Compact X-ray Free-electron Laser Research Articles

Revealing the three-dimensional structure of microbunched plasma-wakefield-accelerated electron beams

Plasma wakefield accelerators use tabletop equipment to produce relativistic femtosecond electron bunches. Optical and X-ray diagnostics have established that their charge concentrates within a micrometre-sized volume, but its sub-micrometre internal distribution, which critically influences gain in free-electron lasers or particle yield in colliders, has proven elusive to characterize. Here, by simultaneously imaging different wavelengths of coherent optical transition radiation that a laser-wakefield-accelerated electron bunch generates when exiting a metal foil, we reveal the structure of the coherently radiating component of bunch charge. The key features of the images are shown to uniquely correlate with how plasma electrons injected into the wake: by a plasma-density discontinuity, by ionizing high-Z gas-target dopants or by uncontrolled laser–plasma dynamics. With additional input from the electron spectra, spatially averaged coherent optical transition radiation spectra and particle-in-cell simulations, we reconstruct coherent three-dimensional charge structures. The results demonstrate an essential metrology for next-generation compact X-ray free-electron lasers driven by plasma-based accelerators.

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.

A High-Flux Compact X-ray Free-Electron Laser for Next-Generation Chip Metrology Needs

Recently, considerable work has been directed at the development of an ultracompact X-ray free-electron laser (UCXFEL) based on emerging techniques in high-field cryogenic acceleration, with attendant dramatic improvements in electron beam brightness and state-of-the-art concepts in beam dynamics, magnetic undulators, and X-ray optics. A full conceptual design of a 1 nm (1.24 keV) UCXFEL with a length and cost over an order of magnitude below current X-ray free-electron lasers (XFELs) has resulted from this effort. This instrument has been developed with an emphasis on permitting exploratory scientific research in a wide variety of fields in a university setting. Concurrently, compact FELs are being vigorously developed for use as instruments to enable next-generation chip manufacturing through use as a high-flux, few nm lithography source. This new role suggests consideration of XFELs to urgently address emerging demands in the semiconductor device sector, as identified by recent national need studies, for new radiation sources aimed at chip manufacturing. Indeed, it has been shown that one may use coherent X-rays to perform 10–20 nm class resolution surveys of macroscopic, cm scale structures such as chips, using ptychographic laminography techniques. As the XFEL is a very promising candidate for realizing such methods, we present here an analysis of the issues and likely solutions associated with extending the UCXFEL to harder X-rays (above 7 keV), much higher fluxes, and increased levels of coherence, as well as methods of applying such a source for ptychographic laminography to microelectronic device measurements. We discuss the development path to move the concept to rapid realization of a transformative XFEL-based application, outlining both FEL and metrology system challenges.

An Ultra-Compact X-ray Regenerative Amplifier Free-Electron Laser

There is a growing interest in designing and building compact X-ray Free Electron Lasers (FELs) for scientific and industry applications. In this paper, we report an X-ray Regenerative Amplifier FEL (XRAFEL) design based on a proposed Ultra Compact X-ray FEL configuration. Our results show that an XRAFEL can dramatically enhance the temporal coherence and increase the spectral brightness of the radiation in the hard X-ray regime without increasing the footprint of the FEL configuration. The proposed compact, fully coherent, and high-flux hard X-ray source holds promise as a valuable candidate for a wide range of high-impact applications in both academia and industry.

Shaping Micro-Bunched Electron Beams for Compact X-ray Free-Electron Lasers with Transverse Gradient Undulators

Laser-modulator-based micro-bunching of electron beams has been applied to many novel operating modes of X-ray free-electron lasers from harmonic generation to attosecond pulse production. Recently, it was also identified as a key enabling technology for the production of a compact XFEL driven by a relatively low-energy beam. In traditional laser modulator schemes with low-energy and high-current bunches, collective effects limit the possible working points that can be employed, and thus it is difficult to achieve optimal XFEL performance. We propose to utilize transverse longitudinal coupling in a transverse gradient undulator (TGU) to shape micro-bunched electron beams so as to optimize their performance in a compact X-ray free-electron laser. We show that a TGU added to a conventional laser modulator stage enables much more flexibility in the design, allowing one to generate longer micro-bunches less subject to slippage effects and even lower the slice emittance of the micro-bunches. We present a theoretical analysis of laser-based micro-bunching with an added TGU, simulation of compression with collective effects in such systems, and finally XFEL simulations demonstrating the gains in peak power enabled by the TGU. Although we focus on the application to compact XFELs, what we propose is a general phase space manipulation that may find utility in other applications as well.

