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 gain medium instead of using stimulated emission from atomic or molecular excitations.
As electron kinetic energy and undulator parameters can be adapted as desired, free-electron lasers are tunable and can be built for a wider frequency range than any other type of laser,[3] currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, ultraviolet, and X-ray.
A beam of electrons is generated by a short laser pulse illuminating a photocathode located inside a microwave cavity and accelerated to almost the speed of light in a device called a photoinjector.
Mirrors at each end of the undulator create an optical cavity, causing the radiation to form standing waves, or alternately an external excitation laser is provided.
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.
K, a dimensionless parameter, defines the wiggler strength as the relationship between the length of a period and the radius of bend,[citation needed] where
The intense pulses from the X-ray laser lies in the principle of self-amplified spontaneous emission (SASE), which leads to microbunching.
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 to Stanford University's Linac Coherent Light Source (LCLS-II) used temperatures around −271 °C to produce 106 pulses/second of near light-speed electrons, using superconducting niobium cavities.
Such a temporally coherent seed can be produced by more conventional means, such as by high 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.
In late 2010, in Italy, the seeded-FEL source FERMI@Elettra[20] started commissioning, at the Trieste Synchrotron Laboratory.
In 2001, at Brookhaven 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.
A similar staging approach, named "Fresh-Slice", was demonstrated at the Paul Scherrer Institut, also at X-ray wavelengths.
[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 diamond monochromator.
The resulting intensity and monochromaticity of the beam were unprecedented and allowed new experiments to be conducted involving manipulating atoms and imaging molecules.
[26][27] Researchers have explored X-ray free-electron lasers as an alternative to synchrotron light sources that have been the workhorses of protein crystallography and cell biology.
The short pulse durations allow images of X-ray diffraction patterns to be recorded before the molecules are destroyed.
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] New biomarkers for metabolic diseases: taking advantage of the selectivity and sensitivity when combining infrared ion spectroscopy and mass 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 understand metabolic diseases and develop novel diagnostic and therapeutic strategies.
[39] These eight surgeries produced results consistent with the standard of care and with the added benefit of minimal collateral damage.
[40] 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.
[41] Rox Anderson proposed the medical application of the free-electron laser in melting fats without harming the overlying skin.
The possible applications of this selective photothermolysis (heating tissues using light) include the selective destruction of sebum lipids to treat acne, as well as targeting other lipids associated with cellulite and body fat as well as fatty plaques that form in arteries which can help treat atherosclerosis and heart disease.
The Thomas Jefferson National Accelerator Facility's FEL has demonstrated over 14 kW power output.
[45] On June 9, 2009 the Office of Naval Research announced it had awarded Raytheon a contract to develop a 100 kW experimental FEL.
