Conventional Undulator Research Articles

Chaotic dynamics in X-ray free-electron lasers with an optical undulator.

In this work, the chaotic motions of relativistic electrons in X-ray free-electron lasers are investigated using an optical undulator in the presence of a magnetized ion-channel background. To miniaturize X-ray light sources, the optical undulator is a promising concept. The optical undulator provides higher optical gain than conventional magnetostatic undulators due to its micrometer wavelength. In addition, it reduces the required electron beam energy from several GeV to the multi-MeV range to produce X-ray pulses. The interaction of an optical undulator with an intense relativistic electron beam is a highly non-linear phenomenon that can lead to chaotic dynamics. At synchrotron radiation sources, the possibility of chaos control for X-ray FELs can be critical for certain classes of experimental studies. The equations of motion for a relativistic electron propagating through the optical undulator in the presence of a magnetized ion-channel can be derived from the Hamiltonian of the interaction region. Simulation results revealed that the intensity of the perturbation route from orderly behavior to chaos depends on the beam density, axial magnetic field strength, ion-channel density parameter, and pump laser undulator. Specific values of parameters were obtained for the transition from regular to chaotic paths. Bifurcation diagrams of the system were plotted to demonstrate the origin of chaos at a critical point, and Poincaré maps were created to distinguish between chaotic and orderly motions of electrons. The proposed new scheme can help to improve X-ray FELs, which have potential usages in basic sciences, medicine, and industry.

Development of an insertion device selectively operational as a helical/figure-8 undulator.

An insertion device capable of switching the operation mode between helical and figure-8 undulators, and thus referred to as a helical-8 undulator, has been developed. It has the advantage that the on-axis heat load can be kept low regardless of the polarization state, even when a high K value is required to lower the fundamental photon energy. This is in contrast to conventional undulators in which the on-axis heat load tends to be significantly high to generate linearly polarized radiation with a high K value, and optical elements can be seriously damaged. The principle of operation, specification and light source performance of the developed helical-8 undulator are presented together with further options to enhance its capability.

Design, construction, and magnetic field characterization of a Segmented Adaptive-Gap In-Vacuum Undulator prototype for NSLS-II

Segmented Adaptive-Gap in-Vacuum Undulator (SAGU) prototype with different period lengths and gaps in segments for production of high flux and brightness hard X-rays is under development at NSLS-II. According to radiation flux and brightness calculation results, SAGU promises a significant spectral performance gain compared to a conventional in-vacuum undulator, especially in hard X-ray range. In order to reduce the cost of the project, three existing mechanical carriages of Mini-Gap Undulators from the former NSLS X-ray ring were partially de-assembled, redesigned and optimized in order to meet the required mechanical performance of the device. The magnetic module assembly of the first segment has been carried out using a sorting technique which progressively optimizes the magnetic field quality of the device. Additional magnetic tuning, using small dipole kick magnets and virtual shimming, is planned to further improve the electron trajectory straightness, in particular to compensate the angular kicks experienced by the electron beam at segment junctions as it travels through the device. This paper will describe the magnetic and mechanical design as well as the field measurement results, and the magnetic adjustment implemented to properly tune the field strength and minimize the total field integral produced by the segments.

Extremely brilliant crystal-based light sources

Brilliance of novel gamma-ray Crystal-based Light Sources (CLS) that can be constructed through exposure of oriented crystals to beams of ultra-relativistic charged particles is calculated basing on the atomistic scale numerical modeling of the channeling process. In an exemplary case study, the brilliance of radiation emitted in a diamond-based Crystalline Undulator LS by a 10 GeV positron beam available at present at the SLAC facility is computed. Intesity of CU radiation in the photon energy range 10^0 - 10^1 MeV, which is inaccessible to conventional synchrotrons, undulators and XFELs, greatly exceeds that of laser-Compton scattering LSs and can be higher than predicted in the Gamma Factory proposal to CERN. Construction of novel CLSs is a challenging task which constitutes a highly interdisciplinary field entangling a broad range of correlated activities. CLSs provide a low-cost altenative to conventional LSs and have enomorous number of applications.

