Editing Linear particle accelerator (section)

== Modern concepts ==
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.

The linear accelerator concepts (often called accelerator structures in technical terms) that have been used since around 1950 work with frequencies in the range from around {{frequency|100|MHz}} to a few gigahertz (GHz) and use the electric field component of electromagnetic waves.

=== Standing waves and traveling waves ===
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 [[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> 
{| class="wikitable"
|+Principle of the acceleration of particle packets
|[[File:Linac schematic (standing wave).gif|center|thumb|220x220px|by a standing wave]]
|[[File:Linac schematic (travelling wave).gif|center|thumb|222x222px|by a traveling wave]]
|}
In the two diagrams, the curve and arrows indicate the force acting on the particles. Only at the points with the correct direction of the electric field vector, i.e. the correct direction of force, can particles absorb energy from the wave. (An increase in speed cannot be seen in the scale of these images.)