High-gain Free-electron Laser Research Articles

High-gain x-ray free-electron laser driven by a laser-plasma accelerator using a vertical gradient optical undulator

Recent advances in the studies of laser-plasma accelerators (LPAs) and optical undulators enable the possibility of designing a compact x-ray free-electron laser (FEL). However, the LPA beams usually have a percent-level energy spread that will degrade the FEL performance. To compensate for the energy spread effect, an optical undulator with a transverse gradient has been proposed, in which a dispersed LPA beam is injected into a horizontally polarized Gaussian laser with a horizontal offset relative to the laser center. Such a scheme, however, leads to an inherent orbit deflection and calls for accurate orbit correction. In this paper, we propose an alternative method that does not induce any orbit deflection and requires less total laser power. In this method, a dispersed LPA beam is sent into an optical undulator with a vertical gradient generated by the combination of a Hermite-Gaussian TEM01 mode laser and a TEM00 mode laser. Simulations suggest that this method is feasible to generate megawatt-level soft x-ray FEL by using an LPA beam with the energy of a few tens of MeV.

Simulation of pre-bunched free electron laser oscillator in the THz regime

One of the long-term objectives in the development of high gain free electron lasers (FEL) is to reduce the necessary electron beam voltage for a strong FEL interaction at a given frequency. FEL oscillators (FELOs) play the main role to this end. In this paper, the simulation of one dimensional FELO with planar wiggler is done at the applicable Tera-hertz regime. The effect of pre-bunched electron beam on the gain improvement or on the laser length is investigated. To study the evolution of system, a set of self-consistent nonlinear differential equations are solved numerically by the Runge-Kutta method and the averaging of electron beam is done by the Simpson method.

Transverse-to-longitudinal emittance-exchange in optical wavelength

Emittance exchange is a promising technique for next-generation accelerator-based applications. A novel technique is proposed in this paper to exchange emittance of the electron beam between transverse and longitudinal planes in optical wavelength. The emittance exchange configuration consists of a dual-tilted-laser modulator sandwiched by two identical doglegs. Analytical and simulation results demonstrate that the emittance exchange for the electrons at the zero-crossing phase of each laser cycle can be easily achieved with this technique. The proposed technique is quite promising for improving the performance of compact high-gain free-electron laser facilities. Meanwhile, it holds the feasibility to flexibly tailor a beam’s longitudinal shape in the optical scale.

Physics issues of the diffraction-limited storage ring light source

<p indent=0mm>The synchrotron radiation light source based on electron storage ring is the most widely used high-performance X-ray source in the field of basic scientific research in the 20th century, and has undergone three generations of development. One important direction of the new generation light source, i.e., the fourth generation light source, is the so-called diffraction-limited storage ring (DLSR) light source. DLSR, with ultra-high average brightness, high repetition frequency, multi-user, high stability, complementary to the free electron laser (FEL), has a great development in recent decades. By adopting lattice design based on compact multi-bend achromats (MBAs), DLSR allows to achieve one or two orders of magnitude lower emittance approaching the diffraction limit of X-rays, and much higher brightness and coherence than available in the third generation light source, while inheriting the high brightness, multi-user and high stability advantages of existing ring light sources. In this paper, the progress made in the last few decades in physics design and optimization of the DLSR, especially on the ultralow-emittance lattice and injection schemes, will be overviewed. To achieve an ultralow emittance, lattice structures of standard MBA and hybrid MBA, and novel magnets of high-gradient quadrupole, antibend, longitudinal gradient dipole and complex bend, have been proposed. To obtain optimized beam dynamics in a DLSR, theoretical and numerical methods have been developed to date. Due to the fact that the nonlinearities are extremely high in a DLSR and the nonlinear dynamics is greatly coupled with the linear optics, the most effective and most commonly used way is global and stochastic optimization of the ring performance evaluated by numerical tracking. To deal with the challenges of injecting to a DLSR with small dynamic acceptance, different injection schemes, such as, pulsed multipole injection, on-axis swap-out injection and on-axis longitudinal injection, have been proposed. Collective effects, especially the intra beam scattering and the Touschek effects, become significant as emittance decreases, requiring methods of bunch lengthening and transverse feedback to ensure the stability of the particle motion at a high beam current. Besides, a few developing topics in DLSR physics will be discussed. Machine learning, the method of transferring disorder data into useful information, is an important branch of artificial intelligence, and has been used in accelerator control system and beam commissioning. This method would be very useful and helpful in the design and optimization, initial commissioning and daily operation of a DLSR. A rational combination of machine learning and multi-objective optimization algorithm, like multi-objective genetic algorithm and multi-objective particle swarm algorithm, can effectively improve the optimization performance, and has been successfully applied in nonlinear optimization of light sources. In addition, improving the performance of synchrotron radiation by combining the beam physics of DLSR and the principle of FEL is one developing research topic which might be able to provide base for further development of the DLSR. Combined with bypass and transverse gradient undulator, high gain FEL could be achieved in the storage ring. Besides, double ring light source, divided into two loops, the inner ring is DLSR, and the other is FEL with more long straight sections, was proposed to improve the utilization rate of the beams.