Simulation and experimental study of short-period strong magnetic field undulator

Further reducing the period length will be a very attractive direction for the future development of undulators, and it is one of the effective ways to realize the miniaturization of free-electron laser facilities and obtain short wavelengths. In this paper, a novel planar hybrid permanent magnet undulator is proposed which can obtain a higher peak field with a short period length. The dimensions of the magnet blocks and the poles are numerically optimized for a sample undulator with a period length of 16 mm and a gap of 5 mm. Compared to the conventional hybrid undulator, the effective peak field can be enhanced by about 48%. An eight-period model is built and the result of the magnetic field measurement by a pulsed wire system shows a good consistent with the simulation result.

Optimization and Design of Cutting-Edge-Technology Tools for Natural Sciences Research in the Twenty-First Century: X-Ray Free Electron Lasers

Regarding the chronologic evolution of accelerator-based light sources, optimization and design issues of cutting-edge-technology insertion devices (i.e., undulators, wigglers, etc.) have been a big competitive concern since the beginning of the twenty-first century. Although these sources are strongly restricted by technological limitations in terms of radiation wavelength, leading X-ray free-electron laser (FEL) facilities are spending considerable efforts to survive in this scientific competition by modifying/updating their insertion devices. In other words, even though they all generate almost identical free-electron laser pulses, advertising a “superconducting undulator based light source” in comparison with a “conventional undulator based light source” may sound more attractive for some scientists. However, in contrast to superconducting undulators, conventional undulators do not require cryogenic cooling systems resulting in a cost-effective operation as a matter of course. Under the circumstances, it is shown that generation of hard X-ray FEL pulses is feasible via unique PPM undulators driven by an electron linear accelerator (linac) within the energy range of 5–10 GeV. The results reveal good consistency with the operating X-ray FEL sources worldwide.

Standard variable short period microwave-plasma undulator

Microwave-plasma undulator (MPU), as a standard short-period length undulator, is a desirable source of hard X-ray with continuous wide spectrum. In this article, the electron wiggling motion under the effect of transverse electric mode, propagating in cold plasma-filled waveguide, is investigated. The deflection parameter and period length of MPU in counter-propagating and co-propagating modes are calculated. Advantages of MPU over conventional and microwave undulators are investigated. It is shown that MPUs generate higher photon flux with overlapping harmonics compared to conventional standard undulators and cover wider energy range of synchrotron radiation. A rectangular MPU with period length of 2.98 mm and deflection parameter of 2.23 and a circular MPU with period length of 2.99 mm and deflection parameter of 2.23 are introduced. Also, a short variable period MPU, with the period length varying from 2.62 mm to 4.12 mm, and a helical MPU is introduced.

Crystal-based intensive gamma-ray light sources

We discuss design and practical realization of novel gamma-ray Crystal-based Light Sources (CLS) that can be constructed through exposure of oriented crystals (linear, bent, periodically bent) to beams of ultrarelativistic charged particles. In an exemplary case study, we estimate brilliance of radiation emitted in a Crystalline Undulator (CU) LS by available positron beams. Intensity of CU radiation in the photon energy range $10^0-10^1$ MeV, which is inaccessible to conventional synchrotrons, undulators and XFELs, greatly exceeds that of laser-Compton scattering LSs and can be higher than predicted in the Gamma Factory proposal to CERN. Brilliance of CU-LSs can be boosted by up to 8 orders of magnitude through the process of superradiance by a pre-bunched beam. Construction of novel CLSs is a challenging task which constitutes a highly interdisciplinary field entangling a broad range of correlated activities. CLSs provide a low-cost alternative to conventional LSs and have enormous number of applications.

Undulator configuration for helicity switching in in-vacuum undulators

We propose a new scheme to control the polarization state of undulator radiation, in which the helicity of circularly polarized radiation can be switched by translating the whole undulator along the horizontal direction. This is in contrast to the conventional method based on a phasing operation (longitudinal motion of magnetic arrays), which gives rise to a large variation of the three-dimensional magnetic force acting on the magnetic array. Because the mechanical design can be much simpler than conventional undulators equipped with a function for the phasing operation, the proposed scheme is well compatible with in-vacuum undulators. Numerical and experimental studies were carried out to reveal the performance and feasibility of the proposed scheme, showing its potential as an attractive option for the in-vacuum undulators.

Enhancing the Radiation Resistance of Undulator Permanent Magnets by Tilting the Easy Axis of Magnetization.

