Speakers from the UK, France, Italy and Japan converged on the University of Southampton on Monday 17th March to discuss the development of cryocoolers and their application in Research and Engineering. Organised by Dr Paul McDonald on behalf of the Institute of Physics (IOP) and the British Cryogenics Council (BCC), the conference was opened by Professor Price, Head of Engineering Sciences in Southampton.
Professor Tom Haruyama from the KEK High Energy Accelerator in Japan described a particle detector developed at the Paul Scherrer Institute in Switzerland for an experiment to measure the decay of muon particles. The detector is filled with extremely dense liquid xenon, approximately 900 litres of it – almost 10% of one year’s worldwide production. A pulse tube cryocooler designed to cater for a heat load of 150W at 165K has been developed and is now finding application on other liquid xenon experiments, in laboratories around the world.
Exceptional organisation and technical complexity were evident in a presentation by Dr Tom Bradshaw, Head of Cryogenics at the Rutherford Appleton Laboratory, on the cooling system for the Planck space project, due to launch in 2008. Planck will measure the cosmic microwave background to unprecedented accuracy and the cryogenics group at Rutherford Appleton has developed a closed-cycle refrigerator with a Joule-Thomson stage to provide cooling to 4K.
This is part of a cryogenic chain which marries with other systems from the US and France, cooling the detectors down to 100mK. With 16 units in orbit, the Rutherford Appleton design is arguably the most successful cooler flown in space, exploiting resonance to run on low power and achieving extremely long life, using flexure springs and non-contact seals – one unit has been operating for sixteen years.
So-called sub-millimetre astronomy – concerning wavelengths of 450μm and 850μm – is relatively unmapped territory for exploring the early universe, according to Dr Adam Woodcraft from the Royal Observatory in Edinburgh. Detectors for this application come in two types, coherent at 4K and incoherent (bolometers) at 300 mK; they need to be colder than the object being observed. Since the ideal sites for sub-mm telescopes are either remote or in space, use of liquid helium for cooling is expensive, inconvenient and limits the operating life of instruments (in space).
Cryocoolers are increasingly being used instead, but are not without their own problems, including vibration, electro-magnetic interference, temperature oscillation and the non-continuous cycling of some systems.
Investigation of the decay of neutrino particles is the mission of the Cryogenic Underground Observatory for Rare Events in the Gran Sasso mountain in Italy. A large cryogenic system is required, comprising a 3m high cryostat cooled by five Cryomech PT415 cryocoolers and a high-power dilution refrigerator from Leiden Cryogenics. The 1.5 ton detector assembly is cooled below 10 mK. In describing CUORE, Dr Angelo Nucciotti from the Milano-Bicooca University touched on yet another snag of depending on cryogens – investigation of a safety incident had apparently resulted in a six-month period of restrictions on supply of liquid helium.
Dr Steve Rawson, Chief Executive of Callisto Space near Toulouse, described the Low Noise Amplifier systems his company has developed for Ground Stations for the European Space Agency in support of deep space missions. The designs all use cryogenically cooled High Energy Mobile Transistor technology, cooled below 20K with commercial closed cycle coolers to achieve optimum noise performance. Dr Rawson also indicated there was little benefit in cooling below 15K.
Nick Kerley, Technical Director of Magnex Scientific, described the evolution of cryocooler applications from the perspective of a magnet builder, starting with two-stage Gifford-McMahon units in the 1990’s and more recently Pulse Tube Refrigerators capable of achieving 4.2K. The G-M systems initially reduced helium consumption and then enabled ‘zero boil-off’ magnets for MRI scanners, which typically contain 15 superconducting switches immersed in liquid helium; lower vibration PT systems are widely used on small bore imaging magnets. A 45 tonne, 9.4 Tesla magnet was illustrated, requiring 870 tonnes of on-site shielding; using twin 10K G-M coolers, it still required 20,000 litres of helium for initial cooldown. Kerley observed that copper conductors used at 20K achieved performance similar to high temperature superconductors, while thermal noise is also reduced at 20-30K -delivering improved sensitivity in NMR detectors.
The conference was rounded off with double contributions from Prof. Ralph Scurlock, presenting the case and potential for improving efficiency of cryocoolers and for employing ‘green’ conservation thinking, with Scurlock, by invitation from Harry Jones of the Clarendon Laboratory in Oxford who chaired the final conference session, giving the meeting a televised cameo of progress in helium liquefaction to mark the centenary of this development by Kammerling Onnes in 1908.
Claridge Scientific, Cryogenic Ltd, Cryox Ltd and Pfeiffer Vacuum staged table-top exhibits which added to the knowledge exchange and networking at the event.