Planar Undulator Research Articles

Enhanced X-ray free-electron laser performance with optical klystron and helical undulators.

This article presents a demonstration of the improved performance of an X-ray free-electron laser (FEL) using the optical klystron mechanism and helical undulator configuration, in comparison with the common planar undulator configuration without optical klystron. The demonstration was carried out at Athos, the soft X-ray beamline of SwissFEL. Athos has variable-polarization undulators, and small magnetic chicanes placed between every two undulators to fully exploit the optical klystron. It was found that, for wavelengths of 1.24 nm and 3.10 nm, the required length to achieve FEL saturation is reduced by about 35% when using both the optical klystron and helical undulators, with each effect accounting for about half of the improvement. Moreover, it is shown that a helical undulator configuration provides a 20% to 50% higher pulse energy than planar undulators. This work represents an important step towards more compact and high-power FELs, rendering this key technology more efficient, affordable and accessible to the scientific community.

Harmonic generation in a terawatt x-ray free-electron laser

Terawatt x-ray free-electron lasers (XFELs) represent the frontier in further development of x-ray sources and require high current densities with strong transverse focusing. In this paper, we investigate the implications/potentialities of TW XFELs on the generation of harmonics at still shorter wavelengths and higher photon energies. The simulations indicate that significant power levels are possible at high harmonics of the XFEL resonance and that these XFELs can be an important coherent source of hard x-rays through the gamma ray spectrum. For this purpose, we use the MINERVA simulation code which self-consistently includes harmonic generation. Both helical and planar undulators are discussed in which the fundamental is at 1.5 Å and study the associated harmonic generation. While tapered undulators are needed to reach TW powers at the fundamental, the taper does not enhance the harmonics because the taper must start before saturation of the fundamental, while the harmonics saturate before this point is reached. Nevertheless, the harmonics reach substantial powers. Simulations indicate that, for the parameters under consideration, peak powers of the order of 180 MW are possible at the fifth harmonic with a photon energy of about 41 keV and still high harmonics may also be generated at substantial powers. Such high harmonic powers are certain to enable a host of enhanced applications.

Permanent helical micro-undulators for x-ray free electron lasers

Helical Halbach undulators, formed by four alternately axially and radially magnetized rare-earth helices, produce a stronger field than two planar Halbach undulators with perpendicular polarization. Hybrid undulators of two longitudinally premagnetized rare-earth helices and two initially unmagnetized steel (or vanadium permendur) helices produce an equal or higher field and are easier to implement. We made sure that the required helices with a period of down to 3 mm or even less could be manufactured with high precision using Wire Electric Discharge Machining. According to the simulation results, when using identical electron bunches with moderate initial spreads in energy and angles, the gain in x-ray free electron laser (XFEL) power from replacing planar micro-undulators with the helical micro-undulators under consideration can be significantly greater than predicted by the simplest 1D theory with a cold electron beam. This makes such micro-undulators promising for creating compact XFELs of various types, both based on modern linear accelerators and plasma wakefield accelerators.

Off-axis spectrum and small signal gain of dual harmonic planar undulator

The spectral properties of the spontaneous undulator radiation generated from the dual harmonic planar undulator configuration are analyzed. The effect of changing the observation point away from the axis on radiation is demonstrated. The expressions for off-axis undulator radiation and off-axis small signal gain are derived using the generalized Bessel function. The emission of radiation at odd and even harmonics and the effect of the additional harmonic field on the intensity and small signal gain are analyzed. The present work can also be used for the diagnostic study of errors in the undulator field and the detector setting for FEL measurements.

Advanced Studies for the Dynamics of High Brightness Electron Beams with the Code MILES

High brightness electron beams enable a wide spectrum of applications ranging from short wavelength radiation sources to high gradient wakefield acceleration. The rich dynamics that are intrinsic in charged particles accelerated in complex systems require a careful description in the analysis and design of a given machine, particularly regarding its stability. Numerous computer codes are in use by the accelerator community for such purposes. In particular, MILES is a simple tracking code we have developed that allows fast evaluations of collective effects in RF linacs. In this paper we extend the simple models previously developed to describe specific, diverse applications that can benefit from the fast simulation tools developed in MILES. Examples of this kind include particle driven acceleration schemes in a plasma where driver and witness beams propagate in the “comb” pulse-train configuration. Specifically, we investigate the self-induced fields excited within the X-band rf-linac stage of EuPRAXIA@SPARC_LAB. Further, we discuss additional advanced topics such as resistive wall wakefield effects in planar FEL undulators and their impact on the radiation emitted.

