Simple field-theoretical approach to critical phenomena is described. In contrast to the Wilson's theory, a description in real 3-dimensions space is used. At the same time the described approach is not the same as Parisi's. Used subtraction scheme is different from the one used by Parisi, but the main point is that we treat changes of $T-T_C$ as explicit perturbations. By such an approach not only the critical domain but also the crossover from critical domain to domain of Landau theory can be described very simply. Consideration is restricted to scalar $\phi^4$ model in one-loop approximation.

A. S. Yurkov

We describe statistical and coherence properties of the radiation from x-ray free electron lasers (XFEL) operating in the post-saturation regime. We consider practical case of the SASE3 FEL at the European XFEL. We perform comparison of the main characteristics of the X-ray FEL operating in the post-saturation regime with and without undulator tapering: efficiency, coherence time and degree of transverse coherence.

E. A. Schneidmiller M. V. Yurkov

The article discusses a theory that allows to describe losses in conductors taking into account the skin effect and the proximity effect. The developed theory is applied to inductance coils. In contrast to the classical Butterworth theory the condition of the strong skin effect, which is frequent in practice, is introduced from the very beginning. This approach makes possible to obtain extremely simple formulas, which show that Q-factor of a coil is determined mainly by the diameter of its winding.

A. S. Yurkov

The problem of an efficiency increase of an FEL amplifier is now of great practical importance. Technique of undulator tapering in the post-saturation regime is used at the existing x-ray FELs LCLS and SACLA, and is planned for use at the European XFEL, Swiss FEL, and PAL XFEL. There are also discussions on the future of high peak and average power FELs for scientific and industrial applications. In this paper we perform detailed analysis of the tapering strategies for high power seeded FEL amplifiers. Application of similarity techniques allows us to derive universal law of the undulator tapering.

E. A. Schneidmiller M. V. Yurkov

To address known privacy problems with the EMV standard, EMVCo have proposed a Blinded Diffie-Hellman key establishment protocol, which is intended to be part of a future 2nd Gen EMV protocol. We point out that active attackers were not previously accounted for in the privacy requirements of this proposal protocol, and demonstrate that an active attacker can compromise unlinkability within a distance of 100cm. Here, we adopt a strong definition of unlinkability that does account for active attackers and propose an enhancement of the protocol proposed by EMVCo. We prove that our protocol does satisfy strong unlinkability, while preserving authentication.

Ross Horne Sjouke Mauw Semen Yurkov

We report the early discovery of the optical afterglow of gamma-ray burst (GRB) 140801A in the 137 deg$^2$ 3-$\sigma$ error-box of the Fermi Gamma-ray Burst Monitor (GBM). MASTER is the only observatory that automatically react to all Fermi alerts. GRB 140801A is one of the few GRBs whose optical counterpart was discovered solely from its GBM localization. The optical afterglow of GRB 140801A was found by MASTER Global Robotic Net 53 sec after receiving the alert, making it the fastest optical detection of a GRB from a GBM error-box. Spectroscopy obtained with the 10.4-m Gran Telescopio Canarias and the 6-m BTA of SAO RAS reveals a redshift of $z=1.32$. We performed optical and near-infrared photometry of GRB 140801A using different telescopes with apertures ranging from 0.4-m to 10.4-m. GRB 140801A is a typical burst in many ways. The rest-frame bolometric isotropic energy release and peak energy of the burst is $E_\mathrm{iso} = 5.54_{-0.24}^{+0.26} \times 10^{52}$ erg and $E_\mathrm{p, rest}\simeq280$ keV, respectively, which is consistent with the Amati relation. The absence of a jet break in the optical light curve provides a lower limit on the half-opening angle of the jet $\theta=6.1$ deg. The observed $E_\mathrm{peak}$ is consistent with the limit derived from the Ghirlanda relation. The joint Fermi GBM and Konus-Wind analysis shows that GRB 140801A could belong to the class of intermediate duration. The rapid detection of the optical counterpart of GRB 140801A is especially important regarding the upcoming experiments with large coordinate error-box areas.

