The term “free electron laser” (FEL) refers to a source of coherent electromagnetic radiation generated by an electron beam, through an amplification, or self-amplification mechanism. Their modern development (and indeed the name “free electron laser”) goes back to the work of Madey and colleagues at Stanford in the early 1970s, although they also have roots in earlier electron beam devices such as the klystron, the travelling wave tube, the undulator and the ubitron, developed between the 1930s and 60s. In a FEL, amplification by stimulated emission occurs when the radiation and a relativistic electron beam propagate together through an undulator magnet. Although the first description was quantum mechanical, it was subsequently shown that FELs can be understood classically as a result of bunching of the electrons on the scale of the radiation wavelength, resulting in coherent emission of radiation (essentially coherent synchrotron radiation). The electrons are ‘free’ in the sense that they are not bound to atoms as in conventional lasers, however they are not completely free since they are under the influence of electromagnetic forces which cause them to radiate.
It has been recognised since the very beginning of the field that the lack of a conventional “lasing medium” gives the FEL enormous advantages: the wavelength of the radiation depends on the electron beam energy and undulator magnetic field strength and hence is continuously tuneable and can reach very short wavelengths. The configuration of the magnetic field also allows variable polarization to be obtained. There are no gain medium breakdown problems and hence there is the possibility of obtaining high peak and average power levels. Finally, the time structure is largely determined by that of the electron beam which therefore permits a flexible pulse length and repetition frequency.
The earliest FELs were oscillators, in which the radiation emitted in the undulator is trapped in an optical cavity whose round-trip time is synchronised to the spacing between successive electron bunches. In this way the intensity builds up after many passes and eventually results in coherent radiation. Oscillator FELs are very successful in the IR region and there are several user facilities in operation. Oscillator FELs have also reached into the VUV, but their extension to shorter wavelengths is currently limited by the availability of suitable mirrors. Most recent developments have therefore concentrated on achievement of much more powerful amplifiers that remove the need for mirrors completely. In its most basic form, the so-called Self Amplified Spontaneous Emission (SASE) mode, the initial incoherent radiation from the electron beam causes the electrons to be bunched, resulting in enhanced emission, which causes more bunching and so on. The radiation intensity grows exponentially until a saturation point is reached. The resulting output intensity is very high, in particular many orders of magnitude higher in peak intensity compared to alternative incoherent synchrotron radiation sources.
The main scientific driver for the development of FELs has been to achieve short wavelengths, going well beyond the realm of conventional laser, and laser-based, sources. Reaching the 1 Å level however requires ~10 GeV electron beams of very high quality and it has taken many years of painstaking development until the required high quality electron beams could be generated, and there was sufficient confidence in other technology, and the underlying physics, that funding agencies could be convinced to embark on such large-scale projects. The era of the Angstrom FEL was finally entered in April 2009 with the first lasing at 1.5 Å of the Linac Coherent Light Source (LCLS) at Stanford, USA. Today, regular experiments are underway using the LCLS in the 25-1.2 Å range, the first lasing at 1.2 Å of the SACLA X-ray facility in Japan was announced in June 2011 and several other facilities are under construction around the world: truly we are standing on the threshold of a new era of amazing discoveries in X-ray science.
New Science with X-ray FELS
The arrival of the first hard X-ray free electron laser facilities promises new advances in structural dynamics and nanoscale imaging that will have impact across the sciences. Free electron lasers operating in the X-ray range offer a unique combination of light source properties of extreme brightness, coherence and ultra-short pulse duration. These properties are anticipated to open important new scientific opportunities for time resolved structural studies and to enable a new class of coherent diffraction X-ray imaging. The potential importance of this new imaging method in the study of nanostructures and biological systems at the sub-cellular and molecular level is evidenced by recent prominent research results such as M.M.Siebert et al Single mimivirus particles intercepted and imaged with an X-ray laser Nature, 470 (2011) 78-81 and H.N. Chapman et al Femtosecond X-ray protein nanocrystallography Nature, 470 (2011) 73-77.
The properties of the light from free electron lasers, are dramatically different from both those of storage rings and conventional lasers. Storage ring synchrotron radiation has enormous spectral coverage and can deliver a high photon fluence (i.e. the average number of photons delivered per second) up to hard X-rays (10’s keV). This has allowed these sources to be the dominant tool for crystallography, X-ray imaging, X-ray spectroscopy and many other areas of X-ray science for the last four decades. Nevertheless these sources have low peak brightness (i.e. peak number of photons delivered per second), especially if a narrow spectral bandwidth or a short pulse is selected. With advanced pulse slicing techniques, synchrotron sources can provide sub-picosecond temporal resolution but only with tiny flux which severely limits the utility for measuring rapid changes. Conventional lasers have advanced hugely in recent years and can produce extremely short pulses (~5 fs) at very high brightness but these capabilities are limited only to the UV/visible/near IR range.
Free electron lasers on the other hand combine the benefits of both types of source, and can deliver high intensity laser-like radiation with variable polarization and flexible time structure across a wide spectral range (1010 times greater than in conventional synchrotrons.
The photon energy range that is covered by LCLS for instance is from 500eV to 10 keV (2.5nm to 0.1nm) so the radiation spans the soft to hard X-ray range. In this photon energy (wavelength) range X-ray scattering/diffraction can be used to extract material structure to the atomic level. This has been widely demonstrated using conventional X-ray sources, especially synchrotrons, where one of the major activities is X-ray structure determination e.g. of proteins through crystallographic methods. As well as elastic scattering of X-ray photons the absorption of X-rays may occur which leads to photoionisation predominately from the atomic inner shells. The atomic specificity of the X-ray spectrum, that carries information on the local chemical environment, is also widely used in research with synchrotrons, e.g. via XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure). The combination of short pulses (~5 -200 fs duration) which are highly energetic (mJ level, 1013-1012 photons per pulse) is now opening the door to time resolved structural and chemical research.
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