Permanent Helical Undulators with Strong Fields

Undulators containing magnetized rare-earth helices can provide a significantly higher oscillatory electron velocity than the widely used planar Halbach undulators. Using Wire Electrical Discharge Machining (WEDM), it is possible to manufacture NdFeB helices with a period of 1 mm or less with high accuracy. In this work, we describe the results of manufacturing and studying prototypes of undulators in the form of one or two axially and radially magnetized helices. More efficient hybrid systems of two axially oppositely magnetized and two steel non-premagnetized helices with a field on the axis of the order of 1 T are also shown. Micro-undulators of this type can significantly increase the efficiency of XFELs and Inverse FELs.

Simulation of Off-Axis Planar Undulator Radiation by Numerical Approach Using SCILAB-Xcos Model

Simulation of Off-Axis Planar Undulator Radiation by Numerical Approach Using SCILAB-Xcos Model

Variably polarized superradiant THz source employing crossed undulator configuration

We propose a variably polarized high-power THz source based on a crossed undulator configuration optimized for superradiant emissions from a short-pulsed electron beam. It generates arbitrary polarization by superimposing orthogonal linear polarization from two planar undulators. The phase difference is adjusted by slightly varying the path length of the first radiation in the phase shifter located between the undulators, enabling continuous and fast polarization manipulation. To realize the crossed undulator scheme based on superradiance in the THz region, it is necessary to maintain an extremely short electron bunch length of the order of 100 fs or less throughout the two undulators. We have designed a prototype system with an R56-variable triple-bend phase shifter that enables isochronous electron beam transport. Through theoretical and numerical analyses, we found that although polarization has significant observation angle dependence, a degree of polarization higher than 0.9 and a significant radiation intensity can be obtained under an appropriate angular limitation. Depending on the electron beam current and the number of undulator periods, it is estimated that the effective intensity under an angular limitation for a degree of circular polarization of 0.9 can reach the order of 100 mW on average.

Development and testing of a three-period, subsize 2G AIMI MgB2 planar undulator

Compared to Nb3Sn- and NbTi-wound superconducting undulators (SCUs), MgB2-wound SCUs are of interest for future electron synchrotron beam light sources owing to their higher temperature operating margin and associated stability. In this study, a three-period undulator consisting of twelve racetrack coils wound with 2nd generation (2 G) multifilamentary advanced-internal-magnesium-infiltration MgB2 strands were fabricated and tested in liquid and gaseous helium (He) over a temperature range of 4.2 K–20 K. The coil winding cross sections (in each coil) were 5 mm wide and 4.8 mm thick. At 4.2 K, a critical current (I c) of 325.7 Amps produced a maximum undulator bore field of 1.19 T. It should be noted that the short, 3-period nature of the coil led to an asymmetry in the field profile (the maximum positive field was 1.19 T, the maximum negative was −0.25 T), suggesting a peak field of 0.72 T in the absence of end effects. Finite element modeling (FEM) results of simulations for a one meter long undulator of otherwise identical design gave 0.85 T (larger because of higher currents enabled by the lower field). But in any case, the I c value coil reached is 94% of that of the short sample (dictated by the 1.19 T positive field for the coil as tested). FEM was performed to study the magnetic field profile, which was validated experimentally. The magnetic field was measured using a Hall probe which was translated along the beam axis during measurement to explore the spatial field variation along the beam travel direction. The spatially alternating field was asymmetric, and the maximum field was more prominent in the positive direction than in the negative direction, the difference being due to broken symmetry, that is, short coil end effects. In this work, we show useful fields are possible for MgB2 undulators; the use of such conductors can allow a larger thermal margin and enable conduction-cooled operation.

Estimation of Screening Current Induced Field on Short Period Planar HTS Undulator

One of the possible obstacles for future high temperature superconductor (HTS) undulator is undesirable field distribution changes due to screening current induced field (SCF). In this study, we investigated the effects of SCF of short period planar undulator magnet using a numerical simulation, specifically H-formulation with nonlinear BH curve of iron core considered. First, we used a periodic screening current analysis model to investigate <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$n$</tex-math></inline-formula> -value and spool-by-spool critical current dependent hysteresis behavior of undulator field change due to SCF. The results with selected <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$n$</tex-math></inline-formula> -value and spool-by-spool critical current values may be used to estimate the possible variations of undulator field due to SCF. After that, the temporal behavior of undulator field with and without the current sweep reversal method is also investigated.

