Laser Beamline Research Articles

Spexwavepy: an open-source Python package for X-ray wavefront sensing using speckle-based techniques.

In situ wavefront sensing plays a critical role in the delivery of high-quality beams for X-ray experiments. X-ray speckle-based techniques stand out among other in situ techniques for their easy experimental setup and various data acquisition modes. Although X-ray speckle-based techniques have been under development for more than a decade, there are still no user-friendly software packages for new researchers to begin with. Here, we present an open-source Python package, spexwavepy, for X-ray wavefront sensing using speckle-based techniques. This Python package covers a variety of X-ray speckle-based techniques, provides plenty of examples with real experimental data and offers detailed online documentation for users. We hope it can help new researchers learn and apply the speckle-based techniques for X-ray wavefront sensing to synchrotron radiation and X-ray free-electron laser beamlines.

Manufacturability-based optical design optimization for advanced Kirkpatrick-Baez X-ray focusing mirrors.

The advanced Kirkpatrick-Baez (AKB) mirror setup is an effective and compelling solution to provide stable X-ray nano-focusing for synchrotron radiation or free-electron laser beamlines. We propose an AKB mirror design optimization approach to mitigate the difficulties associated with mirror fabrication by minimizing the total slope ranges of the four curved mirrors while achieving the expected focusing performance. In the optimization, we have considered geometry constraints to ensure the beam acceptance with the required clear aperture, the diffraction-limited focal size with the adequate numerical aperture, and the desired mirror gaps for adjustment and the necessary working distance for the sample stage. Additionally, practical constraints linked to mirror metrology and fabrication, such as mirror length limits and curvature uncertainty in measurement, are taken into account. Furthermore, progressive objective optimization eliminates the need for any initial guess, fully automating the AKB optimization process. This approach facilitates the development of an elegant Wolter-I or Wolter-III type AKB design solution that satisfies these multiple constraints. In cases where constraints cannot be simultaneously satisfied, the optimization results provide valuable insights into areas where trade-offs need to be considered. Simulations with ray tracing and wavefront propagation validate the optimized AKB design showing high tolerance to the beam incident angle.

Multi-functional gas cell in the vacuum ultraviolet free-electron laser beamline.

A long gas cell, filled with noble gas, is typically positioned between the undulator and the first mirror in the free-electron laser (FEL) beamline to attenuate the laser power as required by the end-stations. In addition to attenuation, the gas cell also serves important functions in various applications, such as spectrometer calibration, resolving power evaluation during beamline commissioning, and filtering of third harmonic in FEL operations. These functions of the gas cell have been successfully tested and implemented at the Dalian Coherent Light Source, a vacuum ultraviolet FEL facility located in Dalian, China. The resolving power of higher than 5000 has been obtained, and accurate calibration has been completed using the gas cell. During operation, the third harmonic of the FEL was attenuated by approximately one order of magnitude with almost the same power of the fundamental. This greatly improved the signal-to-noise ratio at the end-stations.

Optimization of laser-driven quantum beam generation and the applications with artificial intelligence

We have investigated space and astrophysical phenomena in nonrelativistic laboratory plasmas with long high-power lasers, such as collisionless shocks and magnetic reconnections, and have been exploring relativistic regimes with intense short pulse lasers, such as energetic ion acceleration using large-area suspended graphene. Increasing the intensity and repetition rate of the intense lasers, we have to handle large amounts of data from the experiments as well as the control parameters of laser beamlines. Artificial intelligence (AI) such as machine learning and neural networks may play essential roles in optimizing the laser and target conditions for efficient laser ion acceleration. Implementing AI into the laser system in mind, as the first step, we are introducing machine learning in ion etch pit analyses detected on plastic nuclear track detectors. Convolutional neural networks allow us to analyze big ion etch pit data with high precision and recall. We introduce one of the applications of laser-driven ion beams using AI to reconstruct vector electric and magnetic fields in laser-produced turbulent plasmas in three dimensions.

Matrix formulation of the Gaussian expansion of coherent multiple beams in arbitrary dimensions.

Modeling the propagation of beams along laser beamlines is very challenging due to the multidimensional and multiscale configuration of the problem. Spatio-temporal couplings are particularly difficult to address with conventional numerical methods. Here we derive the Wigner function of a sum of Gaussian beams by calculating the multidimensional Fourier transform of the intercorrelation function of the fields. The matrix formulation allows for a simple propagation of the Wigner function in the framework of matrix optics. The relevancy of this approach is assessed by applying this model to one-dimensional and multidimensional configurations and by studying the influence of spatio-temporal couplings when considering propagation and dispersion by a diffraction grating.

Multiline molecular tagging velocimetry of nitric oxide at 100 kHz using an injection-seeded burst-mode OPO.

