Fixed-field alternating gradient accelerator
A Fixed-Field alternating gradient Accelerator (FFA; also abbreviated FFAG) is a circular particle accelerator concept that can be characterized by its time-independent magnetic fields (fixed-field, like in a cyclotron) and the use of alternating gradient strong focusing (as in a synchrotron).[1][2]
In all circular accelerators, magnetic fields are used to bend the particle beam. Since the magnetic force required to bend the beam increases with particle energy, as the particles accelerate, either their paths will increase in size, or the magnetic field must be increased over time to hold the particles in a constant size orbit. Fixed-field machines, such as cyclotrons and FFAs, use the former approach and allow the particle path to change with acceleration.
In order to keep particles confined to a beam, some type of focusing is required. Small variations in the shape of the magnetic field, while maintaining the same overall field direction, are known as weak focusing. Strong, or alternating gradient focusing, involves magnetic fields which alternately point in opposite directions. The use of alternating gradient focusing allows for more tightly focused beams and smaller accelerator cavities.
FFAs use fixed magnetic fields which include changes in field direction around the circumference of the ring. This means that the beam will change radius over the course of acceleration, as in a cyclotron, but will remain more tightly focused, as in a synchrotron. FFAs therefore combine relatively less expensive fixed magnets with increased beam focus of strong focusing machines.[3]
The initial concept of the FFA was developed in the 1950's, but was not actively explored beyond a few test machines until the mid-1980s, for usage in neutron spallation sources, as a driver for muon colliders [1] and to accelerate muons in a neutrino factory since the mid-1990s.
The revival in FFA research has been particularly strong in Japan with the construction of several rings. This resurgence has been prompted in part by advances in RF cavities and in magnet design.[4]
History
First development phase
The idea of fixed-field alternating-gradient synchrotrons was developed independently in Japan by
MURA designed 10 GeV and 12.5 GeV proton FFAs that were not funded.[13] Two scaled down designs, one for 720 MeV[14] and one for a 500 MeV injector,[15] were published.
With the shutdown of MURA which began 1963 and ended 1967,[16] the FFA concept was not in use on an existing accelerator design and thus was not actively discussed for some time.
Continuing development
In the early 1980s, it was suggested by Phil Meads that an FFA was suitable and advantageous as a proton accelerator for an
Conferences exploring this possibility were held at Jülich Research Centre, starting from 1984.
The first proton FFA was successfully construction in 2000,[26] initiating a boom of FFA activities in high-energy physics and medicine.
With
In 2010, after the workshop on FFA accelerators in
Scaling vs non-scaling types
The magnetic fields needed for an FFA are quite complex. The computation for the magnets used on the Michigan FFA Mark Ib, a radial sector 500 keV machine from 1956, were done by Frank Cole at the
This was at the limit of what could be reasonably done without computers; the more complex magnet geometries of spiral sector and non-scaling FFAs require sophisticated computer modeling.The MURA machines were scaling FFA synchrotrons meaning that orbits of any momentum are photographic enlargements of those of any other momentum. In such machines the betatron frequencies are constant, thus no resonances, that could lead to beam loss,[31] are crossed. A machine is scaling if the median plane magnetic field satisfies
- ,
where
- ,
- is the field index,
- is the periodicity,
- is the spiral angle (which equals zero for a radial machine),
- the average radius, and
- is an arbitrary function that enables a stable orbit.
For an FFA magnet is much smaller than that for a cyclotron of the same energy. The disadvantage is that these machines are highly nonlinear. These and other relationships are developed in the paper by Frank Cole.[32]
The idea of building a non-scaling FFA 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 FFA they were working on. This idea had immediate applications in designing better focusing magnets for conventional accelerators,[6] but was not applied to FFA 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. A proof-of-principle linear, non-scaling FFA called (EMMA) (Electron Machine with Many Applications) has been successfully operated at Daresbury Laboratory, UK,.[33][34]
Vertical FFAs
Vertical Orbit Excursion FFAs (VFFAs) are a special type of FFA arranged so that higher energy orbits occur above (or below) lower energy orbits, rather than radially outward. This is accomplished with skew-focusing fields that push particles with higher beam rigidity vertically into regions with a higher dipole field.[35]
The major advantage offered by a VFFA design over a FFA design is that the path-length is held constant between particles with different energies and therefore relativistic particles travel isochronously. Isochronicity of the revolution period enables continuous beam operation, therefore offering the same advantage in power that isochronous cyclotrons have over synchrocyclotrons. Isochronous accelerators have no longitudinal beam focusing, but this is not a strong limitation in accelerators with rapid ramp rates typically used in FFA designs.
