arXiv subjects

Mikhail Yurkov

Publications and source records attributed to Mikhail Yurkov.

17 recordsLinked to original sources

arXiv2024

Attosecond soft X-ray pulses generated by chirp-dispersed manipulation in an XFEL reveal nonlinear core-electron dynamics in neon

Free-electron lasers have demonstrated their capability of generating intense attosecond X-ray pulses, which are the key to studying electron dynamics at their natural time scale and in specifically targeted electronic states, but come at the expanse of complicated generation schemes and stochastic pulse shapes. Here, we demonstrate a novel and simple operation concept based on the manipulation of the electron-bunch chirp-dispersion and working with the full 4.5 MHz repetition rate at the European XFEL in Germany. With a high-fidelity single-shot temporal characterisation, we detect X-ray pulses with durations of down to 200 attoseconds and peak powers reaching into the terawatt regime at ~1 keV photon energy. As a direct application, we present simultaneous measurements of nonlinear X-ray-matter interaction via time-resolved electron spectroscopy. Using the derived temporal pulse information and restricting the durations to a regime where individual X-ray pulses are shorter than the single-core-hole life time in neon atoms, we reveal an otherwise hidden peak-intensity dependence in the nonlinear dynamics of double-core-hole formation. Our results open the field of attosecond science to the investigation of electronic processes not only in the ground state but also in systems driven far off their equilibrium. They shed light on highly transient intermediate steps in complex electronic dynamics and thus promise to help build the conceptual bridge between fundamental physical processes and chemical photo-reactions.

physics.optics↗

arXiv2009

Method for the determination of the three-dimensional structure of ultrashort relativistic electron bunches

We describe a novel technique to characterize ultrashort electron bunches in X-ray Free-Electron Lasers. Namely, we propose to use coherent Optical Transition Radiation to measure three-dimensional (3D) electron density distributions. Our method relies on the combination of two known diagnostics setups, an Optical Replica Synthesizer (ORS) and an Optical Transition Radiation (OTR) imager. Electron bunches are modulated at optical wavelengths in the ORS setup. When these electron bunches pass through a metal foil target, coherent radiation pulses of tens MW power are generated. It is thereafter possible to exploit advantages of coherent imaging techniques, such as direct imaging, diffractive imaging, Fourier holography and their combinations. The proposed method opens up the possibility of real-time, wavelength-limited, single-shot 3D imaging of an ultrashort electron bunch.

physics.optics↗

arXiv2008

Theory of edge radiation

We formulate a complete theory of Edge Radiation based on a novel method relying on Fourier Optics techniques. Similar types of radiation like Transition Undulator Radiation are addressed in the framework of the same formalism. Special attention is payed in discussing the validity of approximations upon which the theory is built. Our study makes consistent use of both similarity techniques and comparisons with numerical results from simulation. We discuss both near and far zone. Physical understanding of many asymptotes is discussed. Based on the solution of the field equation with a tensor Green's function technique, we also discuss an analytical model to describe the presence of a vacuum chamber. In particular, explicit calculations for a circular vacuum chamber are reported. Finally, we consider the use of Edge Radiation as a tool for electron beam diagnostics. We discuss Coherent Edge Radiation, Extraction of Edge Radiation by a mirror, and other issues becoming important at high electron energy and long radiation wavelength. Based on this work we also study the impact of Edge Radiation on XFEL setups and we discuss recent results.

physics.acc-ph↗

arXiv2008

Transverse coherence properties of X-ray beams in third-generation synchrotron radiation sources

