X-ray Free-electron Laser Oscillator Research Articles

Rapidly switchable x-ray orbital angular momentum from a free electron laser oscillator

X-ray vortices carrying tunable orbital angular momentum (OAM) are an emerging tool for x-ray characterization. However, in contrast to the generation of vortex beams in the visible wavelength region, the generation of x-ray vortices in a controlled manner has proved challenging. Here, we demonstrate an x-ray free-electron laser oscillator (XFELO) can adjust only the kinetic energy of the electron beam to produce vortex beams that can be programmed to dynamically change between different OAM modes, without the need for additional optical elements. With the nominal parameters of currently constructing 1 MHz repetition rate facility (i.e., Shanghai high-repetition-rate XFEL and Extreme Light Facility), the active formation of the OAM modes of l=±1 and l=±2 and the rapid switching between them by detuning the electron beam energy of the XFELO are numerically illustrated. The real-time switching can be achieved within 200 μs, while the output pulse energy can reach the 100 μJ level. This result extends the capabilities of XFELOs and paves the way for advanced at-source applications using x-ray vortex beams.

Mirror Vibration Tolerance Studies in X-ray Free-Electron Laser Oscillator

The X-ray free-electron laser oscillator (XFELO) has received significant attention due to its ability to produce fully coherent, high-brightness, and highly stable X-ray beams. Despite these advantages, the operation of the XFELO can be impeded by the surrounding environment. Specifically, vibrations of the optical components within the cavity can lead to poor alignment, which can diminish the interaction between the light and electrons in the undulator. Consequently, the quality of the output X-rays may be compromised. This study aims to investigate the impact of mirror vibrations on the output laser at various vibration frequencies. Firstly, we develop three single-frequency vibration models at 10 Hz, 0.01 MHz, and 1.1 MHz to investigate the changes in energy, spectral width, beam size, and beam divergence angle of the output laser. Secondly, we build a more complex multi-frequency vibration model based on the single-frequency one to simulate the realistic vibration of the mirror. Finally, we utilize the multi-frequency vibration model to investigate the tolerance limits of the output laser to vibration amplitude at different vibration frequencies of the mirror. The results show that the tolerance of the amplitude near the low and middle frequencies has less effect on the output power, which is approximately 250 nrad or more. However, in certain particular instances, particularly in the vicinity of the resonant frequency, there will be deviations from the tolerance limit. These deviations can result in values that are excessively high or excessively low. The study could prove useful in the future installation of XFELOs.

Transverse gradient undulator in a storage ring x-ray free electron laser oscillator

Modern electron storage rings produce bright x rays via spontaneous synchrotron emission, which is useful for a variety of scientific applications. The x-ray free-electron laser oscillator (XFELO) has the potential to amplify this output, both in terms of peak power and photon coherence. However, even current fourth generation storage rings (4GSRs) lack the requisite electron beam brightness to drive the XFELO due to its electron energy spread. The transverse gradient undulator (TGU) can overcome this issue, thus providing a practical means to couple the 4GSR to the XFELO. In this study, we first examine the theoretical basis of the TGU interaction by deriving the 3D TGU gain formula in the low gain approximation. Then, we perform an optimization study of the gain formula in order to determine optimal beam and machine parameters. Finally, we construct a hypothetical storage ring TGU-XFELO based on near-optimal parameters and report on its projected performance using multistage numerical simulation. We also discuss potential implementation challenges associated with the ring-FEL coupling.

Experimental setup for high-resolution characterization of crystal optics for coherent X-ray beam applications.

Stanford Synchrotron Radiation Lightsource serves a wide scientific community with its variety of X-ray capabilities. Recently, a wiggler X-ray source located at beamline 10-2 has been employed to perform high-resolution rocking curve imaging (RCI) of diamond and silicon crystals. X-ray RCI is invaluable for the development of upcoming cavity-based X-ray sources at SLAC, including the cavity-based X-ray free-electron laser and X-ray laser oscillator. In this paper, the RCI apparatus is described and experimental results are provided to validate its design. Future improvements of the setup are also discussed.