The major disadvantages include the fact that VFFAs requires unusual magnet designs and currently VFFA designs have only been simulated rather than tested.
Applications
FFA accelerators have potential medical applications in
Because of their quasi-continuous beam and the resulting minimal acceleration intervals for high energies, FFAs have also gained interest as possible parts of future
Status
In the 1990s, researchers at the KEK particle physics laboratory near Tokyo began developing the FFA concept, culminating in a 150 MeV machine in 2003. A non-scaling machine, dubbed PAMELA, to accelerate both protons and carbon nuclei for cancer therapy has been designed.
See also
- subcritical nuclear reactor which might use an FFA as a neutron source
Further reading
- "The rebirth of the FFAG". CERN Courier. Jul 28, 2004. Retrieved Apr 11, 2012.
References
- ^ a b Ruggiero, A.G. (Mar 2006). "Brief History of FFA Accelerators" (PDF). BNL-75635-2006-Cp.
- PMID 20056871.
- arXiv:1604.05221 [physics.acc-ph].
- ^ Mori, Y. (2004). "Developments of FFA Accelerator" (PDF). Proceedings of FFAG04 /. Archived from the original (PDF) on 2016-12-20. Retrieved 2016-05-04.
- ^ Lawrence W. Jones, Kent M. Terwilliger, A Small Model Fixed Field Alternating Gradient Radial Sector Accelerator, Technical Report MURA-LWJ/KMT-5 (MURA-104), April 3, 1956; contains photos, scale drawings and design calculations.
- ^ hdl:2027.42/87537.
- ^ US patent 2932797, Keith R. Symon, "Imparting Energy to Charged Particles", issued 1960-04-12
- S2CID 5201822.
- ^ US patent 2932798, Donald William Kerst and Keith R. Symon, "Imparting Energy to Charged Particles", issued 1960-04-12
- ^ US patent 2890348, Tihiro Ohkawa, "Particle Accelerator", issued 1959-06-09
- ISBN 9789810209582.
- ^ E. M. Rowe and F. E. Mills, Tantalus I: A Dedicated Storage Ring Synchrotron Radiation Source, Particle Accelerators, Vol. 4 (1973); pages 211-227.
- ^ F. C. Cole, Ed., 12.5 GeV FFA Accelerator, MURA report (1964)
- .
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- ^ "Argonne History: Understanding the Physical Universe". Argonne National Laboratory. Archived from the original on 9 September 2004.
- ^ "COSY - Fundamental research in the field of hadron, particle, and nuclear physics". Institute for Nuclear Physics. Retrieved 12 February 2017.
- ^ Wüstefeld, G. (14 May 1984). "2nd Jülich Seminar on Fixed Field Alternating Gradient Accelerators (FFA)". Jülich. Retrieved 12 February 2017.
- Bibcode:2005pac..conf..261C. Retrieved 12 February 2012.
- ^ "Previous Workshops". BNL. Retrieved 12 February 2017.
- ^ a b Martin, S.; Meads, P.; Wüstefeld, G.; Zaplatin, E.; Ziegler, K. (13 October 1992). "Study of FFAG Options for a European Pulsed Neutron Source (ESS)" (PDF). Proc. XIII National Accelerator Conference, Dubna, Russia.
- ^ Zaplatin, E. (24 March 1992). "Fourth Accelerator Meeting for the EPNS". European Particle Accelerator Conference.
- ^ M. Aiba; et al. (2000). "Development of a FFAG Proton Synchrotron". European Particle Accelerator Conference.
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- ^ S. A. Martin; et al. (24 May 1993). "FFAG Studies for a 5 MW Neutron Source". International Collaboration on Advanced Neutron Sources (ICANS).
- ^ D. Trbojevic, E. Keil, A. Sessler. "Non-Scaling Fixed Field Gradient Accelerator (FFAG) Design for the Proton and Carbon Therapy" (PDF). Retrieved 12 February 2017.
{{cite web}}
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- ^ Typical Designs of High Energy FFA Accelerators, International Conference on High Energy Accelerators, CERN-1959, pp 82-88.
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- ^ S. Machida et al, Nature Physics vol 8 issue 3 pp 243-247
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