The ISIS strong focusing synchrotron also at the Rutherford Appleton Laboratory. Note that ISIS

occupies the same hall as NIMROD used to and re-uses some of the NIMROD magnets. In particular,

beam line magnets are used in the ISIS main ring and the large magnets from the NIMROD ring provide

shielding for the beam extracted from ISIS!

FFixed FField AAlternating GGradient synchrotrons, FFAGs, combine some of the main advantages of both cyclotrons and synchrotrons. Like the latter, they consist of a ring of magnets and use strong focusing. However, like the former, the magnetic field in the bending magnets has a strong gradient as a function of radius. This allows the particles to be contained without having to ramp the magnetic field and hence they can be cycled much more rapidly than a synchrotron, while being able to attain much higher energies than a cyclotron. In addition, they have a large beam acceptance and can be used for high beam currents.

One of the electron model FFAGs constructed by

MURA inthe 1960's.

Although in practice particle accelerators come in a variety of shapes, they can all be put into one of two categories: linear accelerators (linacs) or circular accelerators. Currently, there are two main types of circular accelerator: a cyclotron and a synchrotron. In general, cyclotrons have a large magnet or magnets enclosing the whole machine, with a magnetic field strength that increases as a function of radius. Beam is injected at the centre and accelerated. As the energy increases it spirals out in radius and is contained by the stronger magnetic field. They have the advantage that as the strength of the magnetic field does not change during the acceleration cycle, this can be very fast and they can be almost DC machines, leading to a high average beam current. As a result, the highest power circular accelerator currently in use is the cyclotron at PSI. The main problem with this type of machine is both the maximum beam energy and maximum beam current are ultimately limited.

Why are FFAGs Why are FFAGs Important ? ?

Synchrotrons, on the other hand, consist of a ring of magnets and as the particles are accelerated, the magnetic field strength is ramped up to contain them. In the early days, these were "weak focusing" machines, requiring large beam pipes and large, expensive magnets. However, in the 1950's the concept of "strong focusing" was developed in which quadrupole magnets are employed to give alternate focusing and de-focusing in each transverse plane. Over a wide range of parameters, this leads to a net focusing and results in a compact magnetic ring. The main advantage of such a strong focusing machine is that high energies are possible, at a relatively small cost compared to all other accelerators, limited only by the magnet strength and the size of the ring. The main disadvantage is that the ramping speed of the magnets is limited and the most rapidly cycled machine is currently the 50Hz ISIS synchrotron at the CCLRC Rutherford Appleton Laboratory. In addition, synchrotrons have a limited acceptance for a beam which, combined with the cycle time, leads to a restricted beam current.

The vacuum chamber of the cyclotron at the TRIUMF Laboratory in Canada, the largest in the world.

The 1MW cyclotron at the PSI Laboratory in Switzerland.

The layout of a Neutrino Factory in Japan, which uses a series of four FFAG rings to accelerate the muons to 20 GeV and does not require any cooling of the

muon beam.

Following from the discovery of strong focusing in 1952, FFAGs were proposed independently by Tihiro Ohkawa in Japan, Keith Symon in the US and Andrei Kolomensky in the USSR. The most extensive work was carried out by Symon and others at MURA (the Mid-Western Universities Research Association) in Wisconsin and a number of low energy electron models at around 50 MeV were constructed in the 1960's.However, proposals for proton FFAGs were not successful at that time, nor in the 1980's when 1.5 GeV machines were proposed for spallation neutron sources by the Argonne and Jülich Laboratories.

A bit of history A bit of history ……

Motivated mainly by the requirements of accelerators based on muon storage rings, in particular a Neutrino Factory, FFAG studies re-started in Japan in the late 1990's and resulted in the construction and testing of the first ever proton FFAG in 2000. The primary aim of a Neutrino Factory is to produce intense neutrino beams for long baseline neutrino oscillation searches from muon decays in a storage ring. To make the muons, a high power proton beam is fired into a target to make pions and as many of these pions as possible are captured and allowed to decay. In turn, as largea fraction of the muons from this decay are captured and then need to be accelerated before injection into the storage ring. The problem is they occupy a large volume in phase space, i.e. have a large emittance. It was initially thought that this must be reduced before the acceleration by a large, complex and expensive cooling channel. Studies in Japan, however, have indicated that using a series of four FFAGs sufficient muons can be accelerated without cooling due to the large acceptance and rapid cycling rate of the FFAG rings. These features minimise losses of the muons, in the latter case from muon decay.

The first FFAG built in Japan was the 500 keV proton Proof-of-Principle machine, which, as the name implies, was used to demonstrate the feasibility of this type of accelerator. With the successful test of this machine, a higher energy FFAG was also constructed at KEK, a 150 MeV proton FFAG, and first tested in 2003.

The first proton FFAG ever built: the 500 keV PoP at KEK.

Four further FFAGs are currently under development in Japan:

a proton FFAG for Accelerator Driven Sub-critical Reactor studies in Kyoto PRISM, a muon FFAG for high precision muon experiments at Osaka University a heavy ion therapy FFAG in Chiba a 1 MeV electron FFAG as a prototype for industrial irradiation and CT scanning

A 150 MeV proton FFAG built at KEK in Japan in 2003.

Many applications of FFAGs are being studied world-wide. See the second poster for more details.

FFixed FField AAlternating GGradient Synchrotrons

A new type of particle accelerator - with a wide variety of applications

Introduction and Current Status of Activities

The NIMROD 7GeV weak focusing

synchrotron at theRutherford High

Energy Laboratory.

These advantages have created a great deal of interest in FFAGs over the years, though it is only recently that the first proton machines have been built. Due to the advantages, these machines have a large number of potential applications, some of which are described on the second poster.