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[[File:Wideroe linac en.svg|thumb|300px|Wideroe's linac concept. The voltage from an RF source is connected to a series of tubes which shield the particle between gaps.]]
In 1924, Gustav Ising published the first description of a linear particle accelerator using a series of accelerating gaps. Particles would proceed down a series of tubes. At a regular frequency, an accelerating voltage would be applied across each gap. As the particles gained speed while the frequency remained constant, the gaps would be spaced farther and farther apart, in order to ensure the particle would see a voltage applied as it reached each gap. Ising never successfully implemented this design.<ref name="heibron">{{cite book |last1=Heilbron |first1=J.L. |last2=Seidel |first2=Robert W. |title=Lawrence and His Laboratory: A History of the Lawrence Berkeley Laboratory, Volume I |date=1989 |publisher=University of California Press |___location=Berkeley, CA |url=http://ark.cdlib.org/ark:/13030/ft5s200764/ |access-date=2 February 2022}}</ref>
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The initial Alvarez type linacs had no strong mechanism for keeping the beam focused, and were limited in length and energy as a result. The development of the [[strong focusing]] principle in the early 1950s led to the installation of focusing [[quadrupole magnet|quadrupole magnets]] inside the drift tubes, allowing for longer and thus more powerful linacs. Two of the earliest examples of Alvarez linacs with strong focusing magnets were built at [[CERN]] and [[Brookhaven National Laboratory]].<ref>{{cite report |author=Lapostolle, Pierre |date= July 1989 |title= Proton Linear Accelerators: A Theoretical and Historical Introduction |url= https://www.osti.gov/servlets/purl/6038195 |publisher= Los Alamos National Laboratory |docket=LA-11601-MS |access-date= February 4, 2022}}</ref>
In 1947, at about the same time that Alvarez was developing his linac concept for protons, [[W. W. Hansen|William Hansen]] constructed the first travelling-wave electron accelerator at Stanford University.<ref>{{cite journal |author-last=Ginzton |author-first=Edward L. |date= April 1983| title=Early Accelerator Work at Stanford |journal= SLAC Beam Line | pages=2-16 |url=http://atlas.physics.arizona.edu/~shupe/Physics_Courses/Phys_586_S2015_S2016_S2017/Readings_MS/SLAC_Early_History.pdf
}}</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>
==Construction and operation==
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