V. M. Lipunov J. Gorosabel M. V. Pruzhinskaya A. de Ugarte Postigo V. Pelassa A. E. Tsvetkova I. V. Sokolov D. A. Kann Dong Xu E. S. Gorbovskoy V. V. Krushinski V. G. Kornilov P. V. Balanutsa S. V. Boronina N. M. Budnev Z. Cano A. J. Castro-Tirado V. V. Chazov V. Connaughton C. Delvaux D. D. Frederiks J. F. U. Fynbo A. V. Gabovich A. Goldstein J. Greiner O. A. Gress K. I. Ivanov P. Jakobsson S. Klose F. Knust V. N. Komarova E. Konstantinov A. V. Krylov D. A. Kuvshinov A. S. Kuznetsov G. V. Lipunova A. S. Moskvitin V. D. Pal'shin S. B. Pandey V. A. Poleshchuk S. Schmidl Yu. P. Sergienko E. V. Sinyakov S. Schulze V. V. Sokolov T. N. Sokolova M. Sparre C. C. Thone A. G. Tlatov N. V. Tyurina M. V. Ulanov S. A. Yazev V. V. Yurkov

We propose a technique for the production of attosecond X-ray pulses which is based on the use of X-ray SASE FEL combined with a femtosecond laser system. A few-cycle optical pulse from a Ti:sapphire laser interacts with the electron beam in a two-period undulator resonant to 800 nm wavelength and produces energy modulation within a slice of the electron bunch. Following the energy modulator the electron beam enters the X-ray undulator and produces SASE radiation. Due to energy modulation the frequency is correlated to the longitudinal position within the few-cycle-driven slice of SASE radiation pulse. The largest frequency offset corresponds to a single-spike pulse in the time domain which is confined to one half-oscillation period near the central peak electron energy. The selection of single-spike pulses is achieved by using a crystal monochromator after the X-ray undulator. Our studies show that the proposed technique is capable to produce 300 attoseconds long single pulses with GW-level output power in the 0.1 nm wavelength range, and is applicable to the European X-Ray Laser Project XFEL and the Linac Coherent Light Source at SLAC.

E. L. Saldin E. A. Schneidmiller M. V. Yurkov

Recent theoretical and experimental studies have shown that Self-Amplified Spontaneous Emission Free Electron Laser (SASE FEL) with a planar undulator holds a potential for generation of relatively strong coherent radiation at the third harmonic of the fundamental frequency. Here we present detailed study of the nonlinear harmonic generation in the SASE FEL obtained with time-dependent FEL simulation code. Using similarity techniques we present universal dependencies for temporal, spectral, and statistical properties of the odd harmonics of the radiation from SASE FEL. In particular, we derived universal formulae for radiation power of the odd harmonics at saturation. It was also found that coherence time at saturation falls inversely proportional to harmonic number, and relative spectrum bandwidth remains constant with harmonic number.

E. L. Saldin E. A. Schneidmiller M. V. Yurkov

Since spring, 2005 vacuum ultraviolet free electron laser (VUV FEL) at DESY operates as user facility in the wavelength range around 30 nm. Electron beam formation system at the VUV FEL is essentially nonlinear and naturally results in a formation of a short high-current leading peak in the density distribution that produces FEL radiation. The main feature of the femtosecond mode of operation of the VUV FEL is the production of short, down to 20 fs radiation pulses with GW-level peak power. This paper is devoted to detailed studies of the short-pulse effects in the VUV FEL at DESY operating in the femtosecond regime, and to analysis of first experimental results.

E. L. Saldin E. A. Schneidmiller M. V. Yurkov

We present a comprehensive analysis of coherence properties of the radiation from X-ray free electron laser (XFEL). We consider practically important case when XFEL is optimized for maximum gain. Such an optimization allows to reduce significantly parameter space. Application of similarity techniques to the results of numerical simulations allows to present all output characteristics of the optimized XFEL as functions of the only parameter, ratio of the emittance to the radiation wavelength, $ \hat{\epsilon} = 2\pi \epsilon/\lambda $. Our studies show that optimum performance of the XFEL in terms of transverse coherence is achieved at the value of the parameter $\hat{\epsilon}$ of about unity. At smaller values of $\hat{\epsilon}$ the degree of transverse coherence is reduced due to strong influence of poor longitudinal coherence on a transverse one. At large values of the emittance the degree of transverse coherence degrades due to poor mode selection. Comparative analysis of existing XFEL projects, European XFEL, LCLS, and SCSS is presented as well.

E. L. Saldin E. A. Schneidmiller M. V. Yurkov