FFAG accelerator
Encyclopedia
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
being developed for potential applications in physics, medicine, national security, and energy production, that has features of cyclotron
Cyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...
s and synchrotron
Synchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...
s. 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
Strong focusing
In accelerator physics strong focusing or alternating-gradient focusing is the principle that the net effect on a particle beam of charged particles passing through alternating field gradients is to make the beam converge...
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, built by Lawrence W. Jones and Kent M. Terwilliger at the University of MichiganUniversity of Michigan
The University of Michigan is a public research university located in Ann Arbor, Michigan in the United States. It is the state's oldest university and the flagship campus of the University of Michigan...
was a betatron
Betatron
A betatron is a cyclotron developed by Donald Kerst at the University of Illinois in 1940 to accelerate electrons, but the concepts ultimately originate from Rolf Widerøe and previous development occurred in Germany through Max Steenbeck in the 1930s. The betatron is essentially a transformer with...
, operational in early 1956. That fall, the prototype was moved to the MURA lab at University of Wisconsin, where it was converted to a 500 KeV electron synchrotron
Synchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...
. Symon's patent, filed in early 1956, uses the terms "FFAG accelerator" and "FFAG synchrotron". Ohkawa worked with Symon and the MURA team for several years starting in 1955.
Donald Kerst, working with Symon, filed a patent for the spiral-sector FFAG accelerator at around the same time as Symon'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. This last machine was based on Ohkawa's patent, filed in 1957, for a symmetrical machine able to simultaneously accelerate identical particles in both clockwise and counterclockwise beams. This was one of the first colliding beam accelerators
Collider
A collider is a type of a particle accelerator involving directed beams of particles.Colliders may either be ring accelerators or linear accelerators.-Explanation:...
, although this feature was not used when it was put to practical use as the injector for the Tantalus storage ring
Storage ring
A storage ring is a type of circular particle accelerator in which a continuous or pulsed particle beam may be kept circulating for a long period of time, up to many hours. Storage of a particular particle depends upon the mass, energy and usually charge of the particle being stored...
at what would become the Synchrotron Radiation Center
Synchrotron Radiation Center
The Synchrotron Radiation Center , located at the University of Wisconsin–Madison, is a national synchrotron-radiation light source research facility....
. The 50MeV machine was finally retired in the early 1970s. MURA designed 10 GeV and 12.5 GeV proton FFAGs that were not funded. There were FFAG conferences at Jülich in 1983 and 1985 at which several MURA veterans attended. There was an FFAG conference at Simonskall, Germany in 1991 followed by a series of four at Müden, Germany. Later, there was an FFAG workshop at CERN (2000) and two at KEK(2000, 2003); these have continued roughly yearly.
In 1981 and 1982, Tat Khoe and Phil Meads independently suggested that an FFAG would be an ideal accelerator for an intense neutron source. Thus began multi-year projects at the Argonne National Laboratory (ANL) and at the KFA Laboratory in Jülich working with TU: Garching and Hahn-Meitner Berlin, Germany. The first ANL design [ASPUN] was a spiral machine designed to increase momentum three fold with a modest field index as compared with the MURA machines. Godehard Wüstefeld discovered that with superconducing magnets, a radial FFAG could be made quite small (even "tabletop"). Phil Meads and Gode Wüstefeld invented the DFD and FDF triplet magnet designs for FFAGs that provided substantially greater drift lengths and which has been used for virtually all subsequent scaling FFAGs.
Scaling vs non-scaling types
The magnets needed for an FFAG are quite complex. The computation for the magnets used on the Michigan FFAG Mark Ib, a radial sector 500 KeV machine from 1956, were done by Frank Cole at the University of Illinois on a mechanical calculatorMechanical calculator
A mechanical calculator is a device used to perform the basic operations of arithmetic. Mechanical calculators are comparable in size to small desktop computers and have been rendered obsolete by the advent of the electronic calculator....
built by Friden
Friden, Inc.
Friden Calculating Machine Company was an American manufacturer of typewriters and electronic calculators. It was founded by Carl Friden in San Leandro, California in 1934. Friden electromechanical calculators were robust and popular....
. This was
at the limit of what could be reasonably done without computers;
the more complex magnet geometries of spiral sector and
non-scaling FFAGs require sophisticated computer modeling.
In all early FFAG machines, the bending field increased as a high power of the radius. Such machines are called scaling FFAGs.
In a scaling FFAG, 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.
The idea of building a non-scaling FFAG first occurred to Kent Terwilliger and Lawrence W. Jones in the late 1950s while thinking about how to increase the beam luminosity in the collision regions of the 2-way colliding beam FFAG they were working on. This idea had immediate applications in designing better focusing magnets for conventional accelerators, but was not applied to FFAG design until several decades later.
If acceleration is fast enough, the particles can pass through the betatron 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 therapyProton therapy
Proton therapy is a type of particle therapy which uses a beam of protons to irradiate diseased tissue, most often in the treatment of cancer. The chief advantage of proton therapy is the ability to more precisely localize the radiation dosage when compared with other types of external beam...
for cancer, for non-invasive security inspections of closed cargo containers, for the rapid acceleration of muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
s to high energies before they have time to decay, and as "energy amplifiers", for Accelerator-Driven Sub-critical Reactors
Subcritical reactor
A subcritical reactor is a nuclear fission reactor that produces fission without achieving criticality. Instead of a sustaining chain reaction, a subcritical reactor uses additional neutrons from an outside source...
(ADSRs) in which a neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
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
EMMA (accelerator)
The Electron Machine with Many Applications is a project at Daresbury Laboratory in the UK to build a linear non-scaling FFAG to accelerate electrons from 10 to 20 MeV. A FFAG is a type of accelerator in which the magnetic field in the bending magnets is constant during acceleration...
) 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.