An injection-seeded, burst-mode optical parametric oscillator (OPO) operating at a repetition rate of 100 kHz is used to demonstrate the multiline molecular tagging velocimetry of an underexpanded jet using nitric oxide fluorescence. The very narrow linewidth of the OPO system, along with the relatively high pulse energies of the burst-mode system, enables efficient single-photon excitation of nitric oxide along multiple laser beam lines at a high repetition rate. Simultaneous one-dimensional velocity profile measurements were obtained of an underexpanded jet system at six different locations using a reference initial image and single-shot delayed images. A methodology for calculating the uncertainty of single-shot velocity is also described. Mean and root-mean-square velocity profiles are obtained at multiple locations simultaneously over a sampling time of 1 ms. The high-repetition-rate velocity measurements also appear to capture the onset of velocity oscillations and has the potential to reveal velocity frequency content occurring in the tens of kHz. The demonstrated velocimetry technique could be paired with other emerging burst-mode laser capabilities for a quantitative multiparameter gas property or multicomponent gas velocity measurements for supersonic and hypersonic flows, especially within ground test facilities that are limited to very short run durations.

Spatiotemporal response of concave VLS grating to ultra-short X-ray pulses.

In soft X-ray free-electron laser (FEL) beamlines, variable-line-spacing (VLS) gratings are often used as dispersive components of monochromators and spectrometers due to their combined dispersion and focusing properties. X-ray FEL pulses passing through the VLS grating can result in not only transverse focusing but also spatiotemporal coupling effects, such as pulse front tilt, pulse front rotation, and pulse stretching. In this paper, we present a theoretical study of the spatiotemporal response of concave VLS gratings to ultra-short X-ray pulses. The theoretical analysis indicates that the tilt angle of the non-zero diffraction orders varies with the propagation distance, and disappears at the focus, where the focal lengths and pulse stretching differ for different diffraction orders. The model demonstrates the pulse duration after the concave VLS grating is the convolution of the initial pulse duration and the stretching term induced by dispersion, while the beam size at the focus in x dimension is the convolution of the geometric scaling beam size and the dispersion term. This work provides a mathematical explanation for the spatiotemporal response of concave VLS grating to ultra-short X-ray pulses and offers valuable insights into the design of FEL grating monochromators, spectrometers, pulse compressors, and pulse stretchers.

Linear-to-circular polarization conversion with full-silica meta-optics to reduce nonlinear effects in high-energy lasers

High-energy lasers have benefited from intense efforts to bring light-matter interactions to new standards and to achieve laser fusion ignition. One of the main issues to further increasing laser energy is the resistance of optical materials to high laser fluences, in particular at the final stage of the laser beamline where nonlinear Kerr effects can occur in optical materials and provoke laser filamentation. One promising way to mitigate this process is to reduce the nonlinear susceptibility of the material by switching the polarization from a linear to a circular state. Here, we report a significant reduction in the laser filamentation effect on glass by using a full-silica metamaterial waveplateable to switch the linear-to-circular polarization of high fluence laser beams. This result is achieved through the use of a large size full-silica meta-optics exhibiting nominal polarization conversion associated with an excellent transmission efficiency and wavefront quality, as well as a high laser damage resistance.

Laser Output Performance and Temporal Quality Enhancement at the J-KAREN-P Petawatt Laser Facility

We described the output performance and temporal quality enhancement of the J-KAREN-P petawatt laser facility. After wavefront correction using a deformable mirror, focusing with an f/1.3 off-axis parabolic mirror delivered a peak intensity of 1022 W/cm2 at 0.3 PW power levels. Technologies to improve the temporal contrast were investigated and tested. The origins of pre-pulses generated by post-pulses were identified and the elimination of most pre-pulses by removal of the post-pulses with wedged optics was achieved. A cascaded femtosecond optical parametric amplifier based on the utilization of the idler pulse rather than the signal pulse was developed for the complete elimination of the remaining pre-pulses. The orders of magnitude enhancement of the pedestal before the main pulse were obtained by using a higher surface quality of the convex mirror in the Öffner stretcher. A single plasma mirror was installed in the J-KAREN-P laser beam line for further contrast improvement of three orders of magnitude. The above developments indicate, although it has not been directly measured, the contrast can be as high as approximately 1015 up to 40 ps before the main pulse. We also showed an overview of the digital transformation (DX) of the system, enabling remote and automated operation of the J-KAREN-P laser facility.

