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Scientists develop method for dramatic improvement of X-ray lasers

(Nanowerk News) X-ray free-electron lasers (XFELs) are new, large-scale research facilities that open up completely new insights in the nanoworld. With X-ray flashes of unprecedented brightness and shortness, the XFELs allow direct observation of single molecules, atoms, and even chemical reactions. Several of these new facilities have either just started operation or are currently under construction in the US, Japan, Germany, Switzerland, and Korea. In 2010, researchers from Deutsches Elektronen-Synchrotron (DESY) and European XFEL, both in Hamburg, had devised a way to dramatically improve the already remarkable features of these large machines by placing a special crystal into the path of the radiation. Now their colleagues from Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory in the US and the Technological Institute for Superhard and Novel Carbon Materials in Troitsk, Russia, implemented the setting proposed by their colleagues in Germany and confirmed the predicted outcome. The results are now published in Nature Photonics ("Demonstration of self-seeding in a hard-X-ray free-electron laser").
 Linac Coherent Light Source
To create a precise X-ray band and make the Linac Coherent Light Source (LCLS) even more “laser-like,” researchers installed this chamber with a slice of diamond crystal. The new hardware sits halfway down the 130-meter bank of magnets where the X-rays are generated. (Photo: SLAC)
The facilities, which stretch in tunnels that are several kilometres long, generate short-wave X-rays with laser-like characteristics. Electrons accelerated in a linear accelerator are forced onto a slalom course by a special arrangement of magnets, called undulators, thereby emitting X-ray light. At the end of the tunnel, a sample is placed in the X-ray beam within a special experiment setting. The X-rays are then scattered by the sample. From the resulting patterns, scientists can generate pictures or even movies of the atomic structure and behaviour of the sample. The scientific use of these machines began only recently with the start of user operation at LCLS in late 2009. The invention by the DESY – European XFEL team might lead to higher-quality X-ray flashes and thus to even more sophisticated pictures or movies than expected.
The breakthrough has been obtained by a relatively simple modification of LCLS, improving a parameter of the X-ray beam called “longitudinal coherence” while maintaining a very high intensity. Like with a regular camera, which needs sufficient light to generate a good photo, the quality of the pictures or movies taken by an X-ray FEL significantly improves with the coherence and intensity of the X-ray flashes used. “The new results are a further step towards creating an X-ray beam that, in the future, may take high-resolution pictures of single protein molecules, something that biologists and biochemists can only dream about today”, says Adrian Mancuso, a leading instrument scientist at European XFEL. Until now, to get such pictures, scientists have to grow crystals of proteins, a very time-consuming process that either may take years or, in many important cases, is not possible at all.
The generation of X-ray light in FELs usually starts from what is called electron beam “shot noise”: spontaneous emissions of light by electrons that are randomly distributed in space and time as their path is bent by the undulators. The electrons in the beam interact with the radiation forming microbunches. This induces the electrons to radiate even more and to consequently get even more bunched, resulting in a self-sustaining process known as “self-amplified spontaneous emission” (SASE). “The problem is that this radiation, which started from the electron beam shot noise, is not very coherent and consists of slightly different wavelengths”, explains Gianluca Geloni, a scientist at European XFEL.
What the physicists now did to improve the X-ray flashes was to insert a special crystal between the undulators. When the light generated in the first part of the undulator hits the crystal at a certain angle, most of the light passes through the crystal, keeping the same properties as the incoming beam. However, because of the specific properties of the crystal, the first pulse is followed by a second one, slightly delayed and monochromatic (that is, consisting of only one wavelength of light). These are the X-rays the physicists are interested in: If the electron beam that generates the light is also delayed using a magnetic chicane, and then set on top of the second light pulse, the electron beam will amplify this second pulse. Thus, only the high-quality X-rays seed the generation of very intense and coherent flashes in the second part of the undulator – a procedure that is known as “self seeding”.
When Gianluca Geloni (European XFEL), Vitali Kocharyan, and Evgeni Saldin (both DESY) predicted the generation of these highly intense and coherent single-wavelength flashes through self-seeding by means of the setting described above, fellow scientists at SLAC, who operate the world’s first hard X-ray FEL, were immediately interested. Now they have proven that the laser light generated by the huge X-ray FEL facilities could indeed be improved exactly as foreseen by the team in Hamburg. “Our setting will make X-ray FELs in fact much more laser-like than before”, says Evgeni Saldin. Research still has to be done to further improve the setting, but the team from European XFEL and DESY is very confident that this can be done. “We hope that our discovery will help physicists, chemists, biologists, and physicians to find even more exciting applications of research with high-quality X-rays”, adds Vitali Kocharyan.
Source: DESY
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