Undulator Configuration 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.

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.

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.

An X-ray free-electron laser with a highly configurable undulator and integrated chicanes for tailored pulse properties

X-ray free-electron lasers (FELs) are state-of-the-art scientific tools capable to study matter on the scale of atomic processes. Since the initial operation of X-ray FELs more than a decade ago, several facilities with upgraded performance have been put in operation. Here we present the first lasing results of Athos, the soft X-ray FEL beamline of SwissFEL at the Paul Scherrer Institute in Switzerland. Athos features an undulator layout based on short APPLE-X modules providing full polarisation control, interleaved with small magnetic chicanes. This versatile configuration allows for many operational modes, giving control over many FEL properties. We show, for example, a 35% reduction of the required undulator length to achieve FEL saturation with respect to standard undulator configurations. We also demonstrate the generation of more powerful pulses than the ones obtained in typical undulators. Athos represents a fundamental step forward in the design of FEL facilities, creating opportunities in FEL-based sciences.

A photolithography tailored undulating configuration to confine nanostructures on microtubes via a “stress-induced” effect for boosting sodium ion storage

Undulating structure and internal graphite lattice distorted structure were induced via high-penetrating gamma irradiation. A stress-inducing strategy is supplied to obtain electrode materials. Advanced characterization was used to assess the structural evolution.

Photolithography Tailored Undulating Configuration to Confine Nanostructures on Microtubes Via “Stress-Induced” Effect for Boosting Sodium Ion Storage

Photolithography Tailored Undulating Configuration to Confine Nanostructures on Microtubes Via “Stress-Induced” Effect for Boosting Sodium Ion Storage

Variable polarization states in free-electron lasers

Free-electron lasers (FELs) can emit light with different optical polarizations including linear, elliptic and circular polarizations corresponding to the characteristics of the undulators used. X-ray FELs depend upon long undulator lines consisting of a sequence of short undulators. Linearly polarized undulators are most commonly used; hence the optical output is linearly polarized. Alternately, APPLE-II, Delta undulator designs, or a sequence of linearly polarized undulators with alternating orientations can be used to produce undulating magnetic fields with arbitrary polarizations. We present a three-dimensional, time-dependent formulation that self-consistently includes two optical orientations and, therefore, treats any given sequence or combination of undulator including undulator imperfections and degradation There are two principal characteristics of the formulation that underpin this capability. First, particles are tracked using the full Newton–Lorentz force equations with analytic models of the undulators fields. This permits an accurate model of the interaction of the electrons with a large variety of undulator fields and orientations. Second, the electrons can couple simultaneously to two independent electromagnetic polarizations and, therefore, the optical polarization evolves self-consistently along the undulator line. We present the numerical model and give some examples using prevailing undulator configurations.

Polarization control of an x-ray free electron laser oscillator

High-intensity, fully coherent x-ray pulse with a tunable polarization over a wide spectral range is of great importance to many experiments. In this paper, we propose a tapered crossed-polarized undulator configuration for an x-ray free electron laser oscillator (XFELO) to produce arbitrarily polarized x-ray pulses in the hard x-ray region. A numerical example utilizing the parameters of the Shanghai High-Repetition-Rate XFEL and Extreme Light Facility (SHINE) is presented to demonstrate the generation of polarization controllable, fully coherent hard x-ray pulses with 99.9% polarization degree and 20 kHz polarization switching rate. The analysis presents that this scheme holds the possibility to be used in an energy tunable XFELO built from multiple Bragg mirrors.

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.

Effect of undulating blades on highly loaded compressor cascade performance

Humpback whale’s flipper with leading-edge tubercles has been attracting aerodynamic and hydrodynamic researchers’ attentions by its stall-delayed characteristics. Inspired by this, the undulating configuration is used in a highly loaded compressor cascade as a new type of passive flow control technique. A new model of undulating compressor blade is studied in this paper. To investigate the effect of the undulating configuration on cascade performance without the impact of endwall, steady Reynolds-averaged Navier–Stokes simulations of infinite-span cascades are carried out with and without undulations at an inlet Mach number of 0.5. A parametric study is performed to conclude that, with a suitable wavelength, the undulating blade could achieve a rise in diffusion capacity, accompanied by 12.9% reduction in total pressure loss coefficient at a post-stall incidence angle of 8°, whereas it produces negligible impact in cascade performance at 0° incidence angle. Flow visualization further reveals that wavelength is a crucial parameter, determining the spanwise space for the formation of streamwise vortices. Undulating blades could produce positive effects with maximum magnitude when the counter-rotating streamwise vortices take dominant position along span with an appropriate size.

