A post-Frascati review

The Fifth International Conference on Magnet Technology was held from 21–25 April in Rome, having relocated from the Frascati Laboratory because of increasing attendance. Interest focused on the use of superconducting magnets in high-energy physics, where extreme requirements push the technology further than elsewhere.

Bubble chambers and spectrometers need high fields over large volumes. Pioneering work was done at the Argonne Laboratory where [in 1968] they built a magnet to give a 2 T field in the 12 ft bubble chamber. Another leader was the Omega spectrometer magnet at CERN, with a 1.8 T field in a 1.5 m aperture, achieved in 1973. High field, small volume magnets are required for detecting particles such as hyperons that live for around 10–10s, to manoeuvre them after production to observe their interactions. At CERN, a very compact hyperon beam region was achieved with two short superconducting quadrupoles. Polarized proton targets also require high fields in small volumes to line up the proton spins while allowing the particles to emerge over as wide a range of angles as possible. The first to operate was at the late-lamented Cambridge Electron Accelerator in 1968.

The interest in pulsed magnets is higher peak fields, which would allow synchrotron energies to increase without increasing the machine size. The Fermilab proton synchrotron hopes to reach 500 GeV with 2.25 T fields from the conventional magnets around its 2 km diameter ring. If 4.5 T superconducting magnets could be used, the energy could reach 1 TeV in the same ring; magnets for such an “Energy Doubler” are under development.

Other applications were more prominent than at previous magnet conferences. This was particularly true of fusion research where recent advances coupled with the world energy crisis have refired the desire to bring thermonuclear fusion reactions under control. Magnets for medical applications also drew a good deal of attention with various “medical beam lines” being developed at accelerators.

Practically all the magnets reported use niobium-titanium [NbTi] superconductor. However, NbTi does not retain its superconductivity much beyond current densities of 0.8 kA/mm or temperatures higher than about 5 K. Other materials offer higher field operation and temperature transitions. Niobium-tin [Nb3Sn] can tolerate current densities of about 2 kA/mm, retaining superconductivity up to a temperature of 18 K. It is more difficult and expensive to work since it is very brittle, but during the past year Harwell/ Rutherford has made solenoids using filamentary Nb3Sn superconductor. But even after niobium-tin is mastered, there will still be challenges facing superconducting specialists.

• Compiled from texts on pp147–154.

Protecting the environment around CERN

During the past year, samples of air, water, etc, have been taken around the CERN 400 GeV proton synchrotron site [SPS] to confirm that machine operation will not affect the environment from the point of view of radiation [see the cover thumbnail below].

This photograph was taken in the “Lion stream”, which crosses the Laboratory II site. The stream has to be temporarily rerouted for work on the beam-line to the North Experimental Area. The Franco-Swiss fishing Union was alerted and cleared 700 m of the stream of its population – a haul of 366 trout. The trout were stunned by an electrical fishing rod (a 350 V, 4 A device) and transported to safety further downstream.

• Compiled from texts on pp146 and 160

Compiler’s note

The use of magnets and particle accelerators in medicine is now widespread, and Fermilab did indeed upgrade the Main Ring to become the Tevatron, where the top quark was discovered in 1995. However, endeavours to generate power from nuclear fusion have been less successful, owing to horrendous engineering and technological difficulties in creating and confining the resulting plasma for longer than a few seconds. Collaborative research continues and the first plasma in the 35-nation ITER tokamak in the south of France, using Nb3Sn superconductor technology, is foreseen for 2025.

The first operation of the high-luminosity LHC is also foreseen for 2025. It too will use Nb3Sn superconducting magnets to achieve the necessary increase in performance over the present NbTi-based LHC magnets.