Stability of Bragg reflectors under megahertz heat load at XFELs

Modern X-ray free-electron laser (XFEL) sources can deliver photon pulses with millijoule pulse energies and megahertz repetition rate. As shown by the simulations in this work, for particular cases the dynamical heat load effects for Bragg reflectors could cause problems at these facilities. These problems would be underestimated if only quasi-static thermoelastic simulations are considered. Nevertheless, for the sake of simplicity the quasi-static approach is a common choice for estimating heat load effects. To emphasize the relevance of dynamical thermoelastic effects, the response to the partial absorption of an X-ray pulse, as provided by a saturated X-ray free-electron laser oscillator (XFELO) in a single crystal diamond with a thickness of 100 µm and lateral dimensions in the millimetre range, is discussed in this work. The outcome of the dynamic thermoelastic simulations indicates a clear dominance regarding the strain value reached, which is present for consecutive X-ray matter interactions with megahertz repetition rate.

Design of the nanopositioning and mechanical system for cavity-based X-ray free electron laser

With forming of an X-ray cavity with a set of narrow bandwidth diamond Bragg crystals and by storing and recirculating the output of an amplifier in an X- ray cavity so that the X-ray pulse can interact with following fresh electron bunches over many passes thus enabling the development of full temporal coherence, we will be able to build cavity-based X-ray FELs (CBXFELs) such as the X-ray free-electron laser oscillator (XFELO) and the X-ray regenerative amplifier free-electron laser (XRAFEL) [1, 2]. One of the key challenges to forming the X-ray cavity is the nanopositioning design and construction of the cavity mechanical system. In this paper, we present the design of the nanopositioning and mechanical system that is currently under construction for use in a proof-of-principle cavity-based X-ray free electron laser experiment at the LCLS-II at SLAC.

Misalignment effects on the performance and stability of x-ray free electron laser oscillator

We report on the effects of transverse spatial misalignment on the performance and stability of x-ray free electron laser oscillators (XFELOs). For this study, we adopt the FEL driven paraxial resonator model in which the transverse profile of the radiation field is represented by Gauss-Hermite mode expansion. Then, we apply Gaussian optics for transporting the radiation field along with the misalignment in the optical cavity. We divide the misalignment into relevant categories of static and periodic types to understand their effects on FEL gain, lasing, and saturation. After that, we associate the first order moments of the radiation field to that of driven simple harmonic oscillator models to identify regions of instability. Throughout this report, we validate our models and analyses via theoretical calculations, analytical approximations, and simulations.

Generating X-rays with orbital angular momentum in a free-electron laser oscillator

Light with orbital angular momentum (OAM) provides new insights into a wide range of physical phenomena and has engendered advanced applications in various fields. Additionally, interest in X-ray OAM has been rapidly rising. We present a straightforward method to generate intense OAM beams from an X-ray free-electron laser oscillator (XFELO). The method leverages Bragg mirrors and longitudinal-transverse mode coupling to enable mode selection in a conventional XFELO configuration, thereby natively producing the fully coherent hard X-ray beams carrying OAM. Furthermore, simulation results demonstrate that fully coherent hard X-ray OAM beams at 1 MHz with a pulse energy of about 120 µJ can be generated without the optical mode converters. This simple approach could significantly advance the creation of X-ray OAM while stimulating the development of experimental methods.

Thermal loading on crystals in an x-ray free-electron laser oscillator

X-ray free electron laser oscillators (XFELO) is future light source to produce fully coherent hard X-ray pulses. The X-rays circulate in an optical cavity built from multiple Bragg reflecting mirrors, which has a high reflectance in a bandwidth of ten meV level. The X-ray crystal mirrors exposed to intense X-ray beams in the cavity are subjects to thermal deformations that shift and distort the Bragg reflection. Therefore, the stability of the XFELO operation relies on the abilities of mirrors to preserve the Bragg reflection under such heat load. A new approach was used to analyze the heat load of mirrors and the XFELO operation. The essential light-matter interaction is simulated by the GEANT4 with a dedicated Bragg-reflection physical process to obtain the precise absorption information of the XFELO pulse in the crystals. The transient thermal conduction is analyzed by the finite-element analysis software upon the energy absorption information extract from GEANT4 simulation. A simplified heat-load model is then developed to integrate the heat load in the XFELO. With the help of the heat-load model, the analysis of XFELO operating with several cryogenically cooled diamond mirrors is conducted. The results indicate that the heat load would induce an oscillation when XFELO operates without enough cooling.

