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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 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
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>
==Basic principles of operation==
[[Image:Linear accelerator animation 16frames 1.6sec.gif|thumb|upright=2.5|Animation showing how a linear accelerator works. In this example the particles accelerated (red dots) are assumed to have a positive charge. The graph ''V''(x) shows the [[electrical potential]] along the axis of the accelerator at each point in time. The polarity of the RF voltage reverses as the particle passes through each electrode, so when the particle crosses each gap the electric field ''(E, arrows)'' has the correct direction to accelerate it. The animation shows a single particle being accelerated each cycle; in actual linacs a large number of particles are injected and accelerated each cycle. The action is shown slowed enormously.]]
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=== Radiofrequency acceleration ===
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=== Induction linear accelerator ===
Induction linear accelerators use the electric field induced by a time-varying magnetic field for acceleration—like the [[betatron]]. The particle beam passes through a series of ring-shaped [[ferrite core]]s standing one behind the other, which are magnetized by high-current pulses, and in turn each generate an electrical field strength pulse along the axis of the beam direction. Induction linear accelerators are considered for short high current pulses from electrons but also from heavy ions.<ref>{{Cite web|date=2002-06-25|title=Heavy ions offer a new approach to fusion|url=https://cerncourier.com/a/heavy-ions-offer-a-new-approach-to-fusion/|access-date=2021-01-22|website=CERN Courier|language=en-GB}}</ref> The concept goes back to the work of [[Nicholas Christofilos]].<ref>{{Cite journal|last1=Christofilos|first1=N. C.|last2=Hester|first2=R. E.|last3=Lamb|first3=W. a. S.|last4=Reagan|first4=D. D.|last5=Sherwood|first5=W. A.|last6=Wright|first6=R. E.|date=1964-07-01|title=High Current Linear Induction Accelerator for Electrons|url=https://aip.scitation.org/doi/10.1063/1.1746846|journal=Review of Scientific Instruments|volume=35|issue=7|pages=886–890|doi=10.1063/1.1746846|bibcode=1964RScI...35..886C|issn=0034-6748|url-access=subscription}}</ref> Its realization is highly dependent on progress in the development of more suitable [[Ferrite (magnet)|ferrite]] materials. With electrons, pulse currents of up to 5 kiloamps at energies up to 5 MeV and pulse durations in the range of 20 to 300 nanoseconds were achieved.<ref>{{Cite journal|last=Fraas|first=H.|date=1989|title=Kern- und Elementarteilchenphysik. Von G. Musiol, J. Ranft, R. Reif und D. Seeliger, VCH Verlagsgesellschaft Weinheim, 1988, DM 128|url=http://adsabs.harvard.edu/abs/1989PhuZ...20...31F|journal=Physik in unserer Zeit|volume=20|issue=1|pages=31|doi=10.1002/piuz.19890200109|bibcode=1989PhuZ...20...31F|issn=0031-9252}}</ref>
=== Energy recovery linac ===
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The [[Brookhaven National Laboratory]] and the [[Helmholtz-Zentrum Berlin]] with the project "bERLinPro" reported on corresponding development work. The Berlin experimental accelerator uses superconducting niobium cavity resonators. In 2014, three [[free-electron laser]]s based on ERLs were in operation worldwide: in the [[Thomas Jefferson National Accelerator Facility|Jefferson Lab]] (US), in the [[Budker Institute of Nuclear Physics]] (Russia) and at JAEA (Japan).<ref>{{Cite book|url=https://www.springer.com/gp/book/9783319143934|title=Synchrotron Light Sources and Free-Electron Lasers: Accelerator Physics, Instrumentation and Science Applications|date=2016|publisher=Springer International Publishing|isbn=978-3-319-14393-4|editor-last=Jaeschke|editor-first=Eberhard|language=en|editor-last2=Khan|editor-first2=Shaukat|editor-last3=Schneider|editor-first3=Jochen R.|editor-last4=Hastings|editor-first4=Jerome B.}}</ref> At the [[University of Mainz]], an ERL called MESA is expected to begin operation in 2024.
<ref>{{cite journal |last1=Hug |first1=Florian |last2=Aulenbacher |first2=Kurt |last3=Heine |first3=Robert |last4=Ledroit |first4=Ben |last5=Simon |first5=Daniel |title=MESA - an ERL Project for Particle Physics Experiments |journal=Proceedings of the 28th Linear Accelerator Conf. |date=2017 |volume=LINAC2016 |pages=
=== Compact Linear Collider ===
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The acceleration concepts used today for ''ions'' are always based on electromagnetic [[standing wave]]s that are formed in suitable [[resonator]]s. Depending on the type of particle, energy range and other parameters, very different types of resonators are used; the following sections only cover some of them. ''Electrons'' can also be accelerated with standing waves above a few MeV. An advantageous alternative here, however, is a progressive wave, a traveling wave. The [[phase velocity]] the traveling wave must be roughly equal to the particle speed. Therefore, this technique is only suitable when the particles are almost at the speed of light, so that their speed only increases very little.