On the physics of generation of even and odd harmonics in the undulators of free electron lasers

We theoretically explore the physics of even and odd harmonics generation and radiation in undulators of single pass free electron lasers (FELs). Planar undulators are studied as a most common design. The results are obtained analytically and some of them also numerically. In the analytical form the contributions of various factors to the harmonic generation are distinguished and their effect is analyzed. The results are verified with numerical simulations and compared with reported experimental data; the data for best documented FELs in terms of harmonic generation, LEUTL and LCLS, is analyzed. The reasons for even harmonic generation are identified; we show that they differ for the spontaneous undulator radiation and for the stimulated FEL radiation. Comparative analysis is performed with various theoretical models for the FEL harmonic powers. The advantages and possibilities of amplification and radiation the second FEL harmonic in the LCLS-II FEL with variable deflection parameter is demonstrated for the harmonic lasing self-seed (HLSS) FEL design. The possibility to suppress the radiated second harmonic in a FEL by a guiding field of a weak second harmonic of the main undulator field with negative phase is shown. This can be used to reduce the reflected signal of the second harmonic generation (SHG) when the nonlinear harmonic response of the mater is investigated.

Development of an APPLE III undulator prototype with three-dimensional force compensation for SHINE

The Shanghai high-repetition-rate XFEL and extreme light facility (SHINE) plans to install several elliptically polarizing undulators (EPUs) as afterburners behind the planar undulator section to obtain nearly saturated circularly polarized free-electron laser (FEL) radiation. Therefore, the SHINE R&amp;amp;D project needs to develop a 4-m-long EPU with an effective magnetic field of 1.5 T, a period of 68 mm, and a minimum gap of 3 mm. A magnetic force compensation EPU prototype has recently been built and tested at the Shanghai Synchrotron Radiation Facility (SSRF) based on the addition of permanent magnets. This prototype can realize magnetic compensation under various polarization modes in all working gaps, thereby reducing the deformation of the girders and maintaining sufficient rigidity of the mechanical structure. A girder deformation monitoring system was established to obtain real force compensation feedback. The final magnetic field measurement results meet the physical requirements, and the proposed scheme can be used as an alternative to the EPU with a high magnetic field and large magnetic force.

Status and future of the soft X-ray free-electron laser beamline at the SHINE

The Shanghai High repetition rate XFEL and Extreme light facility (SHINE) is under construction and aims at generating X-rays between 0.4 and 25 keV with three FEL beamlines at repetition rates of up to 1 MHz. The soft X-ray FEL beamline, FEL-II, will be ready for commissioning in 2025. It is designed to cover the photon energy from 0.4 to 3 keV, in which the baselines of the FEL operation modes are self-amplified spontaneous emission (SASE), self-seeding, echo-enabled harmonic generation (EEHG), and polarization control. Therefore, a high repetition-rate external seed laser, large period length modulator, soft X-ray monochromator, planar undulator, and elliptically polarized undulator have been adopted in the FEL-II beamline. Several potentials such as an ultra-short pulse mode and a multi-color mode are also foreseeable without significant equipment changes in the follow-up operation. A dual-period undulator design is suggested for the echo-enabled harmonic generation (EEHG) commissioning, and it has great potential to break through the unreachable energy of the fully coherent X-ray in the future. The FEL-II beamline will deliver SASE radiation and fully coherent radiation in all the wavelengths of interest.

Improvement of undulator radiation intensity using edge radiation

The intensification of undulator radiation caused by the application of edge radiation due to a relativistic electron passing through a straight section is analytically and numerically investigated. Here, the modification of the undulator orbit shape of the cosine wave function to a triangular wave function is shown to increase the peak radiation for an undulator. The principle of the intensification effect is based on the superimposition of the edge radiation field from tilted straight sections, which create the triangular wave orbit. The tilt angles for the straight sections have values of rad for the longitudinal axis of the undulator, where is a relativistic factor. The longitudinal length of a straight section has a value of for a radiation wavelength Using a simple model, an analytical equation for the radiation intensity for an undulator with a triangular orbit is derived. The results obtained using the equation are in agreement with those obtained using a numerical radiation code. The peak radiation intensity is estimated to be about two times larger than that of an ordinary planar undulator with K = 1 and the same longitudinal length.

