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==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 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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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]].
==Application for medical isotope development==
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