Self-amplified spontaneous emission (SASE) is a process within 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 and behaves in many ways like a laser, but instead of using stimulated emission from atomic or molecula ...

(FEL) by which a laser beam is created from a high-energy

electron beam Cathode rays or electron beam (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to ele ...

. The SASE process starts with an electron bunch being injected into an

undulator An undulator is an insertion device from high-energy physics and usually part of a larger installation, a synchrotron storage ring, or it may be a component of a free electron laser. It consists of a periodic structure of dipole magnets. These ca ...

, with a velocity close to the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...

and a uniform density distribution within the bunch. In the undulator the electrons are wiggled and emit light characteristic of the undulator strength but within a certain energy bandwidth. The emitted photons travel slightly faster than the electrons and interact with them each undulator period. Depending on the phase to each other, electrons gain or lose energy (velocity), i.e. faster electrons catch up with slower ones. Thereby the electron bunch density is periodically modulated by the radiation which is called microbunching. The structured electron beam amplifies only certain photon energies at the cost of kinetic energy until the system goes into saturation. SASE energy spectra show a noise-like distribution of intense spikes on top of a lower-amplitude background. The micro-bunch structuring reduces the

phase space In dynamical system theory, a phase space is a space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. For mechanical systems, the phase space usually ...

available to the photons, thus they are also more likely to have a similar phase and the emitted beam is quasi-coherent. This concept has been demonstrated at the

SPring-8 SPring-8 (an acronym of Super Photon Ring – 8 GeV) is a synchrotron radiation facility located in Sayo Town, Sayo District, Hyōgo Prefecture, Japan, which is the main facility of Harima Science Garden City. It was developed jointly by RIKEN ...

FEL SACLA in

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

, the Free electron LASer in Hamburg (

FLASH Flash, flashes, or FLASH may refer to: Arts, entertainment, and media Fictional aliases * Flash (DC Comics character), several DC Comics superheroes with super speed: ** Flash (Barry Allen) ** Flash (Jay Garrick) ** Wally West, the first Kid ...

) and the

Linac Coherent Light Source SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Departme ...

(LCLS) at SLAC.

References

Laser science Synchrotron radiation {{Physics-stub

See also

References