This article describes a complete theory of spatial coherence for undulator radiation sources. Current estimations of coherence properties often assume that undulator sources are quasi-homogeneous, like thermal sources, and rely on the application of the van Cittert-Zernike theorem for calculating the degree of transverse coherence. Such assumption is not adequate when treating third generation light sources, because the vertical(geometrical) emittance of the electron beam is comparable or even much smaller than the radiation wavelength in a very wide spectral interval that spans over four orders of magnitude (from 0.1 Angstrom up to 10^3 Angstrom). Sometimes, the so-called Gaussian-Schell model, that is widely used in statistical optics in the description of partially-coherent sources, is applied as an alternative to the quasi-homogeneous model. However, as we will demonstrate, this model fails to properly describe coherent properties of X-ray beams from non-homogeneous undulator sources. As a result, a more rigorous analysis is required. We propose a technique, based on statistical optics and Fourier optics, to explicitly calculate the cross-spectral density of an undulator source in the most general case, at any position after the undulator. Our theory, that makes consistent use of dimensionless analysis, allows relatively easy treatment and physical understanding of many asymptotes of the parameter space, together with their region of applicability. Particular emphasis is given to the asymptotic situation when the horizontal emittance is much larger than the radiation wavelength, and the vertical emittance is arbitrary. This case is practically relevant for third generation synchrotron radiation sources.

physics.optics↗

arXiv2007

A simple method for timing an XFEL source to high-power lasers

We propose a technique, to be used for time-resolved pump-probe experiments, for timing an x-ray free electron laser (XFEL) to a high-power conventional laser with femtosecond accuracy. Our method takes advantage of the same electron bunch to produce both an XFEL pulse and an ultrashort optical pulse with the help of an optical radiator downstream of the x-ray undulator. Since both pulses are produced by the same electron bunch, they are perfectly synchronized. Application of cross-correlation techniques will allow to determine relative jitter between the optical pulse (and, thus, the XFEL pulse) and a pulse from an external pump-laser with femtosecond resolution. Technical realization of the proposed timing scheme uses an optical replica synthesizer (ORS) setup to be installed after the final bunch-compression stage of the XFEL. The electron bunch is modulated in the ORS setup by an external optical laser. Subsequently, it travels through the main undulator, and produces the XFEL pulse. Finally, a powerful optical pulse of coherent edge radiation is generated as the bunch passes through a long straight section and a separation magnet downstream of the main undulator. Our study shows that at a moderate (about 10%) density modulation of the electron bunch at the location of the optical radiator allows production of high power x-ray and optical pulses. Relative synchronization of these pulses is preserved by using the same mechanical support for both x-ray and optical elements transporting radiation down to the experimental area, where single-shot cross-correlation between optical pulse and pump-laser pulse is performed. We illustrate the potential of the proposed timing technique with numerical examples referring to the European XFEL facility.

physics.acc-ph↗

arXiv2007

Longitudinal impedance and wake from XFEL undulators. Impact on current-enhanced SASE schemes

In this article we derive longitudinal impedance and wake function for an undulator setup with arbitrary undulator parameter, taking into account a finite transverse size of the electron bunch. Earlier studies considered a line density-distribution of electrons instead. We focus our attention on the long-wavelength asymptote (compared with resonance wavelength), at large distance of the electron bunch from the undulator entrance compared to the overtaking length, and for large vacuum-chamber size compared to the typical transverse size of the field. These restrictions define a parameter region of interest for practical applications. We calculate a closed expression for impedance and wake function that may be evaluated numerically in the most general case. Such expression allows us to derive an analytical solution for a Gaussian transverse and longitudinal bunch shape. Finally, we study the feasibility of current-enhanced SASE schemes (ESASE) recently proposed for LCLS, that fall well-within our approximations. Numerical estimations presented in this paper indicate that impedance-induced energy spread is sufficient to seriously degrade the FEL performance. Our conclusion is in contrast with results in literature, where wake calculations for the LCLS case are given in free-space, as if the presence of the undulator were negligible.

physics.acc-ph↗

arXiv2007

Theory of Nonlinear Harmonic Generation in Free-Electron Lasers with Helical Wigglers

Coherent Harmonic Generation (CHG), and in particular Nonlinear Harmonic Generation (NHG), is of importance for both short wavelength Free-Electron Lasers (FELs), in relation with the achievement of shorter wavelengths with a fixed electron-beam energy, and high-average power FEL resonators, in relation with destructive effects of higher harmonics radiation on mirrors. In this paper we present a treatment of NHG from helical wigglers with particular emphasis on the second harmonic. Our study is based on an exact analytical solution of Maxwell's equations, derived with the help of a Green's function method. In particular, we demonstrate that nonlinear harmonic generation (NHG) from helical wigglers vanishes on axis. Our conclusion is in open contrast with results in literature, that include a kinematical mistake in the description of the electron motion.

