Fixed-field alternating gradient accelerator: Difference between revisions

Revision as of 16:46, 12 January 2022

Revision as of 17:31, 11 April 2022

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{{short description|Circular particle accelerator concept}}

{{Expand Russian|FFAG|date=May 2012}}

A '''Fixed-Field alternating gradient Accelerator''' ('''FFA''') (also abbreviated '''FFAG''') is a circular [[particle accelerator]] concept on which development was started in the early 50s, and that can be characterized by its time-independent magnetic fields (''fixed-field'', like in a [[cyclotron]]) and the use of alternating gradient [[strong focusing]] (''alternating gradient'', likeas in a [[synchrotron]]).<ref name=briefhistory>{{Cite journal

| last1 = Ruggiero

| first1 = A.G.

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| date = Mar 2006

| url = http://www.bnl.gov/isd/documents/31130.pdf

}}</ref><ref>{{cite journal | author=Daniel Clery | date=4 January 2010 | title=The Next Big Beam? | journal=[[Science (journal)|Science]] | volume=327 |pages=142–143 | doi=10.1126/science.327.5962.142 | pmid=20056871 | bibcode = 2010Sci...327..142C | issue=5962 }}</ref> Thus, FFA accelerators combine the cyclotron's advantage of continuous, unpulsed operation, with the synchrotron's relatively inexpensive small magnet ring, of narrow bore.

In all circular accelerators, magnetic fields are used to bend the particle beam. Since the [[Lorentz force|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.

Although the development of FFAs had not been pursued for over a decade starting from 1967, interest has been revived since the mid-1980s for usage in [[neutron]] [[spallation]] sources, as a driver for [[muon]] colliders <ref name=briefhistory /> and to accelerate muons in a [[Neutrino Factory|neutrino factory]] since the mid-1990s.

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.<ref>{{cite arXiv |last=Sheehy |first=S.L. |author-link= Suzie Sheehy |eprint= 1604.05221 |title= Fixed-Field Alternating Gradient Accelerators |class= physics.acc-ph|date= April 18, 2016 }}</ref>

AlthoughThe theinitial developmentconcept of FFAsthe hadFFA notwas beendeveloped pursuedin forthe over1950's, abut decadewas startingnot fromactively 1967,explored interestbeyond hasa beenfew revivedtest sincemachines until the mid-1980s, for usage in [[neutron]] [[spallation]] sources, as a driver for [[muon]] colliders <ref name=briefhistory /> and to accelerate muons in a [[Neutrino Factory|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 [[Radio frequency|RF]] cavities and in magnet design.<ref name=mori2004>{{Cite journal

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We note that the acronym for Fixed-Field alternating gradient Accelerator has recently changed from FFAG to FFA. Thus, when searching older literature, one will find more often the original acronym of FFAG.

== History ==