Non-Scaling FFAGs

So-called scaling FFAGs were invented in the 1950s and three electron accelerators were built at the Midwestern Universities Research Association in the late 50s and early 60s. The fundamental feature of these is they have a fixed magnetic field value during acceleration. To keep a wide range of particle momenta in the machine during the acceleration, the value of the magnetic field increases quickly as a function of radius. The particular field variation chosen means that the particle orbits are the same shape at all energies, but the size of the orbit "scales" with energy. Hence, they are called scaling FFAGs. They also employ "alternating gradient" or strong focussing of the beam.
The first FFAG ever built, a 400 keV electron machine first operated at MURA in 1956. The visitors include Niels Bohr and Subrahmanyan Chandrasekhar.

The first proton FFAG ever built, the Proof-of-Principle machine first operated at the KEK Laboratory in Japan in 2000.
Proposals to build proton FFAGs at MURA were not successful as the focus was on going to very high energies available from synchrotrons rather than the superior performance of FFAGs.

However, they were re-invented in Japan in the late 1990s for an entirely new project, the acceleration of muons for a Neutrino Factory. Two of the properties of FFAGs, a very fast rate of acceleration and a large beam acceptance make them particularly well suited for this purpose. This led to the construction of the first proton FFAGs at the KEK laboratory in Japan, both scaling machines.

The successful operation of the Japanese machines has created a great deal of interest world-wide. However, scaling FFAGs have one particular problem: the increase in orbit size during acceleration is very large and this leads to a requirement for large and complex magnets, which may be too expensive for the wide-spread use of these machines.

Recently, a new type of FFAG, the Non-Scaling FFAG has been invented. These were specially designed to have a much smaller increase in orbit radius, by up to a factor of 10 and to have simple magnets. They have an even faster acceleration rate than scaling FFAGs for muons and the same large acceptance. Their other properties also appear to make them ideal for hadron therapy and other applications. Their more compact nature should make them considerably cheaper than scaling FFAGs and comparable to cyclotrons and synchrotrons.

The second proton FFAG built at the KEK laboratory. Note the large size of the magnets.

Nothing comes for free, however, and the special properties of NS-FFAGs come at the cost of novel beam dynamics features. The fact that no machine with these has ever been built has led to the BASROc proposal to construct two machines. The first, EMMA, will be a test machine to study these features in detail and learn how to build NS-FFAGs for a variety of applications. The second, PAMELA, will be an explicit prototype for hadron therapy, but will be complementary to EMMA as taken together they will demonstrate the utility of NS-FFAGs over the entire velocity range of interest.

These two machines represent a huge opportunity for the UK. They will give us a head start in the development of a technology that could revolutionise the use of accelerators in industry, medicine and scientific research. They will also be perfect training machines for a new generation of British accelerator scientists. Finally, they may help to kick-start the treatment of cancer with hadron beams in the UK.

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For more information, please contact: Rob Edgecock - Tel: +44 (0)1235 44 5089