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.

Output coupling from x-ray free-electron laser cavities with intracavity beam splitters

Permeable mirrors are typically used for coupling photons out of laser cavities. A similar approach was proposed for output coupling photons from the cavities of x-ray free-electron laser (XFEL) oscillators. One of the Bragg-reflecting crystal mirrors is thin, just a few extinction length, and is used as a permeable mirror. However, this method is very often limited to extractions of only a few tenths of the intracavity power. Other cavity-based XFELs, such as the high-gain regenerative amplifier XFEL, require much higher outcoupling efficiency. Here, alternative schemes are proposed and analyzed for coupling x-ray photons out of XFEL cavities using intracavity Bragg-reflecting, x-ray-transparent diamond crystal beam splitters. The intracavity beam splitters are efficient and flexible in terms of the amount of the power they are capable of coupling out of the cavity, an amount that can be varied promptly from zero to close to 100%. The schemes can be readily extended to multi-beam outcoupling.

Efficiency enhancement of RF-linac based free electron laser oscillator with electron beam energy ramping

Free electron laser (FEL) oscillator is a very effective method to produce the tunable and high-intense laser in the infrared and terahertz regions. However, in an FEL oscillator, normally only a small portion of the electron beam’s kinetic energy is converted into the optical field. Inspired by the earlier work on the Israeli EA FEL, in this paper we investigate increasing the efficiency of RF-linac based FEL oscillators through ramping up the electron beam energy moderately in the post-saturation regime. Based on theoretical analysis, the optimal beam energy ramping amplitude and the possible maximum efficiency enhancement are presented. Three-dimension simulations with the FELiChEM parameters show that, through ramping the electron energy from the saturation point by 1.9% and 2.68% for the FEL wavelengths of 10μm and 30μm, respectively, about 3.6 and 9.1 times enhancements of the FEL efficiency at the new saturation are achieved. The simulation results are consistent with the analytical prediction. This scheme may also benefit the x-ray FEL oscillators.

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.

The detuning effect of crystal monochromator in self-seeding and oscillator free electron laser.

Self-amplified spontaneous emission (SASE) free electron laser (FEL) is capable of generating ultra-short, high power and high brightness X-ray pulses, but its temporal coherence is poor. Self-seeding scheme is an approach to improve the temporal coherence by employing a crystal monochromator. The crystal detuning effect is the phenomenon that the Bragg angle deviates from the middle of the reflection domain due to the refraction effect, and can affect the seed power of hard X-ray self-seeding (HXRSS) FEL. In this paper, we introduce a novel idea to maximize the seed power by tuning the incident angle off the Bragg condition where the Bragg photon energy is corresponding to the central photon energy of the input X-ray pulse. We present the numerical analysis of the detuning effect in different reflecting atomic planes and different asymmetry angles of diamond crystal. Moreover, we analyze how the detuning affects the seed efficiency of HXRSS FEL, and discuss the application to X-ray FEL oscillator (XFELO). We find when the detuning is much smaller than the bandwidth of input X-ray pulse, we can neglect the detuning effect. However, if the detuning is much larger than or comparable with the bandwidth of input X-ray pulse, the detuning effect can not be ignored. This work can give a guidance to HXRSS FEL and XFELO commissioning for high efficiency FEL output.

BRIGHT: the three-dimensional X-ray crystal Bragg diffraction code

In pursuit of a fully coherent X-ray free-electron laser (FEL), highly reflective Bragg crystals are used and will be used as a highly selective spectral filter in hard X-ray self-seeding FELs and X-ray FEL oscillators (XFELO), respectively. However, currently, when simulating self-seeding and XFELO, the three-dimensional effect of Bragg diffraction is not fully considered. In this paper, we derive a comprehensive solution for the response function of the crystal in Bragg diffraction. A three-dimensional X-ray crystal Bragg diffraction code, named BRIGHT, is introduced, which can be combined with other FEL-related codes, e.g., GENESIS and OPC. Performance and feasibility are assessed using two numerical examples, namely a self-seeding experiment for the linac coherent light source and XFELO options for Shanghai high repetition rate XFEL. The results indicate that BRIGHT provides a new and useful tool for three-dimensional modeling of FEL.

