Science in the spotlight
Inside out: how the cameras roll on JET
The video camera is a ubiquitous tool of modern life. But what role does it play in studying the inside of the world's largest fusion experiment?
“Here we are in the nerve centre of the operations on JET from where the experiments are run,” says Dr Phil Dooley from the EFDA Public Information Office as he leads a group of ‘A' Level students into the JET Control Room.
“What you are looking at here on the computer screen,” he continues, “is a plasma pulse being created inside the vacuum vessel during an experiment.”
Looking intently, the visitors see the formation of a purplish coloured plasma lasting 20 seconds which appears following a robotic sounding 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 countdown.
This real-time visual image allows engineers and scientists in the control room to observe each plasma as it is created, and often give an immediate indication of any unusual events.
Every time a plasma is created inside JET, it is filmed by many different cameras and relayed to the control room. Filming of the pulses in normal visible light is made by an ‘off the shelf' video camera – the kind your favourite technology store would sell for around £500, but with £100,000 of bespoke wide angle specialist optical equipment. The adapted cameras are located two metres away from the vessel and the footage recorded through a special window which has the JET vacuum on one side and open air on the other.
A critical eye may consider that the images captured are not that sharp; this is due to the camera sensors being hit by neutrons created by nuclear fusion reactions in the plasma which make the picture speckled. Nevertheless, the footage clearly shows the edge of the plasma in a purplish colour, and rather surprisingly perhaps, the centre of the ionised gas where it is hottest appears black. To ‘see' the centre needs special equipment sensitive to ultra-violet light or even X-rays.
Since the first experiments were run on JET in 1983 there has always been a camera in situ for filming the plasma, with the quality of pictures improving over the years due to the introduction of wide angle lenses and increasingly sophisticated optics specially developed for the hostile environment of the JET torus.
Following the installation of the ITER-Like Wall and the beginning of the subsequent experimental campaign in August 2011, video cameras have played an increasing and vital role in filming inside the JET vessel both during and between pulses.
To achieve this, an additional 20 video recorders and specialist heat-sensitive cameras (infra-red) have been fitted around the fusion machine wherever suitable windows could be found, to monitor plasma behaviour. They also take thermal measurements of the beryllium and tungsten tiles which make up the new ITER-Like Wall.
These cameras are an essential way of monitoring the wall's interaction with the plasma. Some are very ordinary devices costing around £500 each, while other more specialised scientific cameras can cost up to 20 times more.
This complex equipment is linked to a computer system which will detect any irregularities in the plasma interacting with the wall, such as hotspots. Such a means of detection is important as the new beryllium tiles are less robust that their carbon predecessors.
So fast are the measurements made during a pulse, that upon detection of a problem, action can be taken within a tenth of a second and, if critical, a pulse immediately terminated to avoid excessive damage to the wall.
Viewing Systems Section Leader Dr Gilles Arnoux (pictured in the JET Control Room, above, and with his team, right) was responsible for setting up this real-time monitoring system which consists of seven cameras and nine heat sensors. The footage from inside the vessel is streamed into the control room appearing on an array of screens showing a range of different views.
“These cameras have become a key diagnostic tool for analysing the behaviour of the new JET wall,” says Dr Arnoux. “Detailed processing of the images taken is allowing physics calculations to be made. This data will contribute to the design of ITER, including plans to install a suite of video cameras around the new machine providing inside views of the whole vessel during operations.” As the recent experimental campaign neared completion, record neutral beam heating power of about 26MW was injected into four JET plasmas – exceeding the previous power of 24MW reached in 2009.
“These new viewing and heat detection systems have been key to achieving high current operation and the record heating powers on the new ITER-Like Wall,” says Dr Guy Matthews, Leader of JET Task Force E2.
“The camera-based protection system has been essential to give us the confidence to achieve these goals during pulses in relatively few sessions.”
But what about the time in between pulses? How can scientists tell how the wall has behaved during experiments and if there has been any damage?
To answer these questions, a prototype video camera appropriately named the In Vessel Inspection System Camera (IVIS – pictured right) was developed and assembled in-house at Culham. This was an on-site collaboration between a specialist project team and the CCFE Special Techniques Group.
Four identical devices were assembled for use in the first experimental campaign following the installation of the ITER-Like Wall. They were designed to provide valuable footage which could be added to other experimental data, thereby allowing the detailed study of the new JET tiles.
Dr Guy Matthews believes the regular wall inspections using the new IVIS equipment have worked well, with their usage providing vital understanding into the behaviour of the materials when subjected to experimental conditions.
“On days when there are no plasma operations the IVIS has successfully detected areas of localised melting on a few beryllium tiles,” says Dr Matthews. “This is due to overheating in areas not previously covered by the protection system and their important identification via filming has enabled us to refine our protection systems and strategies.”
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