Afree-electron laser (FEL) is a fourth generation light source producing extremely brilliant and short pulses of radiation. An FEL functions much as alaser but employs relativistic electrons as again medium instead of usingstimulated emission from atomic or molecular excitations.[1][2] In an FEL, abunch of electrons passes through a magnetic structure called anundulator orwiggler to generate radiation, which re-interacts with the electrons to make them emit coherently, exponentially increasing its intensity.
As electron kinetic energy and undulator parameters can be adapted as desired, free-electron lasers aretunable and can be built for a widerfrequency range than any other type of laser,[3] currently ranging inwavelength frommicrowaves, throughterahertz radiation andinfrared, to thevisible spectrum,ultraviolet, andX-ray.[4]
The first free-electron laser was developed byJohn Madey in 1971 atStanford University[5] using technology developed byHans Motz and his coworkers, who built anundulator atStanford in 1953,[6][7] using thewiggler magnetic configuration. Madey used a 43 MeV electron beam[8] and 5 m long wiggler to amplify a signal.
To create an FEL, anelectron gun is used. A beam ofelectrons is generated by a shortlaser pulse illuminating aphotocathode located inside amicrowave cavity and accelerated to almost thespeed of light in a device called aphotoinjector. The beam is further accelerated to a design energy by aparticle accelerator, usually alinear particle accelerator. Then the beam passes through a periodic arrangement ofmagnets with alternatingpoles across the beam path, which creates a side to sidemagnetic field. The direction of the beam is called the longitudinal direction, while the direction across the beam path is called transverse. This array of magnets is called anundulator or awiggler, because theLorentz force of the field forces the electrons in the beam to wiggle transversely, traveling along asinusoidal path about the axis of the undulator.
The transverse acceleration of the electrons across this path results in the release ofphotons, which are monochromatic but still incoherent, because the electromagnetic waves from randomly distributed electrons interfere constructively and destructively in time. The resulting radiation power scales linearly with the number of electrons. Mirrors at each end of the undulator create anoptical cavity, causing the radiation to formstanding waves, or alternately an external excitation laser is provided.The radiation becomes sufficiently strong that the transverseelectric field of the radiation beam interacts with the transverse electron current created by the sinusoidal wiggling motion, causing some electrons to gain and others to lose energy to the optical field via theponderomotive force.
This energy modulation evolves into electron density (current) modulations with a period of one optical wavelength. The electrons are thus longitudinally clumped intomicrobunches, separated by one optical wavelength along the axis. Whereas an undulator alone would cause the electrons to radiate independently (incoherently), the radiation emitted by the bunched electrons is in phase, and the fields add togethercoherently.
The radiation intensity grows, causing additional microbunching of the electrons, which continue to radiate in phase with each other.[9] This process continues until the electrons are completely microbunched and the radiation reaches a saturated power several orders of magnitude higher than that of the undulator radiation.
The wavelength of the radiation emitted can be readily tuned by adjusting the energy of the electron beam or the magnetic-field strength of the undulators.
FELs are relativistic machines since the electrons driving the laser move at relativistic speeds—that is, at velocities very close to thespeed of light. The wavelength of the emitted radiation,, is given by[10]
or when the wiggler strength parameterK, discussed below, is small
where is the undulator wavelength (the spatial period of the magnetic field), is the relativisticLorentz factor and the proportionality constant depends on the undulator geometry and is of the order of 1.
This formula can be understood as a combination of two relativistic effects. Imagine you are sitting on an electron passing through the undulator. Due toLorentz contraction the undulator is shortened by a factor and the electron experiences much shorter undulator wavelength. However, the radiation emitted at this wavelength is observed in the laboratory frame of reference and therelativistic Doppler effect brings the second factor to the above formula. In an X-ray FEL the typical undulator wavelength of 1 cm is transformed to X-ray wavelengths on the order of 1 nm by ≈ 2000, i.e. the electrons have to travel with the speed of 0.9999998c.
K, adimensionless parameter, defines the wiggler strength as the relationship between the length of a period and the radius of bend,[citation needed]
where is the bending radius, is the applied magnetic field, is the electron mass, and is theelementary charge.
Expressed in practical units, the dimensionless undulator parameter is.
In most cases, the theory ofclassical electromagnetism adequately accounts for the behavior of free electron lasers.[11] For sufficiently short wavelengths, quantum effects of electron recoil andshot noise may have to be considered.[12]
Free-electron lasers require the use of an electronaccelerator with its associated shielding, as accelerated electrons can be a radiation hazard if not properly contained. These accelerators are typically powered byklystrons, which require a high-voltage supply. The electron beam must be maintained in avacuum, which requires the use of numerousvacuum pumps along the beam path. While this equipment is bulky and expensive, free-electron lasers can achieve very high peak powers, and the tunability of FELs makes them highly desirable in many disciplines, including chemistry, structure determination of molecules in biology,medical diagnosis, andnondestructive testing.