Demonstration of sub- GV/m accelerating field in a photoemission electron gun powered by nanosecond X -band radio-frequency pulses

Radiofrequency (RF) electron guns operating at high accelerating gradients offer a pathway to producing bright electron bunches. Such beams are expected to revolutionize many areas of science: they could form the backbone of next-generation compact x-ray free-electron lasers or provide coherent ultrafast quantum electron probes. We report on the experimental demonstration of an RF photoemission electron source supporting an accelerating field close to 400~MV/m at the photocathode surface. The gun was operated in an RF transient mode driven by short $\sim 9$~ns X-band (\SI{11.7}{\giga\hertz}) RF pulses. We did not observe any major RF breakdown or significant dark current over a three-week experimental run at high accelerating fields. The demonstrated paradigm provides a viable path to forming relativistic electron beams with unprecedented brightness.

Matching-based two-color X-ray free-electron laser generation utilizing laser–plasma accelerated electron beam

Abstract Laser–plasma accelerators (LPAs) have great potential to realize a compact X-ray free-electron laser (FEL), which is limited by the beam properties currently. Two-color high-intensity X-ray FEL provides a powerful tool for probing ultrafast dynamic systems. In this paper, we present a simple and feasible method to generate a two-color X-ray FEL pulse based on an LPA beam. In this scheme, time-dependent mismatch along the bunch is generated and manipulated by the designed lattice system, enabling FEL lasing at different wavelength within two undulator sections. The time separation between the two pulses can be precisely adjusted by varying the time-delay chicane. Numerical simulations show that two-color soft X-ray FELs with gigawatt-level peak power and femtosecond duration can be generated, which confirm the validity and feasibility of the scheme.

Demonstration of a compact x-ray free-electron laser using the optical klystron effect

We demonstrate the operation of a compact x-ray free-electron laser (FEL) using the optical klystron mechanism. This effect speeds up the FEL process using the dispersion of magnetic chicanes placed between the undulator modules of the FEL beamline. The demonstration was performed at the soft x-ray beamline of SwissFEL, called Athos, which is, as far as we are aware, the only x-ray FEL to date with magnetic chicanes between every two undulator modules. In our measurements, we show that, compared to standard operation without chicanes, the required undulator length to achieve FEL saturation is reduced between 15% and 30% for radiation wavelengths between 1 and 2 nm. Fully exploiting the optical klystron effect represents an important step toward more compact FEL designs, rendering this key technology more affordable and hence accessible to a larger science community.

Imaging by intensity interferometry of x-ray fluorescence at a compact x-ray free-electron laser

A semiclassical theory of incoherent diffractive imaging is given, based on the Hanbury Brown and Twiss effect when used to image inner-shell x-ray fluorescence from heavy atoms excited by the femtosecond pulses of an x-ray laser. Interference between emission from different atoms is expected when the pulse duration is shorter than the fluorescent lifetime. Simulations for atoms at the vertices of an icosahedral virus capsid are given, and reconstructions are presented based on phasing of the pair correlation function between photons emitted independently from many different atoms at two different detector pixels. The dependence of the pair-correlation function on the fluorescence lifetime relative to the pulse duration of the x-ray free-electron laser (XFEL) is computed, and a simple expression is obtained for the contrast of incoherent diffractive imaging speckles as a function of the XFEL's flux and lifetime. This indicates that compact XFELs, with reduced flux but sub-femtosecond pulses, should be ideally suited to atomic-resolution three-dimensional mapping of heavy atoms in materials science, chemistry, and biology.

Free-electron lasing at 27 nanometres based on a laser wakefield accelerator.

