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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>{{Cite journal|author=G. Ising
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]].
The design of a linac depends on the type of particle that is being accelerated: [[electron]]s, [[proton]]s or
==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.]]
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.
[[Rolf Wideroe]] discovered Ising's paper in 1927, and as part of his PhD thesis
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.
The initial Alvarez type linacs had no strong mechanism for keeping the beam focused
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>
==Basic principles of operation==
==Construction and 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.]]
<div style="clear: both"></div>
=== Radiofrequency acceleration ===
When a [[charged particle]] is placed in an [[electromagnetic field]] it experiences a force given by the [[Lorentz force]] law:
:<math>\vec{F} = q \vec{E} + q \vec{v} \times \vec{B}</math>
where <math>q</math> is the charge on the particle, <math>\vec{E}</math> is the electric field, <math>\vec{v}</math> is the particle velocity, and <math>\vec{B}</math> is the magnetic field. The cross product in the magnetic field term means that static magnetic fields cannot be used for particle acceleration, as the magnetic force acts perpendicularly to the direction of particle motion.<ref name="conte">{{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 |pages=1 |edition=2nd}}</ref>
As [[Electrical breakdown|electrostatic breakdown]] limits the maximum constant voltage which can be applied across a gap to produce an electric field, most accelerators use some form of RF acceleration. In RF acceleration, the particle traverses a series of accelerating regions, driven by a source of voltage in such a way that the particle sees an accelerating field as it crosses each region. In this type of acceleration, particles must necessarily travel in "bunches" corresponding to the portion of the oscillator's cycle where the electric field is pointing in the intended direction of acceleration.<ref name="edwards">{{cite book |last1=Edwards |first1=D. A. |last2=Syphers |first2=M.J. |title=An introduction to the physics of high energy accelerators |date=1993 |publisher=Wiley |___location=New York |isbn=9780471551638}}</ref>
If a single oscillating voltage source is used to drive a series of gaps, those gaps must be placed increasingly far apart as the speed of the particle increases. This is to ensure that the particle "sees" the same phase of the oscillator's cycle as it reaches each gap. As particles asymptotically approach the speed of light, the gap separation becomes constant: additional applied force increases the energy of the particles but does not significantly alter their speed.{{r|conte|p=9-12}}
=== Focusing ===
In order to ensure particles do not escape the accelerator, it is necessary to provide some form of focusing to redirect particles moving away from the central trajectory back towards the intended path. With the discovery of [[strong focusing]], [[quadrupole magnets]] are used to actively redirect particles moving away from the reference path. As quadrupole magnets are focusing in one transverse direction and defocusing in the perpendicular direction, it is necessary to use groups of magnets to provide an overall focusing effect in both directions.{{r|conte}}
====Phase stability====
Focusing along the direction of travel, also known as ''phase stability'', is an inherent property of RF acceleration. If the particles in a bunch all reach the accelerating region during the rising phase of the oscillating field, then particles which arrive early will see slightly less voltage than the "reference" particle at the center of the bunch. Those particles will therefore receive slightly less acceleration and eventually fall behind the reference particle. Correspondingly, particles which arrive after the reference particle will receive slightly more acceleration, and will catch up to the reference as a result. This automatic correction occurs at each accelerating gap, so the bunch is refocused along the direction of travel each time it is accelerated.{{r|edwards|pp=30-52}}
▲==Construction and operation==
[[Image:Aust.-Synchrotron,-Quadrupole-Magnets-of-Linac,-14.06.2007.jpg|250px|right|thumb|[[Quadrupole magnet]]s surrounding the linac of the [[Australian Synchrotron]] are used to help [[Focus (optics)|focus]] the electron beam]]
[[File:SLAC_National_Accelerator_Laboratory_Aerial_2.png|thumb|Building covering the 2 mile (3.2 km) beam tube of the [[Stanford Linear Accelerator]] (SLAC) at Menlo Park, California, the second most powerful linac in the world. It has about 80,000 accelerating electrodes and could accelerate electrons to 50 [[GeV]] ]]
A linear particle accelerator consists of the following parts:
*A straight hollow pipe [[vacuum chamber]] which contains the other components.
*The particle source ''(S)'' at one end of the chamber which produces the [[charged particle]]s which the machine accelerates.
*Extending along the pipe from the source is a series of open-ended cylindrical electrodes ''(C1, C2, C3, C4)'', whose length increases progressively with the distance from the source. The particles from the source pass through these electrodes.
*A target
*An [[electronic oscillator]] and [[amplifier]] ''(G)'' which generates a
As shown in the animation, the oscillating voltage applied to alternate cylindrical electrodes has opposite polarity (180° [[out of phase]]), so adjacent electrodes have opposite voltages. This creates an oscillating [[electric field]] ''(E)'' in the gap between each pair of electrodes, which exerts force on the particles when they pass through, imparting energy to them by accelerating them.
The particles are injected at the right time so that the oscillating voltage differential between electrodes is maximum as the particles cross each gap.
:<math>E = qNV_p</math>
electron volts, where <math>N</math> is the number of accelerating electrodes in the machine.
At speeds near the speed of light, the incremental velocity increase will be small, with the energy appearing as an increase in the mass of the particles. In portions of the accelerator where this occurs, the tubular electrode lengths will be almost constant.
