Chris Prior, Rutherford Appleton Laboratory, UK
It is over ten years since the ICFA Beam Dynamics Newsletter was last published on the theme of Fixed-Field alternating gradient Accelerators (now known as FFAs). Back in 2007, FFAs were enjoying a revival led by the demand for rapid acceleration of unstable particles and robust, reliable accelerators for high power applications. I wrote at the time of the interest in EMMA, the world’s first-ever non-scaling FFA under construction at the Daresbury Laboratory. A small (16.6 m circumference) electron FFA designed as a test model of a larger muon accelerator, EMMA was a proof-of-principle machine, used to show that resonances could be crossed sufficiently rapidly to avoid emittance growth and particle loss, and also gave the first verification of what became known as “serpentine” (or ‘out-of-bucket’) acceleration. The latter discovery featured as the cover article in Nature Physics and it was expected that EMMA would go on to become a useful tool for exploring beam dynamics. But financial constraints and budgetary cuts reared their ugly heads and EMMA’s life was short-lived. Now, the only FFAs in an operational state are those in Japan.
Looking back, it is easy to see how the outlook has changed with time. In 2007, there were ideas that FFAs could be used for large-scale projects like the neutrino factory or muon collider, as drivers for ADS, or to generate high-power, multi-megawatt proton beams for, say, spallation neutron sources. These were all highly expensive projects and in that respect nothing has changed. But, as design reports were completed and projects progressively shelved, research has become more detailed and focused on specific aspects. Other possible benefits and applications of FFAs have been identified. Suzie Sheehy gives a wide-ranging overview of the possibilities in her article on FFA applications. The small FFAs in Japan have been used to study beam dynamics in some depth: tune variation, resonances, chromaticity effects, orbit correction, errors, fringe fields. Compact FFAs might be used, say, to generate radio-isotopes. The advantages of FFAs in hadron therapy for cancer treatment are becoming increasingly recognised, and there are proposals for FFA-based proton and ion accelerators small enough to fit in hospitals, with relatively light, flexible gantries using FFA-type magnets that can handle a wide range of energies.
The main FFA-related construction project at the moment is CBETA, a joint initiative between Cornell University and BNL in the United States for a proto-type ERL that features a single FFA return loop capable of transporting accelerated and decelerated beams at four different energies in the same beam pipe. The project is part of the development programme for a future electron-ion collider, such as eRHIC.
In Europe, most of the FFA work is in the U.K. At the Rutherford Appleon Laboratory a long-term study is exploring ideas for a future spallation neutron source. A possible candidate is an accelerator using fixed-field magnets, possibly arranged in a DF-spiral configuration and conceivably designed so that the orbit moves in a vertical, rather than radial, direction as the beam is accelerated. This would reduce the footprint but present constructional challenges. A small prototype ring is planned, using RAL’s existing 3 MeV injector.
Most of these projects (and more) are included in this issue of the Newsletter. We also include details of the announcement of the first successful nuclear transmutation of minor actinides carried out at Kyoto University’s research reactor with a beam accelerated by an FFA. This is reported in Yoshihiro Ishi’s comprehensive article on the experimental FFA programme in Japan.
Understandably, codes to model the beam dynamics in such complicated machines are also important. Traditional synchrotron codes, with the same central orbit at all energies, are not suitable when a beam is accelerating in an FFA structure, and paraxial approximations are seldom valid in this context. The two main codes used in FFA studies, OPAL and Zgoubi, are featured here.
Theme articles aside, I was delighted to receive contributions from our ‘Regular Correspondents’, describing accelerator-related activities in Australia and the Middle East. There are announcements of eleven recent PhD awards in accelerator physics - which may well be a record for the Newsletter. Two have been awarded prizes for the quality of their theses. It is encouraging to see such interest in young people joining the field and good to know that we are investing in training the next generation of accelerator physicists. The Newsletter provides a good opportunity for them to bring their ideas and results to the attention of the beam dynamics community at large.
Finally, the change of acronym to FFA (referring to Fixed-Field alternating gradient Accelerators). The original acronym brought to mind a perjorative term used against members of the LGBT+ community. We have acknowledged this and followed organisations at SLAC and CERN and possibly elsewhere, that have changed their acronyms. Our Japanese colleagues have led the way in the accelerator world by labelling last year’s workshop FFA’18. I have accordingly tried to use “FFA” only throughout this Newsletter; of course, there should be no attempt to re-write history, so references to published works containing the old acronym remain unaltered. We have found it quite easy to make the change and if we can avoid causing offence at little or no cost to ourselves, there is no reason why we should not do it.