Editing Linear particle accelerator (section)

==History==

[[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.]]
[[File:CERN Linac1.jpg|thumb|Alvarez type linac]]
In 1924, [[Gustaf Ising|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>

[[Rolf Wideroe]] discovered Ising's paper in 1927, and as part of his PhD thesis he built an 88-inch long, two gap version of the device. Where Ising had proposed a spark gap as the voltage source, Wideroe used a 25kV [[vacuum tube]] oscillator. He successfully demonstrated that he had accelerated sodium and potassium ions to an energy of 50,000 [[electron volt]]s (50 keV), twice the energy they would have received if accelerated only once by the tube. By successfully accelerating a particle multiple times using the same voltage source, Wideroe demonstrated the utility of [[radio frequency]] (RF) acceleration.<ref>{{cite book |last1=Conte |first1=Mario |last2=MacKay |first2=William |title=An introduction to the physics of particle accelerators |date=2008 |publisher=World Scientific |location=Hackensack, N.J. |isbn=9789812779601 |edition=2nd}}</ref>

This type of linac was limited by the voltage sources that were available at the time, and it was not until after [[World War II]] that [[Luis Walter Alvarez|Luis Alvarez]] was able to use newly developed high frequency oscillators to design the first resonant cavity drift tube linac. An Alvarez linac differs from the Wideroe type in that the RF power is applied to the entire [[Resonator#Cavity resonators|resonant chamber]] through which the particle travels, and the central tubes are only used to shield the particles during the decelerating portion of the oscillator's phase. Using this approach to acceleration meant that Alvarez's first linac was able to achieve proton energies of 31.5 MeV in 1947, the highest that had ever been reached at the time.<ref>{{cite web |title=Alvarez proton linear accelerator |url=https://www.si.edu/es/object/alvarez-proton-linear-accelerator%3Anmah_700150 |website=Smithsonian Institution |access-date=3 February 2022 }}</ref>

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]]s 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>

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 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 |doi-access=free }}</ref> In 1970, Soviet physicists I. M. Kapchinsky and [[Vladimir Teplyakov]] proposed the [[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 allow 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>