Linear particle accelerator: Difference between revisions

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[[Image:Aust.-Synchrotron,-Linac,-14.06.2007.jpg|250px|right|thumb|The linac within the [[Australian Synchrotron]] uses [[radio waves]] from a series of [[Resonator#Cavity resonators|RF cavities]] at the start of the linac to accelerate the electron beam in bunches to energies of 100 MeV.]]

A '''linear particle accelerator''' (often shortened to '''linac''') is a type of [[particle accelerator]] that accelerates charged [[subatomic particle]]s or [[ion]]s to a high speed by subjecting them to a series of [[Oscillation|oscillating]] [[electric potential]]s along a [[Line (geometry)|linear]] [[beamline]]. The principles for such machines were proposed by [[Gustav Ising]] in 1924,<ref>G. Ising: ''Prinzip einer Methode zur Herstellung von Kanalstrahlen hoher Voltzahl.'' In: ''Arkiv för Matematik, Astronomi och Fysik.'' Band 18, Nr. 30, 1924, S. 1–4.</ref> while the first machine that worked was constructed by [[Rolf Widerøe]] in 1928<ref>{{cite journal|last = Widerøe|first = R.|author-link=Rolf Widerøe |date = 17 December 1928 |title=Über Ein Neues Prinzip Zur Herstellung Hoher Spannungen|journal = Archiv für Elektronik und Übertragungstechnik|doi = 10.1007/BF01656341 |volume = 21|number = 4|pages = 387–406|s2cid = 109942448}}</ref> at the [[RWTH Aachen University]].<ref>{{cite conference|last1=Bryant |first1=P J |title=A brief history and review of accelerators |conference=5th General Accelerator Physics Course |publisher=CERN Accelerator School |date=1994 |doi=10.5170/CERN-1994-001.1}}</ref><ref>{{cite book |last1=Mangan |first1=Michelangelo |editor1-last=Brüning |editor1-first=Oliver |editor2-last=Myers |editor2-first=Stephen |title=Challenges and goals for accelerators in the XXI century |date=2016 |publisher=World Scientific |___location=Hackensack, New Jersey |isbn=978-981-4436-39-7 |page=33 |url=https://www.worldscientific.com/worldscibooks/10.1142/8635 |chapter=Particle accelerators and the progress of particle physics|doi=10.1142/8635|bibcode=2016cgat.book.....M }}</ref>

Linacs have many applications: they generate [[X-ray]]s and high energy electrons for medicinal purposes in [[radiation therapy]], serve as particle injectors for higher-energy accelerators, and are used directly to achieve the highest kinetic energy for light particles (electrons and positrons) for [[particle physics]].

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}}</ref> Electrons are sufficiently lighter than protons that they achieve speeds close to the [[speed of light]] early in the acceleration process. As a result, "accelerating" electrons increase in energy, but can be treated as having a constant velocity from an accelerator design standpoint. This allowed Hansen to use an accelerating structure consisting of a horizontal [[waveguide]] loaded by a series of discs. The 1947 accelerator had an energy of 6 MeV. Over time, electron acceleration at the [[SLAC National Accelerator Laboratory]] would extend to a size of {{convert|2|mi|km}} and an output energy of 50 GeV.<ref>{{cite book |last1=Neal |first1=R. B. |title=The Stanford Two-Mile Accelerator |chapter=Chap. 5 |publisher=W.A. Benjamin, Inc |year=1968 |___location=New York, New York |page=59 |chapter-url=http://www.slac.stanford.edu/spires/hep/HEPPDF/twomile/Chapters_4_5.pdf |access-date=2010-09-17}}</ref>

As linear accelerators were developed with higher beam currents, using magnetic fields to focus proton and heavy ion beams presented difficulties for the initial stages of the accelerator. Because the [[Lorentz force|magnetic force]] is dependent on the particle velocity, it was desirable to create a type of accelerator which could simultaneously accelerate and focus low-to-mid energy [[hadron]]s.<ref>{{cite journal |last1=Stokes |first1=Richard H. |last2=Wangler |first2=Thomas P. |title=Radiofrequency Quadrupole Accelerators and their Applications |journal=Annual Review of Nuclear and Particle Science |date=1988 |volume=38 |issue=38 |pages=97–118 |doi=10.1146/annurev.ns.38.120188.000525 |bibcode=1988ARNPS..38...97S |url=https://www.annualreviews.org/doi/pdf/10.1146/annurev.ns.38.120188.000525 |access-date=3 February 2022}}</ref> In 1970, Soviet physicists I. M. Kapchinsky and [[Vladimir Teplyakov]] proposed the [[Radio-frequency quadrupole|radio-frequency quadrupole (RFQ)]] type of accelerating structure. RFQs use vanes or rods with precisely designed shapes in a resonant cavity to produce complex electric fields. These fields provide simultaneous acceleration and focusing to injected particle beams.<ref name = "Reiser 2008, p6">{{cite book |title= Theory and design of charged particle beams |last1= Reiser |first1= Martin |edition = 2nd |date= 2008 |publisher= [[Wiley-VCH]] |___location= Weinheim |isbn= 9783527407415 |page=6 |url= https://books.google.com/books?id=eegK9Mqgpi4C}}</ref>

Beginning in the 1960s, scientists at Stanford and elsewhere began to explore the use of [[superconducting radio frequency]] cavities for particle acceleration.<ref>{{cite arXiv | last=Padamsee | first=Hasan | date= April 14, 2020 | title= History of gradient advances in SRF | class=physics.acc-ph | eprint=2004.06720}}</ref> Superconducting cavities made of [[niobium]] alloys allowed for much more efficient acceleration, as a substantially higher fraction of the input power could be applied to the beam, rather than lost to heat. Some of the earliest superconducting linacs included the Superconducting Linear Accelerator (for electrons) at Stanford<ref>{{cite report | first=Catherine | last=Westfall | title=The Prehistory of Jefferson Lab's SRF Accelerating Cavities, 1962 to 1985 | date=April 1997 | publisher=[[Thomas Jefferson National Accelerator Facility]] | docket=JLAB-PHY-97-35 | url=https://misportal.jlab.org/ul/publications/view_pub.cfm?pub_id=11132}}</ref> and the [[Argonne Tandem Linear Accelerator System]] (for protons and heavy ions) at [[Argonne National Laboratory]].<ref>{{cite journal |last1=Ostroumov |first1=Peter |last2=Gerigk |first2=Frank |title=Superconducting Hadron Linacs |journal=Reviews of Accelerator Science and Technology |date=January 2013 |volume=06 |pages=171–196 |doi=10.1142/S1793626813300089}}</ref>