physics.optics↗

arXiv2007

Undulator Radiation in a Waveguide

We propose an analytical approach to characterize undulator radiation near resonance, when the presence of the vacuum-pipe considerably affects radiation properties. This is the case of the far-infrared undulator beamline at the Free-electron LASer (FEL) in Hamburg (FLASH), that will be capable of delivering pulses in the TeraHertz (THz) range. This undulator will allow pump-probe experiments where THz pulses are naturally synchronized to the VUV pulse from the FEL, as well as the development of novel electron-beam diagnostics techniques. Since the THz radiation diffraction-size exceeds the vacuum-chamber dimensions, characterization of infrared radiation must be performed accounting for the presence of a waveguide. We developed a theory of undulator radiation in a waveguide based on paraxial and resonance approximation. We solved the field equation with a tensor Green's function technique, and extracted figure of merits describing in a simple way the influence of the vacuum-pipe on the radiation pulse as a function of the problem parameters. Our theory, that makes consistent use of dimensionless analysis, allows treatment and physical understanding of many asymptotes of the parameter space, together with their region of applicability.

physics.optics↗

arXiv2006

Longitudinal Wake Field for an Electron Beam Accelerated through a Ultra-High Field Gradient

Electron accelerators with higher and higher longitudinal field gradients are desirable, as they allow for the production of high energy beams by means of compact and cheap setups. The new laser-plasma acceleration technique appears to constitute the more promising breakthrough in this direction, delivering unprecedent field gradients up to TV/m. In this article we give a quantitative description of the impact of longitudinal wake fields on the electron beam. Our paper is based on the solution of Maxwell's equations for the longitudinal field. Our conclusions are valid when the acceleration distance is much smaller than the the overtaking length, that is the length that electrons travel as a light signal from the tail of the bunch overtakes the head of the bunch. This condition is well verified for laser-plasma devices. We calculate a closed expression for the impedance and the wake function that may be evaluated numerically. It is shown that the rate of energy loss in the bunch due to radiative interaction is equal to the energy emitted through coherent radiation in the far-zone. Furthermore, an expression is found for the asymptotic limit of a large distance of the electron beam from the accelerator compared with the overtaking length. Such expression allows us to calculate analytical solutions for a Gaussian transverse and longitudinal bunch shape. Finally, we study the feasibility of Table-Top Free-Electron Lasers in the Vacuum Ultra-Violet (TT-VUV FEL) and X-ray range (TT-XFEL), respectively based on 100 MeV and 1 GeV laser-plasma accelerator drivers. Numerical estimations presented in this paper indicate that the effects of the time-dependent energy change induced by the longitudinal wake pose a serious threat to the operation of these devices.

physics.acc-ph↗

arXiv2006

Fourier Optics treatment of Classical Relativistic Electrodynamics

In this paper we couple Synchrotron Radiation (SR) theory with a branch of physical optics, namely laser beam optics. We show that the theory of laser beams is successful in characterizing radiation fields associated with any SR source. Both radiation beam generated by an ultra-relativistic electron in a magnetic device and laser beam are solutions of the wave equation based on paraxial approximation. It follows that they are similar in all aspects. In the space-frequency domain SR beams appear as laser beams whose transverse extents are large compared with the wavelength. In practical situations (e.g. undulator, bending magnet sources), radiation beams exhibit a virtual "waist" where the wavefront is often plane. Remarkably, the field distribution of a SR beam across the waist turns out to be strictly related with the inverse Fourier transform of the far-field angle distribution. Then, we take advantage of standard Fourier Optics techniques and apply the Fresnel propagation formula to characterize the SR beam. Altogether, we show that it is possible to reconstruct the near-field distribution of the SR beam outside the magnetic setup from the knowledge of the far-field pattern. The general theory of SR in the near-zone developed in this paper is illustrated for the special cases of undulator radiation, edge radiation and transition undulator radiation. Using known analytical formulas for the far-field pattern and its inverse Fourier transform we find analytical expressions for near-field distributions in terms of far-field distributions. Finally, we compare these expressions with incorrect or incomplete literature.

