The Japanese KEK laboratory has for the first time demonstrated an alternative method of accelerating protons to high energy – the fixed-field, alternating gradient (FFAG) synchrotron.
In a normal variable frequency synchrotron, the radio-frequency of the applied electric fields (which accelerate the circulating beam) is increased to remain in step with the beam as it becomes increasingly distorted by relativity.
In such a synchrotron the beams circulate inside a magnetic tube, obviating the need for a large magnet to enclose the whole machine (as had been the case for the cyclotron). In the 1950s the idea of strong focusing (alternating gradient) enabled the dimensions of this magnetic tube to be reduced considerably, cutting still further the expensive magnetic investment needed. (In 1959 CERN’s PS was the first proton synchrotron to operate using this technique.)
Following the strong focusing revolution, several accelerator specialists realized that ingenious magnetic field designs could also enable particles to be accelerated across the machine’s wide aperture without pulsing the magnetic field. This is the FFAG idea, which was first proposed and demonstrated for electrons by the Midwestern Universities Research Association team.
Under the leadership of Yoshiharu Mori, design at KEK started in January 1999 and the first beam was accelerated on 16 June 2000. The fact that these machines use fixed fields allows them to operate at high repetition rates and produce high-intensity beams. The price to pay is large apertures, a larger circumference and consequently massive magnets, which has favoured so far the now classical alternating gradient pulsed synchrotrons and explains why only electron model machines had ever been built.
However, because of their large momentum and transverse acceptances, these machines constitute a promising alternative to the more conventional approaches to muon collection and acceleration (radio-frequency and induction linacs) in a neutrino factory.
This was reported in a poster at the recent European Particle Accelerator Conference in Vienna. An international FFAG workshop was held at CERN immediately after the conference.