Plasma wakefield accelerator driven coherent spontaneous emission from an energy chirped electron pulse

Plasma accelerators (Esary et al 2009 Rev. Mod. Phys. 81 1229) are a potentially important source of high energy, low emittance electron beams with high peak currents generated within a relatively short distance. As such, they may have an important application in the driving of coherent light sources such as the Free Electron Laser (FEL) which operate into the x-ray region (McNeil and Thompson 2010 Nat. Photon. 4 814–21). While novel plasma photocathodes (Hidding et al 2012 Phys. Rev. Lett. 108 035001) may offer orders of magnitude improvement to the normalized emittance and brightness of electron beams compared to Radio Frequency-driven accelerators, a substantial challenge is the energy spread and chirp of beams, which can make FEL operation impossible. In this paper it is shown that such an energy-chirped, ultrahigh brightness electron beam, with dynamically evolving current profile due to ballistic bunching at moderate energies, can generate significant coherent radiation output via the process of Coherent Spontaneous Emission (CSE) (Campbell and McNeil 2012 Proc. FEL2012 (Nara, Japan)). While this CSE is seen to cause some FEL-induced electron bunching at the radiation wavelength, the dynamic evolution of the energy chirped pulse dampens out any high-gain FEL interaction. This work may offer the prospect of a future plasma driven FEL operating in the high-gain Self Amplified CSE mode.

Generating three-color pulses in high-gain harmonic-generation free-electron lasers with atilted electron bunch.

A multi-color light source is a significant tool for nonlinear optics experiments, pump-dump/repump-probe experiments and in other fields. Here, a novel method is proposed to create three-color pulses based on a high-gain harmonic-generation (HGHG) free-electron laser with a tilted electron bunch. In this method, the initial bunch tilt is created by transverse wakefields after the bunch passes through a corrugated structure with an off-axis orbit, and is further enlarged in a following drift section. Then the tilted bunch experiences the off-axis field of a quadrupole magnet to cool down the large transverse velocity induced before. After that, it enters an HGHG configuration adopting a transverse gradient undulator (TGU) as the radiator, where only three separated fractions of the tilted bunch will resonate at three adjacent harmonics of the seed wavelength and are enabled to emit three-color pulses simultaneously. In addition, the use of the natural transverse gradient of a normal planar undulator instead of the TGU radiator to emit three-color pulses is also studied in detail. Numerical simulations including the generation of the tilted bunch and the free-electron laser radiation confirm the validity and feasibility of this scheme both for the TGU radiator and the natural gradient in the extreme-ultraviolet waveband.

High-gain quantum free-electron laser: Emergence and exponential gain

We derive an effective Dicke model in momentum space to describe collective effects in the quantum regime of a free-electron laser (FEL). The resulting exponential gain from a single passage of electrons allows the operation of a Quantum FEL in the high-gain mode and avoids the experimental challenges of an X-ray FEL oscillator. Moreover, we study the intensity fluctuations of the emitted radiation which turn out to be super-Poissonian.