Optical design of micro-focus X-ray photon correlation spectroscopy beamline at Shenzhen Light Source

An optical design study of a beamline proposed for the 3 GeV synchrotron radiation facility, the Shenzhen Light Source, is described. The beamline was designed to cover an energy range from 2.0 to 20 keV with two experimental station, one for X-ray absorption spectroscopy (XAS) experiments and the other for X-ray photon correlation spectroscopy (XPCS) experiments. A 4 m planar undulator with a relatively short magnetic period (20 mm) was used as the radiation source. Horizontally and vertically focusing mirrors separately focus the source along the horizontal and vertical directions, respectively, thus enhancing the flexibility of the beamline. The focus point was employed as the secondary source, and a pair of KB mirrors were used to achieve a near-circular focus spot of about 4.5μm×4.5μm. A Si(111) DCM was selected to produce the monochromatic light that covers the 2.0–20 keV energy range. When the photon energy is lower than 10 keV, the energy resolution can be better than 1.47 × 10−4 and the total flux is larger than 1 × 1014 photons/s. The longitudinal coherence length is in a few μm range. With aperture size adjustment, the transverse coherence length at the sample position is approximately 3μm and the coherent flux is up to 1012 phs/s for the photon energy around 2.5 keV. This excellent coherence performance can be applied for the purpose of X-ray photon correlation spectroscopy related experiments.

Characterization of helical APPLE X undulators with 90 mm period for the European XFEL

European XFEL is going to provide full polarization control in the soft X-ray SASE line (SASE3). For this purpose, four helical APPLE X undulators with 90 mm period are installed downstream with respect to the planar undulators of the SASE3 undulator line consisting of 21 planar undulators with 68 mm period. In this contribution, the measurement technique, as well as the results of the measurements and tuning of the APPLE X undulators performed at European XFEL are presented.

Design and Analyses of Xcos Simulation Model for On-Axis Planar Undulator Radiation

Xcos simulation model designed for the planar undulator radiation (UR) has been analyzed. The detailed explanation of the designed model is given. The Xcos model is designed by using the numerical approach. The intensity calculation results obtained from the Xcos simulation model are compared with analytical results obtained by using generalized Bessel functions (GBFs).

The vacuum chamber for the APPLE-X undulators at the European XFEL

Thanks to the installation of four APPLE X helical undulators, the SASE3 soft X-ray beamline at European XFEL will provide to the experimental stations the polarization control of the X-rays. The helical undulators are located downstream with respect to the already existing planar undulators. The design of the magnetic active parts and of the helical undulator support structure requires a new design of the vacuum chamber and its alignment system. This contribution describes the mechanical design, production process, UHV qualification process and alignment of the vacuum chamber.

Studies of self-design taper algorithms for free-electron lasers with planar and helical undulators

The article presents a numerical study of self-design taper algorithms for free-electron lasers (FELs) with planar and helical undulators. Tapering is a technique that changes the undulator parameters in order to increase the efficiency and the radiation power in FELs. Undulator tapering is one of the methods for designing multi-terawatt X-ray FELs (XFELs), which are necessary for research in the field of non-linear optics and 3D bioimaging. In this article, we study methods to optimize the taper profile K(z) that can enhance the efficiency of extracting energy from the electron beam in FELs with planar and helical undulators. Using the numerical code we apply the Kroll–Morton–Rosenbluth (KMR) model and its modification to a case of the FEL at the MAX IV Laboratory, as well as a case of the European XFEL, compare our results with previous ones and discuss the new results. The program was validated on the PAL-XFEL experiment. The application of the modified KMR method for FELs with helical undulators is also studied.

Parametric Optimization of Undulator Field for Short Period NI HTS Planar Undulator

There are increasing interests in high temperature superconducting(HTS) undulator with a gradual increase of engineering current density of REBCO conductor. Additionally, no-insulation(NI) winding technique may maximize the undulator field of HTS undulators mainly benefited from its high engineering current density. In this study, we present several design options of an NI HTS undulator to investigate its anticipated performance in terms of magnetic field strength. Specifically, maximum magnetic field along the beam axis depending on the magnetic gap, magnetic period length, winding dimension, and operating temperature is investigated. Then, these results are fitted using a scaling law to easily estimate the anticipated performances of NI HTS undulator at different magnetic gaps and magnetic periods. Finally, a comparison between NI HTS undulator, NbTi based low temperature superconducting undulator, and cryogenic permanent magnet undulator is discussed.