Linear particle accelerator: Difference between revisions

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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 [[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]]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 |pages=97–118 |doi=10.1146/annurev.ns.38.120188.000525 |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 | class=physics.acc-ph | 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>

==Basic principles of operation==

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A linear particle accelerator consists of the following parts:

*A straight hollow pipe [[vacuum chamber]] which contains the other components. It is evacuated with a [[vacuum pump]] so that the accelerated particles will not collide with air molecules. The length will vary with the application. If the device is used for the production of X-rays for inspection or therapy the pipe may be only 0.5 to 1.5 meters long.<ref>{{cite book |last1=Podgorsak |first1=E B |title=Radiation Oncology Physics |date=2005 |publisher=[[International Atomic Energy Agency]] |___location=Vienna |isbn=92-0-107304-6 |page=138 |url=https://www.iaea.org/publications/7086/radiation-oncology-physics |language=en |chapter=Treatment Machines for External Beam Radiotherapy}}</ref> If the device is to be an injector for a [[synchrotron]] it may be about ten meters long.<ref>{{cite conference |url=https://s3.cern.ch/inspire-prod-files-b/bf2698b68b2ac10829c592a2137d3ee2 |title=Linear Accelerator Injectors for Proton Synchrotrons |first=J P |last=Blewett |author= |author-link= |date=11 June 1956 |conference=CERN Symposium on High Energy Accelerators and Pion Physics |conference-url= |editor=Edouard Regenstreif |others= |volume=1 |edition= |book-title= |publisher=CERN |archive-url= |archive-date= |___location=Geneva |pages=159–166 |format= |id= |isbn= |bibcode= |oclc= |doi=10.5170/CERN-1956-025 |access-date= |quote= |ref= |postscript= |language= |page= |at= |trans-title=}}</ref> If the device is used as the primary accelerator for nuclear particle investigations, it may be several thousand meters long.<ref>{{cite web |last1=Weise |first1=Hans |last2=Decking |first2=Winfried |title=The world’sworld's longest superconducting linac |url=https://cerncourier.com/a/the-worlds-longest-superconducting-linac/ |website=[[CERN Courier]] |publisher=IOP Publishing |date=10 July 2017}}</ref>

*The particle source ''(S)'' at one end of the chamber which produces the [[charged particle]]s which the machine accelerates. The design of the source depends on the particle that is being accelerated. [[Electron]]s are generated by a [[cold cathode]], a [[hot cathode]], a [[photocathode]], or [[RF antenna ion source|radio frequency (RF) ion sources]]. [[Proton]]s are generated in an [[ion source]], which can have many different designs. If heavier particles are to be accelerated, (e.g., [[uranium]] [[ions]]), a specialized ion source is needed. The source has its own high voltage supply to inject the particles into the beamline.<ref>{{cite arXiv |last=Faircloth |first=D C |author-link= |eprint=2103.13231 |title=Particle Sources |class= physics.acc-ph|date=24 March 2021 }}</ref>

*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. The length of each electrode is determined by the frequency and power of the driving power source and the particle to be accelerated, so that the particle passes through each electrode in exactly one-half cycle of the accelerating voltage. The mass of the particle has a large effect on the length of the cylindrical electrodes; for example an electron is considerably lighter than a proton and so will generally require a much smaller section of cylindrical electrodes as it accelerates very quickly.

*A target ''(not shown)'' with which the particles collide, located at the end of the accelerating electrodes. If electrons are accelerated to produce [[X-rays]] then a water cooled tungsten target is used. Various target materials are used when [[proton]]s or other nuclei are accelerated, depending upon the specific investigation. Behind the target are various detectors to detect the particles resulting from the collision of the incoming particles with the atoms of the target. Many linacs serve as the initial accelerator stage for larger particle accelerators such as [[synchrotron]]s and [[storage ring]]s, and in this case after leaving the electrodes the accelerated particles enter the next stage of the accelerator.