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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|hadrons]].<ref>{{cite journal |last1=Stokes |first1=Richard H. |last2=Wangler |first2=Thomas P. |title=Radiofrequency Quadrupole Accelerators and their Applications |journal=Annual Reviews of Nuclear and Particle Science |date=1988 |volume=38 |pages=97-118 |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 | 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>
==Construction and operation==
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