The undulator is a magnetic device usually consisting of a series of rare-earth permanent magnets (REPMs) arranged to generate a sinusoidal magnetic field, and is installed in synchrotron radiation and free electron laser facilities to periodically deflect high-energy electrons. Because the undulator is operated under a high-radiation environment, it is important to take possible measures to avoid the quality degradation, in particular the radiation damage of REPMs. We present a simple scheme to enhance the radiation resistance of REPMs in undulators, in which the easy axis of each REPM is tilted by 45°. Experimental studies have revealed that the radiation resistance of REPMs in this configuration is enhanced by an order of magnitude compared to that in the conventional undulator.

Development of mini-undulators for a table-top free-electron laser

AbstractThe development of laser wakefield accelerators (LWFA) over the past several years has led to an interest in very compact sources of X-ray radiation – such as “table-top” free electron lasers. However, the use of conventional undulators using permanent magnets also implies system sizes which are large. In this work, we assess the possibilities for the use of novel mini-undulators in conjunction with a LWFA so that the dimensions of the undulator become comparable with the acceleration distances for LWFA experiments (i.e., centimeters). The use of a prototype undulator using laser machining of permanent magnets for this application is described and the emission characteristics and limitations of such a system are determined. Preliminary electron propagation and X-ray emission measurements are taken with a LWFA electron beam at the University of Michigan.

Two-Frequency Undulators for Generation of X-Ray Radiation in Free-Electron Lasers

We present a theoretical study and a computer simulation of characteristics of the undulator radiation in single-pass free-electron lasers (FELs). Using a phenomenological model describing the dynamics of the radiated power in FELs with allowance for the basic loss, we study generation of harmonics in the X-ray range in a FEL with a two-frequency undulator. We study the possibility to achieve a hundredfold increase in the radiation intensity of the nth harmonic in a FEL, in which the electron-phase shift by kπ/n with respect to photons occurs between undulator sections, where k = 2, 4, . . . . The advantages of using a two-frequency undulator in a single-pass FEL and the possibility of generating the high-power X-ray radiation by the FEL at the harmonic wavelengths 2.3–3.3 nm in the linear regime are demonstrated. The FEL is compared with the two-frequency undulator and the conventional plane undulator. Additionally, generation of radiation having a power of tens of megawatts is studied at the wavelength λ ≈ 3.27 nm in a multistage FEL with a length of 40 m, an off-the-shelf excimer ultraviolet seed laser, which operates at a wavelength of 157 nm, and an electron beam having an energy of about 0.6 GeV.

Development of PAL-XFEL undulator system

Pohang Accelerator Laboratory (PAL) have developed a 0.1 nm SASE based FEL based on 10 GeV S-band linear accelerator named PAL-XFEL. At the first stage, PAL-XFEL needs two undulator lines for photon source. The hard X-ray undulator line requires 20 units of 5 m long hybrid-type conventional planar undulator and soft X-ray line requires 7 units of 5 m long hybrid type planar undulators. PAL have developed undulator magnetic structure based on EU-XFEL concepts. In this report, the impact of the background field, alignment issues, the results of pole height tuning, and final magnetic measurement results are summarized.

Results of test of prototype of variable period undulator

The development of the permanent-magnet variable-period undulator (VPU) is aimed at improving the parameters of the Novosibirsk free electron laser (FEL). The key features of this design are the possibility of increasing the number of poles for shorter periods with constant undulator length and wider radiation wavelength tuning range as compared with conventional undulators. As the idea of the permanent-magnet VPU has not been properly tested yet, there are several issues to be solved before THE manufacture of the device. Two short prototypes of the VPU were made for the purpose of testing solutions to existing problems. We present here the results of mechanical and magnetic measurements of the undulator prototypes and compare the characteristics of the prototypes with those predicted by simulations.

Lightweight-compact variable-gap undulator with force cancellation system based on multipole monolithic magnets.

A lightweight-compact variable-gap undulator (LCVGU) having the force-cancellation system based on the multipole monolithic magnets (MMMs) has been developed. The LCVGU is free from the heavy mechanical frames, which is a fundamental element specific to conventional variable-gap undulators (VGUs) because of a strong attractive force, and thus the cost and time for construction and installation are expected to be significantly reduced; the MMMs counteract the strong attractive force in a cost-effective manner. Results of mechanical tests and magnetic-field measurements of two prototype LCVGUs equipped with the proposed force cancellation system have revealed the comparable performance with the conventional VGUs.

