Fixed-field alternating gradient accelerator

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Layout of the EMMA prototype electron FFAG, currently under construction as part of the ALICE complex at Daresbury Laboratory in the U.K. for cancer therapy.

A Fixed-Field Alternating Gradient accelerator (FFAG) is a type of circular particle accelerator being developed for potential applications in physics, medicine, national security, and energy production, that has features of cyclotrons and synchrotrons. FFAG accelerators combine the cyclotron's advantage of continuous, unpulsed operation, with the synchrotron's relatively inexpensive small magnet ring, of narrow bore.

This is achieved by using magnets with strong focusing alternating-gradient quadrupole fields to confine the beam, accompanied by a dipole bending magnetic field which bends the beam to close the orbital ring. By the use of a strong radial magnetic field gradient in the dipole component, yet with a time-constant "fixed field" as the particles are accelerated, particles with larger energies move successively to slightly larger orbits, where the bending field is larger. The beam thus remains confined to a narrow ring, as in a synchrotron, yet without the synchrotron's requirement that the machine be operated in pulsed acceleration cycles.

History

The idea of fixed-field alternating-gradient synchrotrons was developed independently in Japan, the United States, and Russia by Tihiro Ohkawa, Keith Symon and Andrei Kolomensky. The first prototype FFAG electron accelerator was a 400 keV machine built by the Midwest Universities Research Association (MURA) in the 1956. Simon's patent, filed in early 1956, uses the terms "FFAG accelerator" and "FFAG synchrotron". Ohkawa worked with Simon and the MURA team for several years starting in 1955. The first prototype had radial sectors. Donald Kerst, working with Symon, filed a patent for the spiral-sector FFAG accelerator at around the same time as Simon's Radial Sector patent. A very small spiral sector machine was built in 1957, and a 50 MeV radial sector machine was operated in 1961. Development of FFAG synchrotrons then went dormant for over 40 years.

Scaling vs non-scaling types

The magnets needed for an FFAG are quite complex, and their design has become feasible only in recent years with improved computer modeling and magnet technology. In scaling FFAGs, the bending field increases at a high power of the radius, such that the higher energy orbits move outwards but without changing shape. This is useful to avoid so-called betatron oscillations, resonances in transverse beam stability that have long plagued the designers of cyclic accelerators. If however the acceleration is fast enough, the particles can pass through the resonances before they have time to build up to a damaging amplitude. In that case the dipole field can be linear with radius, making the magnets smaller and simpler to construct. These newer, non-scaling FFAGs are under development.

Applications

Such machines have potential medical applications in proton therapy for cancer, for non-invasive security inspections of closed cargo containers, for the rapid acceleration of muons to high energies before they have time to decay, and as "energy amplifiers", for Accelerator-Driven Sub-critical Reactors [ADSRs) in which a neutron beam derived from a FFAG drives a slightly sub-critical fission reactor. Such ADSRs would be inherently safe, having no danger of accidental exponential runaway, and relatively little production of transuranium waste, with its long life and potential for nuclear weapons proliferation.

Status

In the 1990s, researchers at the KEK particle physics laboratory near Tokyo began developing the FFAG concept, culminating in a 150 MeV machine in 2003. The Electron Machine with Many Applications (EMMA) is a project at Daresbury Laboratory in the UK to build a prototype linear non-scaling FFAG to accelerate electrons from 10 to 20 MeV. It is expected to be come operational in March 2010. A follow-on non-scaling machine, dubbed PAMELA, to accelerate both protons and carbon nuclei for cancer therapy, is in design. Meanwhile, an ADSR operating at 100 MeV was demonstrated in Japan in March 2009 at the Kyoto University Critical Assembly (KUCA), achieving "sustainable nuclear reactions" with the critical assembly's control rods inserted into the reactor core to damp it below criticality.