Studies of a terawatt x-ray free-electron laser

The resolution in x-ray coherent diffractive imaging applications can be improved by increasing the number of photons in the optical pulse. An x-ray free-electron laser (XFEL) producing pulses with terawatt (TW) peak power and about 10 femtosecond duration can satisfy this requirement. In this paper, we consider the conditions necessary for achieving powers in excess of 1 TW in a 1.5 Å FEL. Using the MINERVA simulation code, an extensive steady-state analysis has been conducted using a variety of undulator and focusing configurations. In particular, strong focusing using FODO lattices is compared with the natural, weak focusing inherent in helical undulators. It is found that the most important requirement to reach TW powers is extreme transverse compression of the electron beam in a strong FODO lattice in conjunction with a tapered undulator. We find that when the current density reaches extremely high levels, that the characteristic growth length in the tapered undulator becomes shorter than the Rayleigh range giving rise to optical guiding. We also show that planar undulators can reach near-TW power levels. In addition, preliminary time-dependent simulations are also discussed and show that TW power levels can be achieved both for self-seeding and pure self-amplified spontaneous emission. This result shows that high-resolution, single molecule diffractive imaging may be realized using XFELs.

Matching-based fresh-slice method for generating two-color x-ray free-electron lasers

A two-color free-electron laser pulse can be generated in a split undulator configuration, where a controllable quadrupole wakefield induces a time-dependent mismatch along the bunch.

Terawatt-Isolated Attosecond X-ray Pulse Using a Tapered X-ray Free Electron Laser

High power attosecond (as) X-ray pulses are in great demand for ultrafast dynamics and high resolution microscopy. We numerically demonstrate the generation of a ~230 attosecond, 1.5 terawatt (TW) pulse at a photon energy of 1 keV, and a 115 attosecond, 1.2 TW pulse at a photon energy of 12.4 keV, using the realistic electron beam parameters such as those of Korean X-ray free electron laser (XFEL) in a tapered undulator configuration. To compensate the energy loss of the electron beam and maximize its radiation power, a tapering is introduced in the downstream section of the undulator. It is found that the tapering helps in not only amplifying a target radiation pulse but also suppressing the growth of satellite radiation pulses. Tapering allows one to achieve a terawatt-attosecond pulse only with a 60 m long undulator. Such an attosecond X-ray pulse is inherently synchronized to a driving optical laser pulse; hence, it is well suited for the pump-probe experiments for studying the electron dynamics in atoms, molecules, and solids on the attosecond time-scale. For the realization of these experiments, a high level of synchronization up to attosecond precision between optical laser and X-ray pulse is demanded, which can be possible by using an interferometric feedback loop.

Three-dimensional, time-dependent simulation of free-electron lasers with planar, helical, and elliptical undulators

Free-electron lasers (FELs) have been built ranging in wavelength from long-wavelength oscillators using partial wave guiding through ultraviolet through hard x-ray that are either seeded or start from noise. In addition, FELs that produce different polarizations of the output radiation ranging from linear through elliptic to circular polarization are currently under study. In this paper, we develop a three-dimensional, time-dependent formulation that is capable of modeling this large variety of FEL configurations including different polarizations. We employ a modal expansion for the optical field, i.e., a Gaussian expansion with variable polarization for free-space propagation. This formulation uses the full Newton–Lorentz force equations to track the particles through the optical and magnetostatic fields. As a result, arbitrary three-dimensional representations for different undulator configurations are implemented, including planar, helical, and elliptical undulators. In particular, we present an analytic model of an APPLE-II undulator to treat arbitrary elliptical polarizations, which is used to treat general elliptical polarizations. To model oscillator configurations, and allow propagation of the optical field outside the undulator and interact with optical elements, we link the FEL simulation with the optical propagation code OPC. We present simulations using the APPLE-II undulator model to produce elliptically polarized output radiation, and present a detailed comparison with recent experiments using a tapered undulator configuration at the Linac Coherent Light Source. Validation of the nonlinear formation is also shown by comparison with experimental results obtained in the Sorgente Pulsata Auto-amplificata di Radiazione Coerente SASE FEL experiment at ENEA Frascati, a seeded tapered amplifier experiment at Brookhaven National Laboratory, and the 10 kW upgrade oscillator experiment at the Thomas Jefferson National Accelerator Facility.