X-ray free-electron lasers can generate intense and coherent radiation at wavelengths down to the sub-ångström region1-5, and have become indispensable tools for applications in structural biology and chemistry, among other disciplines6. Several X-ray free-electron laser facilities are in operation2-5; however, their requirement for large, high-cost, state-of-the-art radio-frequency accelerators has led to great interest in the development of compact and economical accelerators. Laser wakefield accelerators can sustain accelerating gradients more than three orders of magnitude higher than those of radio-frequency accelerators7-10, and are regarded as an attractive option for driving compact X-ray free-electron lasers11. However, the realization of such devices remains a challenge owing to the relatively poor quality of electron beams that are based on a laser wakefield accelerator. Here we present an experimental demonstration of undulator radiation amplification in the exponential-gain regime by using electron beams based on a laser wakefield accelerator. The amplified undulator radiation, which is typically centred at 27 nanometres and has a maximum photon number of around 1010 per shot, yields a maximum radiation energy of about 150 nanojoules. In the third of three undulators in the device, the maximum gain of the radiation power is approximately 100-fold, confirming a successful operation in the exponential-gain regime. Our results constitute a proof-of-principle demonstration of free-electron lasing using a laser wakefield accelerator, and pave the way towards the development of compact X-ray free-electron lasers based on this technology with broad applications.

Near-GeV Electron Beams at a Few Per-Mille Level from a Laser Wakefield Accelerator via Density-Tailored Plasma.

A simple, efficient scheme was developed to obtain near-gigaelectronvolt electron beams with energy spreads of few per-mille level in a single-stage laser wakefield accelerator. Longitudinal plasma density was tailored to control relativistic laser-beam evolution, resulting in injection, dechirping, and a quasi-phase-stable acceleration. With this scheme, electron beams with peak energies of 780-840MeV, rms energy spreads of 2.4‰-4.1‰, charges of 8.5-23.6pC, and rms divergences of 0.1-0.4mrad were experimentally obtained. Quasi-three-dimensional particle-in-cell simulations agreed well with the experimental results. The dechirping strength was estimated to reach up to 11 TeV/mm/m, which is higher than previously obtained results. Such high-quality electron beams will boost the development of compact intense coherent radiation sources and x-ray free-electron lasers.

Overview of SwissFEL dual-photocathode laser capabilities and perspectives for exotic FEL modes

Abstract SwissFEL is a compact, high-brilliance, soft and hard X-ray free electron laser (FEL) facility that started user operation in 2019. The facility is composed of two parallel beam lines seeded by a common linear accelerator (LINAC), and a two-bunch photo-injector. For the injector, an innovative dual-photocathode laser scheme has been developed based on state-of-the-art ytterbium femtosecond laser systems. In this paper, we describe the performance of the SwissFEL photocathode drive lasers (PCDLs), the pulse-shaping capabilities as well as the versatility of the systems, which allow many different modes of operation of SwissFEL. The full control over the SwissFEL electron bunch properties via the unique architecture of the PCDLs will enable in the future the advent of more-advanced FEL modes; these modes include, but are not restricted to, the generation of single or trains of sub-femtosecond FEL pulses, multi-color FEL and finally, the generation of fully coherent X-ray pulses via laser-based seeding.

A compact and cost-effective hard X-ray free-electron laser driven by a high-brightness and low-energy electron beam

We present the first lasing results of SwissFEL, a hard X-ray free-electron laser (FEL) that recently came into operation at the Paul Scherrer Institute in Switzerland. SwissFEL is a very stable, compact and cost-effective X-ray FEL facility driven by a low-energy and ultra-low-emittance electron beam travelling through short-period undulators. It delivers stable hard X-ray FEL radiation at 1-A wavelength with pulse energies of more than 500 μJ, pulse durations of ~30 fs (root mean square) and spectral bandwidth below the per-mil level. Using special configurations, we have produced pulses shorter than 1 fs and, in a different set-up, broadband radiation with an unprecedented bandwidth of ~2%. The extremely small emittance demonstrated at SwissFEL paves the way for even more compact and affordable hard X-ray FELs, potentially boosting the number of facilities worldwide and thereby expanding the population of the scientific community that has access to X-ray FEL radiation. The first lasing results at SwissFEL, an X-ray free-electron laser, are presented, highlighting the facility’s unique capabilities. A general comparison to other major facilities is also provided.

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.

An ultra-compact x-ray free-electron laser

In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV m−1. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 m. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by 50 nm-rad normalized emittances. The emittance, we note, is the effective area in transverse phase space (x, p x /m e c) or (y, p y /m e c) occupied by the beam distribution, and it is relevant to achievable beam sizes as well as setting a limit on FEL wavelength. These beams, when injected into innovative, short-period (1–10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine—including the imaging of virus particles—which may profit from this new model of performing XFEL science.

Ultracold Electrons via Near-Threshold Photoemission from Single-Crystal Cu(100).

