Posted on: 17th June 2013

It sounds like a kind of gothic torture – being put in a large bucket and lowered into a 9m deep hole at the centre of a huge machine. But it’s all in a day’s work for JET’s inspection team, as part of maintenance of JET’s central magnet, the P1 solenoid.

The solenoid itself is made up of 1440 turns of copper, separated into 14 “pancakes” – sections of coils stacked on top of each other. During the course of experiments these carry up to 60 000 amps, and are subjected to huge magnetic forces, which causes them to shift around slightly. A set of spring-loaded keys pull the pancakes back into alignment, but over the course of thousands of plasma pulses you would expect these to wear and lose their precision.

“Actually no refurbishment was needed, even though this procedure was last done 8 years ago,” says project leader Michael Porton.

“In the intervening time, a number of key staff had retired, so we were partly re-learning how to do the work,” says Mr Porton. “It was very interesting – it’s not often you get to peer down the insides of JET!”

Not surprisingly pulling the large iron core out of the heart of JET and accurately measuring the rotation of the pancakes was a complex task, which involved cooperation from many different areas, such as the lifting team, and the photogrammetry team, whose expertise in locating and surveying the exact positions of the components was vital.

There were also safety contingencies developed – rescue plans, in case the gothic horror tale came true. But there was a happy ending to this tale, no rescues were needed and after 80 000 pulses or more, JET’s central solenoid is in still great shape, ready for 80 000 more!