending that it was a telescope. Of course it does not magnify the image that you see, but it simply limits your field of view, blocking out the surrounding bigger picture. This toy telescope is really a collimator. The main difference between this and JET’s new collimator is that light is stopped by a thin layer of card, but the energetic neutrons from the JET plasma need layers of low density, hydrogen rich, polythene to slow them down. Additionally, layers of higher density lead is used to cut down high energy gamma rays.
The installation process was almost like an exercise in choreography. Different teams of people were required to complete the work at precisely scheduled times. The shielding doors had to be opened and the shielding beams lifted. 4,000 tonnes of concrete has to be moved to achieve this! The main crane was then able to carry the collimator and its stand into the torus hall. The crane operations team, the ‘riggers’ had to be ready. A team of scaffolders built a tower a few metres high, for the team of fitters to work from. First the stand was lifted into place and bolted to an existing component. The positions of the feet were then marked on the concrete floor and the stand was lifted away so that the floor could be drilled. Then the stand was re-installed, bolted into position, and the collimator lifted into position, aligned and secured.
At the end of the job, the crane had to be driven out of the torus hall and the shielding elements closed again, ready for normal operations to resume at 06:30 on Monday morning.
Now one of the gamma ray spectrometers has a narrower and more discriminating field of view which will reveal new physics during the coming months.
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.