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Connectors at the forefront of science

01 August 2007

Les Hunt was recently granted access to the ISIS facility to discover how Multi-Contact’s ‘Multilam’ connector technology has overcome a particularly troublesome problem with high current connections on the new ‘synchrotron extraction kicker’ magnet supply installation

The STFC Rutherford Appleton Laboratory site in Oxfordshire is home to ISIS, the world’s leading pulsed spallation neutron source. This impressive facility provides high-energy beams of neutrons and muon particles for a huge variety of experiments, ranging from the detailed study of complex molecules to the authentication of archaeological artefacts. Some 1,600 researchers from over 30 countries visit this site annually to conduct academic- and industry-led experiments into materials technology, engineering components and microbiological processes.

The scope of this article doesn’t allow a detailed description of the ISIS facility - a better source of information can be found at www.isis.rl.ac.uk. In simple terms, it is designed to accelerate a beam of protons within a toroidal shaped chamber and thence along a proton beam line towards a tungsten target. Here, upon impact, it releases neutrons by the process of spallation that are subsequently directed towards experimental stations, placed radially with respect to the target. At impact, the protons have an energy of 800MeV (approximately 13x10-11 joule).

In order to steer the proton beam along its circular path and maintain its ‘focus’ within the synchrotron chamber, a number of magnets have to be ‘fired’ at very precise intervals. When the beam has reached its maximum energy within the synchrotron (within 20ms), it is deflected into the Extracted proton beamline via a ‘septum’ bending magnet towards the target. This deflection is provided by a set of ‘kicker’ magnets, which effectively ‘lifts’ the beam out of its circular trajectory.

By the time the proton beam is ready for extraction into the beamline it is travelling at an enormous speed, and the kicker magnet must reach its maximum capacity within a very short time period in order to be effective. Should the kicker magnet not perform to this very tight specification, a part of the beam can impact with internal parts of the chamber, reducing impact energy at the target and providing an unwanted source of radiation.

As part of a recent upgrade at the facility, ISIS engineers have been hard at work improving the power systems supplying the synchrotron magnets. A key part of this work, carried out under the supervision of electrical engineer, Adrian McFarland, has been the installation of an entirely new kicker magnet power system, which will increase the deflection of the beam and ensure the efficiency of the beam extraction process for years to come.

The kicker magnet power system is made up of six identical installations, each comprising a 60kV/600mA power supply, a capacitor bank, a gas-filled thyratron, a matching resistor (to block transients reflected back along the lengthy cables between the thyratron and kicker magnet coils) and oil insulation and cooling circuits. In operation, the capacitors are charged to 60kV in 12ms and the power supply is switched off. A precise timing sequence ensues, with the capacitors being discharged through the thyratron, which, on firing, provides a fast rising (less than 100ns) 7,000A current pulse to the kicker magnet. The six installations are configured in pairs, each providing a negative and positive going current with respect to ground, thus effectively doubling the current available to the kicker magnet. The extraction process within the synchrotron occurs 50 times per second, so the pairs operate sequentially to allow time for recharging prior to each firing.

The thyratron is clearly an important component of this system, and these e2v supplied units are normally expected to operate reliably in a demanding 24/7 duty cycle. They do have a limited life in this application and usually require replacement at approximately one-year intervals. Occasionally, they may need to be removed from the kicker supply assembly for other maintenance purposes during their service life, and this additional handling places considerable demands on the contact strips that provide the internal connections for the device.

As Mr McFarland explained when DPA visited the facility back in June, the standard finger-strip contact bands that were previously used worked satisfactorily so long as the make or break was a straightforward insertion without any twisting. Unfortunately, some rotational movement inevitably occurs and this was causing elements of the finger-strip to break off, not only reducing the effective contact area but also presenting the team with further time-consuming maintenance problems.

Mr McFarland’s team and other engineering groups within ISIS had used Multi-Contact’s Multilam technology in the past, so it seemed logical to approach the company to see if it had a solution to this particular problem. Multi-Contact’s UK managing director, Selwyn Corns takes up the story: “We identified one of our standard product ranges as a possible replacement for the finger-strips. This was the Multi-Contact Multilam LA-CUT contact band, which we developed for large circular bus bar connectors, During installation, these have to endure a degree of physical abuse and some misalignment, and this seemed to offer a natural solution to the problems that Adrian and his team were experiencing.”

LA-CUT comprises an array of plated copper louvres (contact elements) that is articulated and retained in position by means of a stainless steel pressing that runs the full length of the contact. This pressing provides hinge points at both ends of the louvre and maintains the relative position of each with respect to its neighbour – an arrangement that tolerates the mechanical stresses imposed by frequent make and break actions. Each individual louvre is capable of a 50A continuous load, so the entire connector is more than capable of handling the nominal 7,000A kicker current pulse provided by the thyratron.

The LA-CUT Multilam bands are incorporated into several sizes of connector supplied to this project for the four main thyratron connections. Views of these connectors are shown on this page. Multi-Contact has over 50 different shapes and sizes of Multilam, which it is able to call upon when designing contacts, whether they be flat, cylindrical or spherical contact profiles. Applications of this tough, flexible contact systems are numerous, ranging from rail transportation, through renewable energy generating units to sophisticated medical imaging systems such as MRI scanners.


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