Stanford Accelerator Conference
The sessions on superconductivity at the IXth International Conference on High Energy Accelerators, held at the Stanford Linear Accelerator Centre from 2–7 May, were somehow rather frustrating. For many years, the potential of superconductivity, both in radio-frequency and magnet applications, has seemed on the brink of opening new doors. A lot has been achieved in practical realisation and in increasing basic understanding but, for many factors important for the future big projects, it is extremely difficult to get convincing answers. There was a woolliness about many of the discussions which needs to be cleared up.
On the r.f. side, superconducting cavities could give high accelerating voltage gradients and low power absorption, allowing cavities to be operated for longer times resulting in high duty cycle linacs and separators. In r.f. conditions the losses do not disappear completely but fall exponentially with temperature near absolute zero. Hence there is interest in pushing temperatures lower than is adequate for superconducting magnets. The currents flow in the surface layer of superconductors and the major problems have been concerned with achieving good quality surfaces in large r.f. structures and retaining their properties in operation.
The news of the work on superconducting magnets was equally frustrating. On the one hand, the past few years have seen d.c. superconducting magnets being brought into reliable operation at accelerators (for example, the Optique à Grande Acceptance OGA quadrupoles at Saclay and beam-line magnets at Berkeley and Brookhaven). These magnets have thousands of hours of physics use under their belts. Also many pulsed magnets have been through their paces in the laboratory with reasonable success.
On the other hand, all the magnets have exhibited training to some degree. In other words, we still do not manage to avoid small mechanical movements of the superconductor and because of this, a multi-magnet project would need to design the machine for a field considerably lower than the optimum, since we would not be sure to what fields the magnets would “train”.
Among other factors which could lead to accepting lower performance figures is the temperature sensitivity of the superconductor. Under usual operating conditions, with niobium-titanium superconductor at liquid helium temperature of 4.2 K giving fields of about 4.5 T, fluctuations of a few tenths of a degree can flip the magnets out of their superconducting state. There is a need to develop other superconducting materials, such as vanadium-gallium or niobium-tin. These materials have a much higher critical temperature (about 17 K) and could be operated at higher current densities to give fields of 6 T or above with comfortable temperature stability. The materials are however extremely brittle and the metallurgical problems of using them are not yet solved.
• Compiled from texts on pp202–205.
Not by physics alone doth man live…
The CERN rugby team won the final of the Swiss Cup on 26 May, having already taken the Swiss league championship for 1973–74. It was a strong year for CERN rugby – the reserve team also won their championship and the junior team won all the matches in its category.
Professor Jentschke, Director-General of CERN Lab I, fires the starting pistol for the annual team race around the CERN site. Forty-five teams raced on 29 May 1974. A Theory Division team won in 12 minutes 5 seconds, comfortably beating the track record.
Notwithstanding the somewhat pessimistic note struck at the Stanford Conference, in 1983 the first superconducting particle accelerator went into operation. The Tevatron at Fermilab was a 6.3 km circular synchrotron with 990 magnets at 4.4 K giving fields of 4.4 T. In 1995, the first large-scale superconducting r.f. accelerator came on air. The Continuous Electron Beam Accelerator Facility (CEBAF) at the Jefferson Lab consisted of two linacs with 1.5 GHz Nb cavities operating at 2 K. And in 2008, CERN’s Large Hadron Collider (LHC) became the largest scientific instrument in the world. Around the 27 km circumference, 1706 main superconducting magnets cooled to 1.9 K provided fields of 8.3 T.
The CERN Rugby Club – now the Rugby Club of CERN, Meyrin and St Genis (RC CMSG) – is one of the oldest in Switzerland and today it has male and female teams across all ages who take part in the Swiss leagues. Players span 23 nationalities, representing almost a quarter of the 100 or so countries that play the game worldwide. The CERN annual relay race also remains a popular social event, organised by the Running Club and Staff Association. In May 2016, 127 teams competed, with the winners scoring a record time of 10 minutes 19 seconds.