Precision motion systems supplied by Delta Tau have played a crucial role in accurately measuring the in-vessel geometry of JET, the Joint European Torus, a tokamak plasma containment device that is being used for leading-edge research into the generation of energy from nuclear fusion.

Sited at Culham, near Oxford and operational since the early 1980s, JET has been upgraded many times. The next major upgrade, scheduled for 2008, is the installation of a beryllium wall to face the plasma in the containment vessel. This replaces the existing carbon fibre wall and will enhance the capabilities of JET for use with tritium, greatly increasing its usefulness as well as extending its useful life.

An essential preliminary to the installation of the new plasma-facing wall is the accurate determination of the geometry of the containment vessel. This is a far from trivial task, as the shape of the vessel is constantly changing as a consequence of the extreme forces and high thermal loads generated during JET’s operation.

After a detailed theoretical study, the first stage of the vessel measuring process was the installation of 300 photogrammetry targets, incorporating 1,500 small circular retroreflective dots. After this, a special camera with tightly controlled optical characteristics and a resolution of one-tenth of a pixel was mounted on the existing remote handling boom.

This five-metre long boom is driven by DC brushed servomotors on 18 axes, each of which is controlled using Delta Tau’s PMAC high-precision motion controller. The control system works in conjunction with a graphical operator interface created by the JET team’s software engineers, which provides a full 3D graphical model of the boom and its surrounding environment.

Using the facilities provided by the Delta Tau motion control systems, the boom was manipulated to accurately position the camera at 90 different pre-calculated positions within the containment vessel. From each position, pictures were taken of the targets. The pictures, which incorporate a high degree of redundancy in the target images, were subsequently analysed to produce an accurate map of the vessel’s contours.

By using this advanced technique, and exploiting the inherent flexibility and high positional accuracy offered by the Delta Tau motion control system, it has proved possible for the JET engineers to measure the position of each target with an accuracy of the order of 0.01mm. This data will make possible the accurate location of the new beryllium plasma-facing wall when it is installed during 2008.

For further information, contact: Andy Joslin,
Delta Tau UK Limited,
Holland Cottage, Kirby Road,
Great Holland, Essex
CO13 0HZ

Telephone: 01255 670196
Fax: 01255 850768
e-mail: ajoslin@deltatau.com