Storage ring two-color free-electron laser

We report a systematic experimental study of a storage ring two-color free-electron laser (FEL) operating simultaneously in the infrared (IR) and ultraviolet (UV) wavelength regions. The two-color FEL lasing has been realized using a pair of dual-band high-reflectivity FEL mirrors with two different undulator configurations. We have demonstrated independent wavelength tuning in a wide range for each lasing color, as well as harmonically locked wavelength tuning when the UV lasing occurs at the second harmonic of the IR lasing. Precise power control of two-color lasing with good power stability has also been achieved. In addition, the impact of the degradation of FEL mirrors on the two-color FEL operation is reported. Furthermore, we have investigated the temporal structures of the two-color FEL beams, showing simultaneous two-color micropulses with their intensity modulations displayed as FEL macropulses.

Applicability of a double-undulator configuration

The applicability of the double-undulator concept for an electron storage ring of 3-GeV class is evaluated based on the parameters of Taiwan Photon Source. In the soft X-ray case, the fundamental harmonic is mainly used, the interference effect is preserved at some level, which means that the brilliance from a double-undulator is expected to be much greater than that of a single undulator. In the hard X-ray case, harmonics number greater than five are generally used, the interference effect cannot, however, be preserved, which means that a double undulator configuration can be assumed to comprise two independent and uncorrelated sources. The total coherent flux obtained from a double-undulator configuration is found to be much less than twice that of a single undulator. The double-undulator concept is hence inapplicable in the hard X-ray region from the viewpoint of high coherent flux performance.

The formation mechanism and impact of streamwise vortices on NACA 0021 airfoil's performance with undulating leading edge modification

Wings with tubercles have been shown to display advantageous loading behavior at high attack angles compared to their unmodified counterparts. In an earlier study by the authors, it was shown that an undulating leading-edge configuration, including but not limited to a tubercled model, induces a cyclic variation in circulation along the span that gives rise to the formation of counter-rotating streamwise vortices. While the aerodynamic benefits of full-span tubercled wings have been associated with the presence of such vortices, their formation mechanism and influence on wing performance are still in question. In the present work, experimental and numerical tests were conducted to further investigate the effect of tubercles on the flow structure over full-span modified wings based on the NACA 0021 profile, in the transitional flow regime. It is found that a skew-induced mechanism accounts for the formation of streamwise vortices whose development is accompanied by flow separation in delta-shaped regions near the trailing edge. The presence of vortices is detrimental to the performance of full-span wings pre-stall, however renders benefits post-stall as demonstrated by wind tunnel pressure measurement tests. Finally, primary and secondary vortices are identified post-stall that produce an enhanced momentum transfer effect that reduces flow separation, thus increasing the generated amount of lift.

A SASE free electron laser optimization based on new generation in-vacuum undulators

Optimization studies for an accelerator based light source, namely self-amplified spontaneous emission (SASE) free electron laser (FEL), based on new generation in-vacuum hybrid and superconducting undulator configurations, are compared and discussed. It is shown that the FEL wavelength should be down to soft X-rays ( ∼ 3 nm ) part of the spectrum while keeping the same linear accelerator (linac) energy about 1 GeV. On the other hand, numerical calculations and simulation results of the main performance parameters for SASE operation (1D gain length, saturation power and saturation length), are optimized. Finally, technological advantages and challenges for both cases, are briefly mentioned.

Pulse Splitting in Short Wavelength Seeded Free Electron Lasers

We investigate a fundamental limitation occurring in vacuum ultraviolet and extreme ultraviolet seeded free electron lasers (FELs). For a given electron beam and undulator configuration, an increase of the FEL output energy at saturation can be obtained via an increase of the seed pulse duration. We put in evidence a complex spatiotemporal deformation of the amplified pulse, leading ultimately to a pulse splitting effect. Numerical studies of the Colson-Bonifacio FEL equations reveal that slippage length and seed laser pulse wings are core ingredients of the dynamics.

Coherent Harmonic Generation experiments on UVSOR-II storage ring

Coherent Harmonic Generation (CHG) is of great interest for new Free Electron Laser light sources in the VUV—soft X-ray region. In this configuration, an external laser is focused inside a first undulator. The interaction between the electron beam and the external laser leads to a density modulation resulting in coherent emission with harmonic content in a second undulator. Experiments have been carried out on the UVSOR-II FEL (Okasaki, Japan) seeded with a Ti:Sa laser at 800 nm wavelength, 2.5 mJ, 1 kHz repetition rate and 1.2 ps pulse duration. In this paper, generation of the second, third and fourth harmonics of the fundamental (400, 266 and 200 nm) using planar undulators is reported, together with angular distribution measurement of the second harmonic. We also present coherent generation of the second and third harmonic using helical undulators. These investigations on even harmonic generation with both undulator configurations may provide interesting insight in nonlinear harmonic generation with FELs for next generation FELs, aiming at GW pulse delivery at sub-nm wavelengths.