The development of high-frequency oscillators and power amplifiers from the 1940s, especially the klystron, was essential for these two acceleration techniques . The first larger linear accelerator with standing waves - for protons - was built in 1945/46 in the [[Lawrence Berkeley National Laboratory]] under the direction of [[Luis Walter Alvarez|Luis W. Alvarez]]. The frequency used was {{frequency|200|MHz}}. The first electron accelerator with traveling waves of around {{frequency|2|GHz}} was developed a little later at [[Stanford University]] by [[W. W. Hansen|W.W. Hansen]] and colleagues.<ref>{{Cite journal|last1=Ginzton|first1=E. L.|last2=Hansen|first2=W. W.|last3=Kennedy|first3=W. R.|date=1948-02-01|title=A Linear Electron Accelerator|url=https://aip.scitation.org/doi/10.1063/1.1741225|journal=Review of Scientific Instruments|volume=19|issue=2|pages=89–108|doi=10.1063/1.1741225|pmid=18908606|bibcode=1948RScI...19...89G|issn=0034-6748|url-access=subscription}}</ref>
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[[File:Aerial view of a model of a linear accelerator.jpg|thumb|Aerial view of the Little LINAC Model]]
In 2019 a Little Linac model kit, containing 82 building blocks, was developed for children undergoing radiotherapy treatment for cancer. The hope is that building the model will alleviate some of the stress experienced by the child before undergoing treatment by helping them to understand what the treatment entails. The kit was developed by Professor David Brettle, [[Institute of Physics and Engineering in Medicine]] (IPEM) in collaboration with manufacturers Best-Lock Ltd. The model can be seen at the [[Science Museum, London]].
An MR-LINAC is a medical linac integrated with a [[Magnetic resonance imaging]] scanner, which allows for real-time imaging during treatment, as well as patient motion management, and on-table adaptive planning.<ref>{{Cite journal |last1=Ng |first1=John |last2=Gregucci |first2=Fabiana |last3=Pennell |first3=Ryan T. |last4=Nagar |first4=Himanshu |last5=Golden |first5=Encouse B. |last6=Knisely |first6=Jonathan P. S. |last7=Sanfilippo |first7=Nicholas J. |last8=Formenti |first8=Silvia C. |date=2023-01-27 |title=MRI-LINAC: A transformative technology in radiation oncology |journal=Frontiers in Oncology |volume=13 |doi=10.3389/fonc.2023.1117874 |doi-access=free |issn=2234-943X |pmc=9911688 |pmid=36776309}}</ref>
==Application for medical isotope development==
The expected shortages of [[Mo-99]], and the [[technetium-99m]] [[medical isotope]] obtained from it, have also shed light onto linear accelerator technology to produce Mo-99 from non-enriched [[Uranium]] through neutron bombardment. This would enable the medical isotope industry to manufacture this crucial isotope by a sub-critical process. The aging facilities, for example the [[Chalk River Laboratories]] in Ontario, Canada, which still now produce most Mo-99 from [[Enriched uranium#Highly enriched uranium (HEU)|highly enriched uranium]] could be replaced by this new process. In this way, the sub-critical loading of soluble [[Uranyl nitrate|uranium salts]] in [[heavy water]] with subsequent photo neutron bombardment and extraction of the target product, Mo-99, will be achieved.<ref>Gahl and Flagg (2009).[http://www.slideshare.net/Flagg707/subcritical-fission-mo99-production Solution Target Radioisotope Generator Technical Review]. Subcritical Fission Mo99 Production. Retrieved 6 January 2013.</ref>{{better source needed|date=August 2020}}
==Disadvantages==
*The device length limits the locations where one may be placed.<ref name="Pichoff">{{cite journal |last1=Pichoff |first1=N. |title=Introduction to RF Linear accelerators |journal=CAS - CERN Accelerator School: Intermediate Accelerator Physics |date=2006 |pages=105–128 |doi=10.5170/CERN-2006-002.105 |url=https://cds.cern.ch/record/941324/files/p105.pdf |access-date=6 March 2025}}</ref>
*A great number of driver devices and their associated power supplies are required, increasing the construction and maintenance expense of this portion.{{r|Pichoff}}
*If the walls of the accelerating cavities are made of normally conducting material and the accelerating fields are large, the wall resistivity converts electric energy into heat quickly.
*[[Superconductivity| * Due to these limitations, high energy accelerators such as [[SLAC]], still the longest in the world (in its various generations), are run in short pulses, limiting the average current output and forcing the experimental detectors to handle data coming in short bursts. ==See also==
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