Gain-guided X-ray free-electron laser oscillator

The X-ray free-electron laser oscillator (XFELO) has recently come to be reconsidered as a promising candidate for producing high-brightness, fully coherent pulses in the hard X-ray regime. This letter proposes a gain-guided XFELO scheme, without the need for the external focusing elements that are required in a conventional configuration. Self-consistent theoretical analysis and three-dimensional numerical simulation results verify that the X-ray transverse mode in such an “unstable” cavity is stable and robust owing to electron beam gain-guiding. The output pulse energy and the transverse and longitudinal coherence are comparable to those from conventional XFELOs. This promising scheme is expected to contribute significantly to the construction and operation of a real XFELO.

High brightness fully coherent x-ray amplifier seeded by a free-electron laser oscillator

X-ray free-electron laser oscillator (XFELO) is expected to be a cutting-edge tool for fully coherent x-ray laser generation, and undulator taper technique is well-known for considerably increasing the efficiency of free-electron lasers (FELs). In order to combine the advantages of these two schemes, FEL amplifier seeded by XFELO is proposed by simply using a chirped electron beam. With the right choice of the beam parameters, the bunch tail is within the gain bandwidth of XFELO, and lase to saturation, which will be served as a seeding for further amplification. Meanwhile, the bunch head which is outside the gain bandwidth of XFELO, is preserved and used in the following FEL amplifier. It is found that the natural ``double-horn'' beam current, as well as residual energy chirp from chicane compressor, are quite suitable for the new scheme. Inheriting the advantages from XFELO seeding and undulator tapering, it is feasible to generate nearly terawatt level, fully coherent x-ray pulses with unprecedented shot-to-shot stability, which might open up new scientific opportunities in various research fields.

Systematic design and three-dimensional simulation of X-ray FEL oscillator for Shanghai Coherent Light Facility

The Shanghai Coherent Light Facility (SCLF) is a quasi-continuous wave hard X-ray free electron laser facility, which is currently under construction. Due to the high repetition rate and high-quality electron beams, it is straightforward to consider X-ray free electron laser oscillator (XFELO) operation for the SCLF. In this paper, the main processes for XFELO design, and parameter optimization of the undulator, X-ray cavity, and electron beam are described. A three-dimensional X-ray crystal Bragg diffraction code, named BRIGHT, was introduced for the first time, which can be combined with the GENESIS and OPC codes for the numerical simulations of the XFELO. The performance of the XFELO of the SCLF is investigated and optimized by theoretical analysis and numerical simulation.

Efficiency and coherence preservation studies of Be refractive lenses for XFELO application.

Performance tests of parabolic beryllium refractive lenses, considered as X-ray focusing elements in the future X-ray free-electron laser oscillator (XFELO), are reported. Single and double refractive lenses were subject to X-ray tests, which included: surface profile, transmissivity measurements, imaging capabilities and wavefront distortion with grating interferometry. Optical metrology revealed that surface profiles were close to the design specification in terms of the figure and roughness. The transmissivity of the lenses is >94% at 8 keV and >98% at 14.4 and 18 keV. These values are close to the theoretical values of ideal lenses. Images of the bending-magnet source obtained with the lenses were close to the expected ones and did not show any significant distortion. Grating interferometry revealed that the possible wavefront distortions produced by surface and bulk lens imperfections were on the level of ∼λ/60 for 8 keV photons. Thus the Be lenses can be succesfully used as focusing and beam collimating elements in the XFELO.

X-ray free-electron laser oscillator with nuclear-resonant cavity stabilization and quantum-optical applications

X-ray free-electron-laser oscillators with nuclear-resonant cavity stabilization hold the promise for providing X-rays with unprecedented coherence properties that will enable interesting quantum-optical and metrologial applications. Among these are atom optics with X-ray-based optical elements providing high momentum transfer, or a frequency standard far surpassing the best state-of-the-art atomic clocks.