physics.optics↗

arXiv2006

Statistical Optics approach to the design of beamlines for Synchrotron Radiation

In this paper we analyze the image formation problem for undulator radiation through an optical system, accounting for the influence of the electron beam emittance. On the one hand, image formation with Synchrotron Radiation is governed by the laws of Statistical Optics. On the other hand, the widely used Gaussian-Shell model cannot be applied to describe the coherence properties of X-ray beams from third generation Synchrotron Radiation sources. As a result, a more rigorous analysis of coherence properties is required. We propose a technique to explicitly calculate the cross-spectral density of an undulator source, that we subsequently propagate through an optical imaging system. At first we focus on the case of an ideal lens with a non-limiting pupil aperture. Our theory, which makes consistent use of dimensionless analysis, also allows treatment and physical understanding of many asymptotes of the parameter space, together with their applicability region. Particular emphasis is given to the asymptotic situation when the horizontal emittance is much larger than the radiation wavelength, which is relevant for third generation Synchrotron Radiation sources. First principle calculations of undulator radiation characteristics (i.e. ten-dimensional integrals) are then reduced to one-dimensional convolutions of analytical functions with universal functions specific for undulator radiation sources. We also consider the imaging problem for a non-ideal lens in presence of aberrations and a limiting pupil aperture, which increases the dimension of the convolution from one to three. In particular we give emphasis to cases when the intensity at the observation plane can be presented as a convolution of an impulse response function and the intensity from an ideal lens.

physics.acc-ph↗

arXiv2005

Exact Solution for the Second Harmonic Generation in XFELs

The generation of harmonic radiation through a non-linear mechanism driven by bunching at fundamental frequency is an important option in the operation of high gain Free-Electron Lasers (FELs). The use of harmonic generation at a large scale facility may result in achieving shorter radiation wavelengths for the same electron beam energy. This paper describes a theory of second harmonic generation in planar undulators with particular attention to X-Ray FELs (XFELs). Our study is based on an exact analytical solution of Maxwell equations, derived with the help of the Green's function method. On the contrary, up-to-date theoretical understanding of the second harmonic generation is only limited to some estimation of the total radiation power based on the source part of the wave equation. Moreover, we find that such part of the wave equation is presented with several incorrect manipulations among which is the omission of an important contribution. Our work yields correct parametric dependencies and specific predictions of additional properties such as polarization, angular distribution of the radiation intensity and total power. The most surprising prediction is the presence of a vertically polarized part of the second harmonic radiation, whereas up-to-date understanding assumes that the field is horizontally polarized. Altogether, this paper presents the first correct theory of second harmonic generation for high gain FELs.

physics.acc-ph↗

arXiv2005

Understanding transverse coherence properties of X-ray beams in third generation Synchrotron Radiation sources

This paper describes a theory of transverse coherence properties of Undulator Radiation. Our study is of very practical relevance, because it yields specific predictions of Undulator Radiation cross-spectral density in various parts of the beamline. On the contrary, usual estimations of coherence properties assume that the undulator source is quasi-homogeneous, like thermal sources, and rely on the application of van Cittert-Zernike (VCZ) theorem, in its original or generalized form, for calculating transverse coherence length in the far-field approximation. The VCZ theorem is derived in the frame of Statistical Optics using a number of restrictive assumptions: in particular, the quasi-homogeneous assumption is demonstrated to be inaccurate in many practical situations regarding undulator sources. We propose a technique to calculate the cross-spectral density from undulator sources in the most general case. Also, we find the region of applicability of the quasi-homogeneous model and we present an analytical expression for the cross-spectral density which is valid up to the exit of the undulator. For the case of more general undulator sources, simple formulas for the transverse coherence length, interpolated from numerical calculations and suitable for beamline design applications are found. Finally, using a simple vertical slit, we show how transverse coherence properties of an X-ray beam can be manipulated to obtain a larger coherent spot-size on a sample. This invention was devised almost entirely on the basis of theoretical ideas developed throughout this paper.