Achieving a low mean transverse energy or temperature of electrons emitted from the photocathode-based electron sources is critical to the development of next-generation and compact x-ray free electron lasers and ultrafast electron diffraction, spectroscopy, and microscopy experiments. In this Letter, we demonstrate a record low mean transverse energy of 5meV from the cryo-cooled (100) surface of copper using near-threshold photoemission. Further, we also show that the electron energy spread obtained from such a surface is less than 11.5meV, making it the smallest energy spread electron source known to date: more than an order of magnitude smaller than any existing photoemission, field emission, or thermionic emission based electron source. Our measurements also shed light on the physics of electron emission and show how the energy spread at few meV scale energies is limited by both the temperature and the vacuum density of states.

Performance improvement of a 200TW/1Hz Ti:sapphire laser for laser wakefield electron accelerator

For investigations on laser wakefield electron acceleration, there is a strong demand for driving laser exhibiting high stability and good spatial-temporal quality. Here we present an upgraded 200TW/1Hz Ti:sapphire laser with following key characteristics. The fluctuation of pulse energy and beam pointing for consecutive 5400 pulses in 90 min are as low as 0.55% and 1.5μrad respectively. The compressed pulse duration is 23.7 fs, which is about 1.1 times of the Fourier-transform limit. The focal spot obtained by an f/30 off-axis parabolic mirror is 54 μm × 52 μm at 1/e2, which is very close to the diffraction limit. Moreover, in the laser wakefield electron acceleration experiments driven by this upgraded 200TW/1Hz laser, quasi-mono-energetic electron beams are reproduced in consecutive shots. For successive 300 shots, the average peak energy of produced electron beams is 667 MeV, and the peak energy fluctuation is only 3.4%. The progresses on both driving laser and laser wakefield electron acceleration make the realization of compact X-ray free electron laser possible.

Generation and Characterization of Intense Ultralow-Emittance Electron Beams for Compact X-Ray Free-Electron Lasers.

The transverse emittance of the electron beam is a fundamental parameter in linac-based x-ray free-electron lasers (FELs). We present results of emittance measurements carried out at SwissFEL, a compact x-ray FEL facility at the Paul Scherrer Institute in Switzerland, including a description of the novel high-resolution measurement techniques and the optimization procedure. We obtained slice emittance values at the undulator entrance down to 200nm for an electron beam with a charge of 200pC and an rms duration of 30-40fs. Furthermore, we achieved slice emittances as low as 100nm for 10pC beams with few fs duration. These values set new standards for electron linear accelerators. The quality, verification, and control of our electron beams allowed us to generate high-power FEL radiation for a wavelength as short as 0.1nm using an electron beam with an energy of only 6GeV. The emittance values demonstrated at SwissFEL would allow producing hard x-ray FEL pulses with even lower-energy beams, thus paving the way for even more compact and cost-effective FEL facilities.

High-quality 5 GeV electron bunches with resonant multi-pulse ionization injection

The production of high-quality electron bunches in laser wakefield acceleration relies on the possibility of injecting ultra-low emittance bunches in the plasma wave. A new bunch injection scheme (resonant multi-pulse ionization, ReMPI) has been conceived and studied, in which electrons extracted by ionization are trapped by a large-amplitude plasma wave driven by a train of resonant ultrashort pulses. Such a train of pulses can be obtained in a very efficient, compact and stable way, by phase manipulation in the laser front-end. The ReMPI injection scheme relies on currently available laser technology and is being considered for the implementation of future compact x-ray free electron laser schemes. Simulations show that high-quality electron bunches with an energy of up to 5 GeV and a peak current exceeding 2 kA, with normalized emittance of below 0.1 mm × mrad and a slice energy spread of below 0.1%, can be obtained with a single stage.

Compression of Ultra-High Brightness Beams for a Compact X-ray Free-Electron Laser

The creation of the first X-ray free-electron laser at SLAC in 2009 introduced the scientific community to coherent photons of unprecedented high brightness. These photons were produced, however, at the cost of billion-dollar-class price tags and kilometer-scale machine footprints. This has meant that getting access to these photons is very difficult, and those who do get access do so with a strict time budget. Now, the development of critical enabling technologies, in particular high-field cryogenically cooled accelerating cavities and short-period, high-field undulator magnets, opens the door to an X-ray free-electron laser less than 30 m in length. We present here critical potential design elements for such a soft X-ray free-electron laser. To this end, simulation results are presented focusing on the problems associated with the process of bunch compression and novel ways in which those problems can be resolved.