An energy recovery linac (ERL) is a type of

linear particle accelerator A linear particle accelerator (often shortened to linac) is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of Oscillation, oscillating electric potentials along ...

that provides a beam of electrons used to produce x-rays by

synchrotron radiation Synchrotron radiation (also known as magnetobremsstrahlung) is the electromagnetic radiation emitted when relativistic charged particles are subject to an acceleration perpendicular to their velocity (). It is produced artificially in some types ...

. First proposed in 1965 the idea gained interest since the early 2000s.

Spectral radiance

The usefulness of an x-ray beam for scientific experiments depends upon the beam's

spectral radiance In radiometry, spectral radiance or specific intensity is the radiance of a surface per unit frequency or wavelength, depending on whether the Spectral radiometric quantity, spectrum is taken as a function of frequency or of wavelength. The Interna ...

, which tells how much power of a given wavelength is concentrated on a spot. Most scientific literature on x-ray sources uses a closely related term called '' brilliance'', which counts the rate of photons produced, rather than their power. The energy of a photon is inversely proportional to the photon's wavelength. Very high power is usually achieved by delivering the energy in short pulses, allowing the apparatus to work within reasonable power demands and cooling limits. Depending upon the pulse length and repetition rate, the average spectral radiance will be much lower than the peak spectral radiance. The peak spectral radiance and the average spectral radiance are both important properties of an x-ray beam. For some experiments, the peak value is most important, but for other experiments, the average value is most important. As a synchrotron light source, the performance of an energy recovery linac falls between a

storage ring A storage ring is a type of circular particle accelerator in which a continuous or pulsed particle beam may be kept circulating, typically for many hours. Storage of a particular particle depends upon the mass, momentum, and usually the charge o ...

and a

free-electron laser A free-electron laser (FEL) is a fourth generation light source producing extremely brilliant and short pulses of radiation. An FEL functions much as a laser but employs relativistic electrons as a active laser medium, gain medium instead of using ...

(FEL). Energy recovery linacs have high repetition rates and therefore high average spectral radiance, but lower peak spectral radiance than a FEL.

Mechanism

While using a recirculating

charged particle beam A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same position, kinetic energy (resulting in the same velocity), and direction. The kinetic energies of the particles are much lar ...

with a magnet lattice resembling that of a

, each particle travels through the recirculating arc before being decelerated in a

linac A linear particle accelerator (often shortened to linac) is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of oscillating electric potentials along a linear bea ...

structure. The same linac structure also accelerates new low-energy particles that are continuously injected into the linac. Thus, instead of recycling the particle beam continuously, while its emittance increases by

emission, only its kinetic energy is recycled, enabling a low beam emittance while maintaining high repetition rates comparable to

synchrotron A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The strength of the magnetic field which bends the particle beam i ...

s. #Charged particles (usually electrons) are injected into a linear accelerator (linac), where the particles are accelerated by a radio frequency (RF) field. #The beam of accelerated particles exits the linac, and passes through a series of magnets, that guides the beam back to the beginning of the linac. #The length of the beam path is such that the returning particles are about 180 degrees out of phase with particles being accelerated by the linac. #The phase difference causes the returning particles to be decelerated, while newly injected particles are accelerated. The kinetic energy of the decelerated particles increases the intensity of the RF field, which is used by the particles being accelerated.

Energy Recovery Linacs around the world

Brookhaven National Laboratory

BNL-ERL is aimed at 500mA at 20MeV. It is now under commissioning at the Collider Accelerator Department at Brookhaven National Laboratory. One of the main feature of this ERL is a superconducting laser photocathode RF gun powered by a 1MW CW klystron and equipped with a load-lock system for the insertion of high quantum efficiency photocathodes. This ERF gun will provide high brightness electron beams at an unprecedented average power. The objective of this ERL is to serve as a platform for R&D into high current ERL. In particular issues of halo generation and control, Higher-order mode issues, coherent emissions for the beam and high brightness, high power beam generation and preservation. Following its completion we plan to use it for various applications, such as the generation of THz radiation and high power x rays through compton scattering of laser light off its electron beam.

Cornell University

Cornell University, in partnership with Brookhaven National Laboratory, are in the process of constructing CBETA, an ERL built using FFAG optics and superconducting RF cavities, targeting up to 100 mA of CW electron beam at up to 150 MeV, as part of a research program for a future electron-ion collider.

A project to improve CERN's LHC into an LHeC

A recent study suggests to improve

CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in Meyrin, western suburb of Gene ...

's Large Hadron Collider (

LHC The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008, in collaboration with over 10,000 scientists, and ...

), the largest accelerator existing at present (2013), by adding to the large storage ring of the LHC a tangential construction of two electron Energy recovery linacs, each of 1008 m length, producing thus the possibility to obtain not only Hadron-Hadron smashes, but also, e.g., Hadron-Electron ones, and thus to improve the LHC into some kind of " LHeC". For this suggestion, originating from a special committee of CERN physicists, M. Klein (Liverpool university), on the suggestion of the UK's

Institute of Physics The Institute of Physics (IOP) is a UK-based not-for-profit learned society and professional body that works to advance physics education, physics research, research and applied physics, application. It was founded in 1874 and has a worldwide ...

, received the 2013 mutual Max Born Prize of the British and the German Physical Societies.O. Brüning, M. Klein: ''Preparations for a future Proton-Lepton-collider in the TeV center of mass system''
, CERN, internal report; called 2013, 17 Sept.

References

{{Reflist Accelerator physics