== Concepts in development ==
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=== Induction linear accelerator ===
Induction linear accelerators use the electric field induced by a time-varying magnetic field for
=== Energy
In previous electron linear accelerators, the accelerated particles are used only once and then fed into an absorber ''(beam dump)'', in which their residual energy is converted into heat. In an
The [[Brookhaven National Laboratory]]
<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=313–316 |doi=10.18429/JACoW-LINAC2016-MOP106012 |url=https://inspirehep.net/literature/1633150 |access-date=18 August 2024}}</ref>
=== Compact Linear Collider ===
The concept of the [[Compact Linear Collider]] (CLIC) (original name
=== Kielfeld accelerator (plasma accelerator) ===
In cavity resonators, the dielectric strength limits the maximum acceleration that can be achieved within a certain distance. This limit can be circumvented using accelerated waves in plasma to generate the accelerating field in [[Plasma acceleration|Kielfeld accelerators]]: A laser or particle beam excites an oscillation in a [[Plasma (physics)|plasma]], which is associated with very strong electric field strengths. This means that significantly (factors of 100s to 1000s ) more compact linear accelerators can possibly be built. Experiments involving high power lasers in metal vapour plasmas suggest that a beam line length reduction from some tens of metres to a few cm is quite possible.
===Compact
The LIGHT program (Linac
The project aim is to make proton therapy a more accessible mainstream medicine as an alternative to existing radio therapy.
== Modern
The higher the frequency of the acceleration voltage selected, the more individual acceleration thrusts per path length a particle of a given speed experiences, and the shorter the accelerator can therefore be overall. That is why accelerator technology developed in the pursuit of higher particle energies, especially towards higher frequencies.
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When it comes to energies of more than a few MeV, accelerators for ions are different from those for electrons. The reason for this is the large mass difference between the particles. Electrons are already close to the [[speed of light]], the absolute speed limit, at a few MeV; with further acceleration, as described by [[relativistic mechanics]], almost only their energy and [[momentum]] increase. On the other hand, with ions of this energy range, the speed also increases significantly due to further acceleration.
The acceleration concepts used today for ''ions'' are always based on electromagnetic [[
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>
{| class="wikitable"
|+Principle of the acceleration of particle packets
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[[File:Linear Accelerator.jpg|thumb|right|250px|Steel casting undergoing x-ray using the linear accelerator at [[Goodwin Steel Castings Ltd]]]]
The linear accelerator could produce higher particle energies than the previous [[electrostatic particle accelerator]]s (the [[
High power linacs are also being developed for production of electrons at relativistic speeds, required since fast electrons traveling in an arc will lose energy through [[synchrotron radiation]]; this limits the maximum power that can be imparted to electrons in a synchrotron of given size. Linacs are also capable of prodigious output, producing a nearly continuous stream of particles, whereas a synchrotron will only periodically raise the particles to sufficient energy to merit a "shot" at the target. (The burst can be held or stored in the ring at energy to give the experimental electronics time to work, but the average output current is still limited.) The high density of the output makes the linac particularly attractive for use in loading storage ring facilities with particles in preparation for particle to particle collisions. The high mass output also makes the device practical for the production of [[antimatter]] particles, which are generally difficult to obtain, being only a small fraction of a target's collision products. These may then be stored and further used to study matter-antimatter annihilation.
===Medical linacs===
[[Image:External beam radiotherapy retinoblastoma nci-vol-1924-300.jpg|thumb|left|Historical image showing Gordon Isaacs, the first patient treated for [[retinoblastoma]] with linear accelerator radiation therapy (in this case an electron beam), in 1957, in the U.S. Other patients had been treated by linac for other diseases since 1953 in the UK. Gordon's right eye was removed on January 11, 1957 because cancer had spread there. His left eye, however, had only a localized tumor that prompted [[Henry Kaplan (physician)|Henry Kaplan]] to treat it with the electron beam.]]
Linac-based [[radiation therapy]] for cancer treatment began with the first patient treated in 1953 in London, UK, at the [[Hammersmith Hospital]], with an 8 MV machine built by [[Metropolitan-Vickers]] and installed in 1952, as the first dedicated medical linac.<ref>{{Cite journal|author=Thwaites, DI and Tuohy J
[[Medical linear accelerators]] accelerate electrons using a tuned-cavity waveguide, in which the RF power creates a [[standing wave]]. Some linacs have short, vertically mounted waveguides, while higher energy machines tend to have a horizontal, longer waveguide and a bending magnet to turn the beam vertically towards the patient. Medical linacs use monoenergetic electron beams between 4 and 25 MeV, giving an X-ray output with a spectrum of energies up to and including the electron energy when the electrons are directed at a high-density (such as [[tungsten]]) target. The electrons or X-rays can be used to treat both benign and malignant disease. The LINAC produces a reliable, flexible and accurate radiation beam. The versatility of LINAC is a potential advantage over [[cobalt therapy]] as a treatment tool. In addition, the device can simply be powered off when not in use; there is no source requiring heavy shielding – although the treatment room itself requires considerable shielding of the walls, doors, ceiling etc. to prevent escape of scattered radiation. Prolonged use of high powered (>18 MeV) machines can induce a significant amount of radiation within the metal parts of the head of the machine after power to the machine has been removed (i.e. they become an active source and the necessary precautions must be observed).
[[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
==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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==External links==
{{
*[https://web.archive.org/web/20110726014831/http://www.ionactive.co.uk/multi-media_video.html?m=8 Linear Particle Accelerator (LINAC) Animation by Ionactive ]
*[http://www.rcp.ijs.si/mic/general/accelerator.php 2MV Tandetron linear particle accelerator in Ljubljana, Slovenia]
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[[Category:Accelerator physics]]
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