Polarization characteristics of radiation in both ‘light’ and conventional undulators

As a rule, an intensity spectrum of undulator radiation (UR) is calculated by using the classical approach, even for electron energy higher than 10GeV. Such a spectrum is determined by an electron trajectory in an undulator while neglecting radiation loss. Using Planck’s law, the UR photon spectrum can be calculated from the obtained intensity spectrum, for both linear and nonlinear regimes.The electron radiation process in a field of strong electromagnetic waves is considered within the quantum electrodynamics framework, using the Compton scattering process or radiation in a ‘light’ undulator. A comparison was made of the results from using these two approaches, for UR spectra generated by 250-GeV electrons in an undulator with a 11.5-mm period; this comparison shows that they coincide with high accuracy. The characteristics of the collimated UR beam (i.e. spectrum and circular polarization) were simulated while taking into account the discrete process of photon emission along an electron trajectory in both undulator types. Both spectral photon distributions and polarization dependence on photon energy are ‘smoothed’, in comparison to that expected for a long undulator—the latter of which considers the ILC positron source (ILC Technical Design Report).

The prototype of new variable period undulator for Novosibirsk free electron laser

To improve the parameters of the second stage Novosibirsk free electron laser we plan to replace the existing electromagnetic undulator by permanent-magnet variable-period undulator (VPU). The VPUs have several advantages compared to conventional undulators, which include wider radiation wavelength tuning range and an option to increase the number of poles. Both these advantages will be realized in the new undulator under development in Budker INP. There are some technical problems, which have to be solved before this idea can be implemented in practice. To check the solution of these problems we designed and manufactured a small undulator prototype, which has just several periods. In this paper, the results of mechanical and magnetic measurements of this undulator prototype are presented and compared with simulations.

Current status of the TwinMic beamline at Elettra: asoft X-ray transmission and emission microscopy station.

The current status of the TwinMic beamline at Elettra synchrotron light source, that hosts the European twin X-ray microscopy station, is reported. The X-ray source, provided by a short hybrid undulator with source size and divergence intermediate between bending magnets and conventional undulators, is energy-tailored using a collimated plane-grating monochromator. The TwinMic spectromicroscopy experimental station combines scanning and full-field imaging in a single instrument, with contrast modes such as absorption, differential phase, interference and darkfield. The implementation of coherent diffractive imaging modalities and ptychography is ongoing. Typically, scanning transmission X-ray microscopy images are simultaneously collected in transmission and differential phase contrast and can be complemented by chemical and elemental analysis using across-absorption-edge imaging, X-ray absorption near-edge structure or low-energy X-ray fluorescence. The lateral resolutions depend on the particular imaging and contrast mode chosen. The TwinMic range of applications covers diverse research fields such as biology, biochemistry, medicine, pharmacology, environment, geochemistry, food, agriculture and materials science. They will be illustrated in the paper with representative results.

Radiation emitted by transverse-gradient undulators

Conventional undulators are used in synchrotron light sources to produce radiation with a narrow relative spectral width as compared to bending magnets or wigglers. The spectral width of the radiation produced by conventional undulators is determined by the number of undulator periods and by the energy spread and emittance of the electron beam. In more compact electron sources like for instance laser plasma accelerators the energy spread becomes the dominating factor. Due to this effect these electron sources cannot in general be used for high-gain free electron lasers (FELs). In order to overcome this limitation, modified undulator schemes, so-called transverse gradient undulators (TGUs), were proposed and a first superconducting TGU was built at Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany. In this paper simulations of the expected synchrotron radiation spectral distribution are presented. An experimental test with that device is under preparation at the laser wakefield accelerator at the JETI laser at the University of Jena, Germany.

Radiation from relativistic electrons in "light" and in conventional undulators. Classical and quantum approaches

A photon spectrum of undulator radiation (UR) is calculated in the semi-classical approach. The UR intensity spectrum is determined by an electron trajectory in the undulator neglecting by energy losses for radiation. Using the Planck's law, the UR photon spectrum can be calculated from the classical intensity spectrum both for linear and nonlinear regimes. The radiation of an electron in a field of strong electromagnetic wave (radiation in the "light" undulator) is considered in the quantum electromagnetic frame. Comparison of results obtained by both approaches has been shown that UR spectra in the whole cone coincide with high accuracy for the case x<<1. Characteristics of the collimated UR beam were simulated with taking into account the discrete process of photon emission along an electron trajectory in both kinds of undulators.