TheFritz Haber Institute in Berlin completed a mid-infrared andterahertz FEL in 2013.[13][14]
AtHelmholtz-Zentrum Dresden - Rossendorf two terahertz and mid-infrared FEL-based sources are in operation. FELBE is an FEL equipped with a cavity with continuous pulsing with a repetition rate of 13 MHz, pulsing with 1 kHz by applying a pulse picker, and macrobunch operation with bunch length > 100 μs and macrobunch repetition rates ≤ 25 Hz. Pulse duration and pulse energy vary with wavelength and lie in the range from 1 - 25 ps and 100 nJ - few μJ, respectively.[15] The TELBE facility is based on asuperradiant undulator offering THz pulses ranging from 0.1 THz to 2.5 THz at repetition rates up to 500 kHz.[16]
The lack ofmirror materials that can reflectextreme ultraviolet andx-rays means that X-ray free electron lasers (XFEL) need to work without aresonant cavity. Consequently, in an X-ray FEL (XFEL) the beam is produced by a single pass of radiation through theundulator. This requires that there be enough amplification over a single pass to produce an appropriate beam.
Hence, XFELs use long undulator sections that are tens or hundreds of meters long. This allows XFELs to produce the brightest X-ray pulses of any human-made x-ray source. The intense pulses from the X-ray laser lies in the principle ofself-amplified spontaneous emission (SASE), which leads to microbunching. Initially all electrons are distributed evenly and emit only incoherent spontaneous radiation. Through the interaction of this radiation and the electrons'oscillations, they drift into microbunches separated by a distance equal to one radiation wavelength. This interaction drives all electrons to begin emitting coherent radiation. Emitted radiation can reinforce itself perfectly whereby wave crests and wave troughs are optimally superimposed on one another. This results in an exponential increase of emitted radiation power, leading to high beam intensities and laser-like properties.[17]
Examples of facilities operating on the SASE FEL principle include the:
In 2022, an upgrade toStanford University'sLinac Coherent Light Source (LCLS-II) used temperatures around −271 °C to produce 106 pulses/second of near light-speed electrons, using superconducting niobium cavities.[19]
One problem with SASE FELs is the lack oftemporal coherence due to anoisy startup process. To avoid this, one can "seed" an FEL with a laser tuned to the resonance of the FEL. Such a temporally coherent seed can be produced by more conventional means, such as byhigh harmonic generation (HHG) using an optical laser pulse. This results in coherent amplification of the input signal; in effect, the output laser quality is characterized by the seed. While HHG seeds are available atwavelengths down to the extreme ultraviolet, seeding is not feasible atx-ray wavelengths due to the lack of conventional x-ray lasers.
In late 2010, in Italy, the seeded-FEL source FERMI@Elettra[20] started commissioning, at theTrieste Synchrotron Laboratory. FERMI@Elettra is a single-pass FEL user-facility covering the wavelength range from 100 nm (12 eV) to 10 nm (124 eV), located next to the third-generation synchrotron radiation facility ELETTRA in Trieste, Italy.
In 2001, atBrookhaven national laboratory, a seeding technique called "High-Gain Harmonic-Generation" that works to X-ray wavelength has been developed.[21] The technique, which can be multiple-staged in an FEL to achieve increasingly shorter wavelengths, utilizes a longitudinal shift of the radiation relative to the electron bunch to avoid the reduced beam quality caused by a previous stage. This longitudinal staging along the beam is called "Fresh-Bunch".[22] This technique was demonstrated at x-ray wavelength[23][24] atTrieste Synchrotron Laboratory.