Realizing quantum free-electron lasers: a critical analysis of experimental challenges and theoretical limits

We examine the experimental requirements for realizing a high-gain quantum free-electron laser (QFEL). Beyond fundamental constraints on electron beam and undulator, we discuss optimized interaction geometries, include coherence properties along with the impact of diffraction, space-charge and spontaneous emission. Based on desired QFEL properties, as well as current experimental capabilities, we provide a procedure for determining a corresponding set of experimental parameters. Even for an idealized situation, the combined constraints on space-charge and spontaneous emission put strong limits on sustaining the quantum regime over several gain lengths. Guided by these results we propose to shift the focus towards seeded QFELs instead of continuing to aim for self-amplified spontaneous emission. Moreover, we point out the necessity of a rigorous quantum theory for spontaneous emission as well as for space-charge in order to identify possible loopholes in our line of argument.

One way only to synchrotron light sources upgrade?

The last decade has seen a renaissance of machine-physics studies and technological advancements that aim to upgrade at least 15 synchrotron light sources worldwide to diffraction-limited storage rings. This is expected to improve the average spectral brightness and transversally coherent fraction of photons by several orders of magnitude in the soft- and hard-X-ray wavelength range, at the expense of pulse durations longer than ∼80 ps FWHM. This paper discusses the compatibility of schemes for the generation of sub-picosecond photon-pulse durations in synchrotron light sources with standard multi-bunch user operation and, in particular, diffraction-limited electron optics design. The question of this compatibility is answered taking into consideration the storage ring beam energy and the constraint of existing synchrotrons' infrastructure. An alternative scheme for the upgrade of medium-energy synchrotron light sources to diffraction-limited storage rings and the simultaneous production of picosecond-long photon pulses in a high-gain free-electron laser scheme are illustrated.

Single-pass high-gain tapered free-electron laser with transverse diffraction in the postsaturation regime

It has been well known that the resonant interaction of an ultrarelativistic electron beam and the radiation field in the single-pass high-gain free electron laser (FEL) amplifier leads to the optical gain guiding. The transverse Laplacian term of the slowly varying wave equation in the linear regime can be approximated as a constant detuning parameter, i.e., |∇⊥2|∼kR/zR where kR is the resonant wave number and zR is the Rayleigh range of the laser. In the post-saturation regime, the radiation power begins to oscillate about an equilibrium for the untapered case while continues to grow by undulator tapering. Moreover, in this regime the gain guiding decreases and the simple constant detune is no longer valid. In this paper we study the single-pass high-gain FEL performance in the post-saturation regime with inclusion of diffraction effect and undulator tapering. Our analysis relies upon two constants of motion, one from the energy conservation and the other from the adiabatic invariant of the action variable. By constructing a two-dimensional axisymmetric wave equation and the coupled one-dimensional electron dynamical equations, the performance of a tapered FEL in the postsaturation regime can be analyzed, including the fundamental mode profile, the power efficiency and the scaled energy spread. We begin the analytical investigation with two different axisymmetric electron beam profiles, the uniform and bounded parabolic ones. It is found that the tapered FEL power efficiency can be smaller but close to the taper ratio provided the resonant phase remains constant and the beam-wave is properly matched. Such a tapered efficiency is nearly independent of transverse electron beam size before significant electron detrapping occurs. This is essentially different from the untapered case, where the power extraction efficiency is around the essential FEL gain bandwidth (or ρ, the Pierce or FEL parameter) and depends on the beam size. It is also found that the power enhancement due to undulator tapering is attributed more by the field increase outside the transverse electron beam than that inside the transverse electron beam. Several scaling properties on the taper ratio and the transverse electron beam size are also discussed in this paper.3 MoreReceived 29 January 2018DOI:https://doi.org/10.1103/PhysRevAccelBeams.21.060702Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsSynchrotron radiation & free-electron lasersAccelerators & Beams

Sub-picosecond electron bunch length measurement using coherent transition radiation at SXFEL

Longitudinal electron bunch length plays a significant role in single-pass free-electron lasers (FEL), as the high-gain FEL process depends strongly on the high peak current of electron bunches. Longitudinal electron bunch length was measured by detecting the interferogram of coherent transition radiation generated by electron bunches using a THz interferometer and a Golay cell (spectral range 0.02–20 THz) at Shanghai X-ray free-electron laser. The detailed process of measurement and data analysis are discussed herein. Furthermore, the electron bunch length was estimated based on the dispersive strength $$ R_{56} $$ of the bunch compressor and the energy spread $$ \delta $$ of electron bunches, which were obtained via experiments. The comparison showed that the measured bunch length was consistent with the estimated bunch length.

