Current Issue : April - June Volume : 2014 Issue Number : 2 Articles : 5 Articles
Discussion of some of the most important principles of single-crystal X-ray diraction, including:\r\nBragg''s law, fundamentals of diraction, components of a typical instrument, a comparison of powder\r\nand single-crystal analyses, qualities of an ideal crystal, and methods for growing single crystals for\r\nanalysis....
An x-ray diffraction reciprocal spacing mapping (RSM) technique was applied to investigate crystal\r\nquality of Nb films and bulk materials, relevant to superconducting radio frequency applications. The\r\nRSM study used different x-ray penetration depths to study material processes after two different surface\r\ntreatments: mechanical polishing or energetic condensation (a plasma process). The study revealed the\r\nmaterialâ��s microstructure evolution for the two different processes. The RSM plots revealed crystal quality\r\nof the materials at different thickness. The novel, differential-depth RSM technique presented in this study\r\nsubstantiated the following facts: (1) For a heteroepitaxial Nb film Nb�°100�ž=MgO�°100�ž, a film thickening\r\nprocess, via a cathodic arc-discharge Nb ion deposition (energetic condensation), created a near-perfect\r\nsingle crystal Nb on the surface top layer. (2) For a fine mechanically polished single-crystal bulk Nb\r\nmaterial, the microstructure on the top surface layer is more disordered than that at a greater depth....
Fully coherent x rays can be generated by self-seeding x-ray free electron lasers (XFELs). Self-seeding\r\nby a forward Bragg diffraction (FBD) monochromator has been recently proposed [G. Geloni, V.\r\nKocharyan, and E. Saldin, J. Mod. Opt. 58, 1391 (2011)] and demonstrated [J. Amann et al., Nat.\r\nPhotonics 6, 693 (2012)]. Characteristic time T 0 of FBD determines the power, spectral, and time\r\ncharacteristics of the FBD seed [Yu. Shvyd�ko and R. Lindberg, Phys. Rev. ST Accel. Beams 15, 100702\r\n(2012)]. Here we show that for a given electron bunch with duration e the spectral flux of the self-seeding\r\nXFEL can be maximized, and the spectral bandwidth can be respectively minimized by choosing T 0 \r\ne= and by optimizing the electron bunch delay e. The choices of T 0 and e are not unique. In all\r\ncases, the maximum value of the spectral flux and the minimum bandwidth are primarily determined\r\nby e. Two-color seeding takes place if T 0 e=. The studies are performed, for a Gaussian\r\nelectron bunch distribution with the parameters, close to those used in the short-bunch (e � 5 fs) and\r\nlong-bunch (e � 20 fs) operation modes of the Linac Coherent Light Source XFEL....
Baseline design of a typical x-ray free electron laser (FEL) undulator assumes a planar configuration\r\nwhich results in a linear polarization of the FEL radiation. However, many experiments at x-ray FEL user\r\nfacilities would profit from using a circularly polarized radiation. As a cheap upgrade, one can consider an\r\ninstallation of a short helical (or cross-planar) afterburner, but then one should have an efficient method to\r\nsuppress the powerful linearly polarized background from the main undulator. In this paper we propose a\r\nnew method for such a suppression: an application of the reverse taper in the main undulator. We discover\r\nthat in a certain range of the taper strength, the density modulation (bunching) at saturation is practically\r\nthe same as in the case of a nontapered undulator while the power of linearly polarized radiation is\r\nsuppressed by orders of magnitude. Then strongly modulated electron beam radiates at full power in the\r\nafterburner. Considering the SASE3 undulator of the European XFEL as a practical example, we\r\ndemonstrate that soft x-ray radiation pulses with peak power in excess of 100 GW and an ultimately\r\nhigh degree of circular polarization can be produced. The proposed method is rather universal, i.e., it can\r\nbe used at SASE FELs and seeded (self-seeded) FELs, with any wavelength of interest, in a wide range of\r\nelectron beam parameters, and with any repetition rate. It can be used at different x-ray FEL facilities, in\r\nparticular at Linac Coherent Light Source after installation of the helical afterburner in the near future....
To meet the demand from the growing number of user experiments, multi-beam-line\r\noperation with a wide spectral range is seriously considered in x-ray free-electron laser\r\nfacilities. In a conventional design, the beam line of low photon energies branches off from\r\nthe middle of the accelerator to take out low energy electron beams. Here in this paper, a novel\r\nmethod is proposed to deliver bunch-to-bunch energy changed electron beams at the end of the\r\naccelerator. Since all accelerator components are operated in steady state, this method enables\r\nquasisimultaneous operation of multi-beam-line in the same undulator hall without degrading\r\nthe stability and performance of the electron beam....
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