A similar staging approach, named "Fresh-Slice", was demonstrated at thePaul Scherrer Institut, also at X-ray wavelengths. In the Fresh Slice the short X-ray pulse produced at the first stage is moved to a fresh part of the electron bunch by a transverse tilt of the bunch.[25]
In 2012, scientists working on the LCLS found an alternative solution to the seeding limitation for x-ray wavelengths by self-seeding the laser with its own beam after being filtered through a diamondmonochromator. The resulting intensity and monochromaticity of the beam were unprecedented and allowed new experiments to be conducted involving manipulating atoms and imaging molecules. Other labs around the world are incorporating the technique into their equipment.[26][27]
Researchers have explored X-ray free-electron lasers as an alternative tosynchrotron light sources that have been the workhorses of protein crystallography andcell biology.[28]
Exceptionally bright and fast X-rays can image proteins usingx-ray crystallography. This technique allows first-time imaging of proteins that do not stack in a way that allows imaging by conventional techniques, 25% of the total number of proteins. Resolutions of 0.8 nm have been achieved with pulse durations of 30femtoseconds. To get a clear view, a resolution of 0.1–0.3 nm is required. The short pulse durations allow images of X-ray diffraction patterns to be recorded before the molecules are destroyed.[29] The bright, fast X-rays were produced at theLinac Coherent Light Source at SLAC. As of 2014, LCLS was the world's most powerful X-ray FEL.[30]
Due to the increased repetition rates of the next-generation X-ray FEL sources, such as theEuropean XFEL, the expected number of diffraction patterns is also expected to increase by a substantial amount.[31] The increase in the number of diffraction patterns will place a large strain on existing analysis methods. To combat this, several methods have been researched to sort the huge amount of data that typical X-ray FEL experiments will generate.[32][33] While the various methods have been shown to be effective, it is clear that to pave the way towards single-particle X-ray FEL imaging at full repetition rates, several challenges have to be overcome before the next resolution revolution can be achieved.[34][35]
One remaining challenge includes an efficient way to deliver single particles to the X-ray beam. Several different approaches are in development to address this issue, which includeselectrospray ionization,gas-dynamic virtual nozzles andliquid sheet jets, which aim to provide stable and reproducible sample streams.[36][37]
It is of significance to consistently ensure that the particles intersect with the X-ray beam. As a result of introducing singular particles at a time, the hit rate on the beam is typically very low. This represents a major limitation with single-particle X-ray FEL imaging. Improvements to sample concentration and the precision of the delivery method is therefore crucial for increasing the efficiency of data collection.[38]
Newbiomarkers for metabolic diseases: taking advantage of the selectivity and sensitivity when combining infrared ion spectroscopy andmass spectrometry scientists can provide a structural fingerprint of small molecules in biological samples, like blood or urine. This new and unique methodology is generating exciting new possibilities to better understandmetabolic diseases and develop novel diagnostic and therapeutic strategies.
Research by Glenn Edwards and colleagues atVanderbilt University's FEL Center in 1994 found that soft tissues including skin,cornea, and brain tissue could be cut, orablated, usinginfrared FEL wavelengths around 6.45 micrometres with minimal collateral damage to adjacent tissue.[39][40] This led to surgeries on humans, the first ever using a free-electron laser. Starting in 1999, Copeland and Konrad performed three surgeries in which they resectedmeningiomabrain tumors.[41] Beginning in 2000, Joos and Mawn performed five surgeries that cut a window in the sheath of theoptic nerve, to test the efficacy for optic nerve sheathfenestration.[42] These eight surgeries produced results consistent with thestandard of care and with the added benefit of minimal collateral damage. A review of FELs for medical uses is given in the 1st edition of Tunable Laser Applications.[43]
Several small, clinical lasers tunable in the 6 to 7 micrometre range with pulse structure and energy to give minimal collateral damage in soft tissue have been created.[citation needed] At Vanderbilt, there exists a Raman shifted system pumped by an Alexandrite laser.[44]
Rox Anderson proposed the medical application of the free-electron laser in melting fats without harming the overlying skin.[45] Atinfraredwavelengths, water in tissue was heated by the laser, but at wavelengths corresponding to 915, 1210 and 1720nm, subsurfacelipids were differentially heated more strongly than water. The possible applications of this selective photothermolysis (heating tissues using light) include the selective destruction of sebum lipids to treatacne, as well as targeting other lipids associated withcellulite and body fat as well as fatty plaques that form in arteries which can help treatatherosclerosis andheart disease.[46]
FEL technology is being evaluated by theUS Navy as a candidate for ananti-aircraft and anti-missiledirected-energy weapon. TheThomas Jefferson National Accelerator Facility's FEL has demonstrated over 14 kW power output.[47] Compact multi-megawatt class FEL weapons are undergoing research.[48] On June 9, 2009, theOffice of Naval Research announced it had awardedRaytheon a contract to develop a 100 kW experimental FEL.[49] On March 18, 2010Boeing Directed Energy Systems announced the completion of an initial design for U.S. Naval use.[50] A prototype FEL system was demonstrated, with a full-power prototype scheduled by 2018.[51]
The FEL prize is given to a person who has contributed significantly to the advancement of the field of free-electron lasers. In addition, it gives the international FEL community the opportunity to recognize its members for their outstanding achievements. The prize winners are announced at the FEL conference, which currently takes place every two years.
The Young Scientist FEL Award (or "Young Investigator FEL Prize") is intended to honor outstanding contributions to FEL science and technology from a person who is less than 37 years of age at the time of the FEL conference.[52]