The detrimental effect of spontaneous emission in quantum free electron lasers: A discrete Wigner model

We study the spontaneous emission in high-gain free-electron lasers operating in the quantum regime and its detrimental effect on coherent emission. A quantum model describing the coherent and spontaneous emission in free electron lasers has been recently proposed and investigated [G. R. M. Robb and R. Bonifacio, Phys. Plasmas 19, 073101 (2012)]. The model is based on a Wigner distribution describing the electron beam dynamics, coupled to Maxwell equations for the emitted radiation field. Here, we rephrase the model in a more rigorous way, considering a discrete Wigner distribution defined for a periodic space coordinate for which the electron momentum is discrete. From its numerical solution, we find good agreement with the approximate continuous model. In the quantum regime of the free-electron laser, we obtain a simple density matrix equation for two momentum states, where the role of the spontaneous emission has a clear interpretation in terms of coherence decay and population transfer.

Generation of coherent soft X-ray radiation in short FEL with harmonic multiplication cascades and two-frequency undulator

The generation of powerful nanometer x-ray radiation in cascaded high-gain free-electron lasers (FELs) with two-frequency undulator is studied. An advanced phenomenological model of single pass FEL is developed, which describes both linear and nonlinear harmonic generation from seed and initial shot noise; it includes calibrated corrections for all loss factors, specific for each FEL harmonic in each cascade. In this way, it differs from other phenomenological models, which account for the energy spread or divergence equally for all FEL harmonics. The new model, calibrated with a FEL experiment and numerical 3D simulations, is employed for the analysis of the FEL power dynamics to obtain maximum high-harmonic power in the X-ray band at the shortest possible FEL length with a low seed frequency and electron energy. The advantages of the two-frequency undulators in FEL prebunchers are demonstrated; their use in the cascaded FELs is proposed. Several such FELs with two-frequency undulators are simulated. The nanometer X-ray radiation production is studied in high-gain cascaded FELs with the seed wavelengths, matching peak reflectivities of MoRu/Be and Mo/Si. Dozens of megawatts of X-ray radiation are modeled at ≈40 m in a multi-cascade FEL, seeded from commercially available F2 excimer laser.

Sideband instability analysis based on a one-dimensional high-gain free electron laser model

When an untapered high-gain free electron laser (FEL) reaches saturation, the exponential growth ceases and the radiation power starts to oscillate about an equilibrium. The FEL radiation power or efficiency can be increased by undulator tapering. For a high-gain tapered FEL, although the power is enhanced after the first saturation, it is known that there is a so-called second saturation where the FEL power growth stops even with a tapered undulator system. The sideband instability is one of the primary reasons leading to this second saturation. In this paper, we provide a quantitative analysis on how the gradient of undulator tapering can mitigate the sideband growth. The study is carried out semianalytically and compared with one-dimensional numerical simulations. The physical parameters are taken from Linac Coherent Light Source-like electron bunch and undulator systems. The sideband field gain and the evolution of the radiation spectra for different gradients of undulator tapering are examined. It is found that a strong undulator tapering ($\ensuremath{\sim}10%$) provides effective suppression of the sideband instability in the postsaturation regime.

Fractional Derivatives, Memory Kernels and Solution of a Free Electron Laser Volterra Type Equation

The high gain free electron laser (FEL) equation is a Volterra type integro-differential equation amenable for analytical solutions in a limited number of cases. In this note, a novel technique, based on an expansion employing a family of two variable Hermite polynomials, is shown to provide straightforward analytical solutions for cases hardly solvable with conventional means. The possibility of extending the method by the use of expansion using different polynomials (two variable Legendre like) expansion is also discussed.