physics.optics↗

arXiv2005

Paraxial Green's functions in Synchrotron Radiation theory

This work contains a systematic treatment of single particle Synchrotron Radiation and some application to realistic beams with given cross section area, divergence and energy spread. Standard theory relies on several approximations whose applicability limits and accuracy are often forgotten. We begin remarking that on the one hand, a paraxial approximation can always be applied without loss of generality and with ultra relativistic accuracy. On the other hand, dominance of the acceleration field over the velocity part in the Lienard-Wiechert expressions is not always granted and constitutes a separate assumption, whose applicability is discussed. Treating Synchrotron Radiation in paraxial approximation we derive the equation for the slow varying envelope function of the Fourier components of the electric field vector. Calculations of Synchrotron Radiation properties performed by others showed that the phase of the Fourier components of the electric field vector differs from the phase of a virtual point source. In this paper we present a systematic, analytical description of this phase shift, calculating amplitude and phase of electric field from bending magnets, short magnets, two bending magnet system separated by a straight section (edge radiation) and undulator devices. We pay particular attention to region of applicability and accuracy of approximations used. Finally, taking advantage of results of analytical calculation presented in reduced form we analyze various features of radiation from a complex insertion device (set of two undulators with a focusing triplet in between) accounting for the influence of energy spread and electron beam emittance.

physics.acc-ph↗

arXiv2004

Theory of space-charge waves on gradient-profile relativistic electron beam: an analysis in propagating eigenmodes

We developed an exact analytical treatment for space-charge waves within a relativistic electron beam in terms of (self-reproducing) propagating eigenmodes. This result is of obvious theoretical relevance as it constitutes one of the few exact solution for the evolution of charged particles under the action of self-interactions. It also has clear numerical applications in particle accelerator physics where it can be used as the first self-consistent benchmark for space-charge simulation programs. Today our work is of practical relevance in FEL technology in relation with all those schemes where an optically modulated electron beam is needed and with the study of longitudinal space-charge instabilities in magnetic bunch compressors.

physics.acc-ph↗

arXiv2003

Transverse self-interactions within an electron bunch moving in an arc of a circle (generalized)

Transverse self-interactions within a "line" bunch of electrons moving in an arc of a circle have been studied extensively by us from an electrodynamical viewpoint. Here we treat an electron bunch with a given vertical (i.e. perpendicular to the orbital plane) size by a generalization of our previous work to the case of a test particle with vertical displacement interacting with a line bunch. In fact, since a bunch with vertical extent can always be thought of as a superposition of displaced charge lines, all the relevant physical aspects of the problem are included in the study of that simple model. Our generalization results in a physically meaningful and quantitative explanation of the simulations obtained with the code $TraFiC^4$ as well as in successful cross-checking of the code.

physics.acc-ph↗

arXiv2002

Transverse self-fields within an electron bunch moving in an arc of a circle

As a consequence of motions driven by external forces, self-fields (which are different from the static case) originate within an electron bunch. In the case of magnetic external forces acting on an ultrarelativistic beam, the longitudinal self-interactions are responsible for CSR (Coherent Synchrotron Radiation)-related phenomena, which have been studied extensively. On the other hand, transverse self-interactions are present too. At the time being, existing theoretical analysis of transverse self-forces deal with the case of a bunch moving along a circular orbit only, without considering the situation of a bending magnet with a finite length. In this paper we propose an electrodynamical analysis of transverse self-fields which originate, at the position of a test particle, from an ultrarelativistic electron bunch moving in an arc of a circle. The problem will be first addressed within a two-particle system. We then extend our consideration to a line bunch with a stepped density distribution, a situation which can be easily generalized to the case of an arbitrary density distribution. Our approach turns out to be also useful in order to get a better insight in the physics involved in the case of simple circular motion and in order to address the well known issue of the partial compensation of transverse self-force.

physics.acc-ph↗