High-harmonic x-ray undulator radiation for nanoscale-wavelength free-electron lasers

We explore the undulator radiation (UR) from relativistic electrons in two-frequency undulators intended for use in x-ray free-electron lasers (FELs). We use an exact analytical account for the major sources of spectral line broadening of the spontaneous UR to describe the UR spectrum, intensity and emission line shape. We also show possible partial compensation of the off-axis divergence by ad hoc imposed weak transverse constant magnetic components. The harmonic radiation of the two-frequency undulator is studied. High-harmonics amplification in the two-frequency undulator is underlined for possible use in x-ray FELs and the generation of powerful x-ray radiation is demonstrated. The evolution of high harmonic power is studied with the help of the developed phenomenological self-amplified spontaneous emission FEL model. The performance of a high-gain harmonic generation FEL with a two-frequency undulator–prebuncher is explored. Harmonic multiplication cascades are introduced in FELs to obtain nanometer x-ray FEL radiation in reduced size FELs at ~30 m with seed from commercially available excimer laser and ~0.5 GeV electrons.

Eliminating the microbunching-instability-induced sideband in a soft x-ray self-seeding free-electron laser

Soft x-ray self-seeding has been proved to be a feasible method to improve the longitudinal coherence of high gain free-electron laser. However, a pedestal-like sideband in the spectrum has been observed in the experiment, which generally limits the purity of the radiation pulse and the user’s application. The previous theoretical study indicates that the pedestal-like sideband is mainly induced by microbunching instability generated from LINAC. In this paper, three dimensional simulations have been performed to confirm the analytical results and show the formation process of the spectral sideband. A probable method is proposed to eliminate the pedestal-like sideband by simply inserting a magnetic chicane before the self-seeding FEL undulator. Theoretical and numerical simulations have been performed and the results show that the proposed method can efficiently eliminate the microbunching-instability-induced sideband in a soft x-ray self-seeding FEL.

Compact compressive arc and beam switchyard for energy recovery linac-driven ultraviolet free electron lasers

High gain free electron lasers (FELs) driven by high repetition rate recirculating accelerators have received considerable attention in the scientific and industrial communities in recent years. Cost-performance optimization of such facilities encourages limiting machine size and complexity, and a compact machine can be realized by combining bending and bunch length compression during the last stage of recirculation, just before lasing. The impact of coherent synchrotron radiation (CSR) on electron beam quality during compression can, however, limit FEL output power. When methods to counteract CSR are implemented, appropriate beam diagnostics become critical to ensure that the target beam parameters are met before lasing, as well as to guarantee reliable, predictable performance and rapid machine setup and recovery. This article describes a beam line for bunch compression and recirculation, and beam switchyard accessing a diagnostic line for EUV lasing at 1 GeV beam energy. The footprint is modest, with 12 m compressive arc diameter and $\ensuremath{\sim}20\text{ }\text{ }\mathrm{m}$ diagnostic line length. The design limits beam quality degradation due to CSR both in the compressor and in the switchyard. Advantages and drawbacks of two switchyard lines providing, respectively, off-line and on-line measurements are discussed. The entire design is scalable to different beam energies and charges.

Angular Trajectory Kicks in a High-Gain Free-Electron Laser

Angular Trajectory Kicks in a High-Gain Free-Electron Laser

Effect of an angular trajectory kick in a high-gain free-electron laser

In a free-electron laser, a transverse momentum offset (or ``kick'') results in an oscillation of the centroid of the electron beam about the undulator axis. Studying the influence of this effect on the free-electron laser (FEL) interaction is important both from a tolerance point of view and for its potential diagnostic applications. In this paper, we present a self-consistent theoretical analysis of a high-gain FEL driven by such a ``kicked'' beam. In particular, we derive a solution to the three-dimensional, linearized initial value problem of the FEL through an orthogonal expansion technique and also describe a variational method for calculating the average FEL growth rate. Our results are benchmarked with genesis simulations and provide a robust theoretical background for a comparison with previous analytical results.