Characterization in the extreme ultraviolet (XUV) domain of microchannel plate based device using synchrotron radiation

Innovative, low-loss, and compact optical systems are essential to meet the experimental requirements of emerging novel radiation sources. The microchannel plate (MCP), a metamaterial-based optical device, shows promising potential for shaping, condensing, and focusing soft x-ray radiation at synchrotron radiation (SR) facilities. This study highlights the impact of MCP optical devices on SR beam condensing capability and their sensitivity to the degree of coherence by investigating the profile of transmitted beams through single and double MCP optical devices. Transmitted diffraction patterns of soft x-ray SR radiation change with energy and radiation modes. At 92 eV, the double MCP-based device affects the beam divergence and degree of coherence more than the single MCP. Moreover, the double MCP device shows potential as a condensing optics at shorter wavelengths, i.e., 480 eV. Experiments were performed at the available end-station of the Circular Polarization beamline at the Elettra synchrotron facility in Trieste, using a high-vacuum chamber with a hexapod system, providing the precise movement necessary to align these diffractive optics. The findings contribute to the development of innovative optical systems for SR and free-electron laser beamlines, paving the way for advanced experiments in spectroscopy, microscopy, and imaging in a wide energy range.

Real-time machine-learning-driven control system of a deformable mirror for achieving aberration-free X-ray wavefronts.

A neural-network machine learning model is developed to control a bimorph adaptive mirror to achieve and preserve aberration-free coherent X-ray wavefronts at synchrotron radiation and free electron laser beamlines. The controller is trained on a mirror actuator response directly measured at a beamline with a real-time single-shot wavefront sensor, which uses a coded mask and wavelet-transform analysis. The system has been successfully tested on a bimorph deformable mirror at the 28-ID IDEA beamline of the Advanced Photon Source at Argonne National Laboratory. It achieved a response time of a few seconds and maintained desired wavefront shapes (e.g., a spherical wavefront) with sub-wavelength accuracy at 20 keV of X-ray energy. This result is significantly better than what can be obtained using a linear model of the mirror's response. The developed system has not been tailored to a specific mirror and can be applied, in principle, to different kinds of bending mechanisms and actuators.

Status and future of the soft X-ray free-electron laser beamline at the SHINE

The Shanghai High repetition rate XFEL and Extreme light facility (SHINE) is under construction and aims at generating X-rays between 0.4 and 25 keV with three FEL beamlines at repetition rates of up to 1 MHz. The soft X-ray FEL beamline, FEL-II, will be ready for commissioning in 2025. It is designed to cover the photon energy from 0.4 to 3 keV, in which the baselines of the FEL operation modes are self-amplified spontaneous emission (SASE), self-seeding, echo-enabled harmonic generation (EEHG), and polarization control. Therefore, a high repetition-rate external seed laser, large period length modulator, soft X-ray monochromator, planar undulator, and elliptically polarized undulator have been adopted in the FEL-II beamline. Several potentials such as an ultra-short pulse mode and a multi-color mode are also foreseeable without significant equipment changes in the follow-up operation. A dual-period undulator design is suggested for the echo-enabled harmonic generation (EEHG) commissioning, and it has great potential to break through the unreachable energy of the fully coherent X-ray in the future. The FEL-II beamline will deliver SASE radiation and fully coherent radiation in all the wavelengths of interest.

Data-driven modeling and control of an X-ray bimorph adaptive mirror.

Adaptive X-ray mirrors are being adopted on high-coherent-flux synchrotron and X-ray free-electron laser beamlines where dynamic phase control and aberration compensation are necessary to preserve wavefront quality from source to sample, yet challenging to achieve. Additional difficulties arise from the inability to continuously probe the wavefront in this context, which demands methods of control that require little to no feedback. In this work, a data-driven approach to the control of adaptive X-ray optics with piezo-bimorph actuators is demonstrated. This approach approximates the non-linear system dynamics with a discrete-time model using random mirror shapes and interferometric measurements as training data. For mirrors of this type, prior states and voltage inputs affect the shape-change trajectory, and therefore must be included in the model. Without the need for assumed physical models of the mirror's behavior, the generality of the neural network structure accommodates drift, creep and hysteresis, and enables a control algorithm that achieves shape control and stability below 2 nm RMS. Using a prototype mirror and ex situ metrology, it is shown that the accuracy of our trained model enables open-loop shape control across a diverse set of states and that the control algorithm achieves shape error magnitudes that fall within diffraction-limited performance.

Commissioning results from the high-repetition rate nanosecond-kilojoule laser beamline at the extreme light infrastructure

High-power laser–matter interaction and the related high-energy density physics (HEDP) are two important topics in modern physics. However, for decades, a very restrictive limitation prevents researchers from studying these topics efficiently: nanosecond-kilojoule class lasers are typically characterized by a low repetition rate (1 shot h−1) that restricts the data acquisition and limits the available statistics for analysis. Here, we describe the first results obtained using the high-repetition rate nanosecond-kilojoule (L4n laser beamline) experimental platform commissioned at the Extreme Light Infrastructure—Beamlines. We proved the capability to deliver hundreds of joules shots every three min with a very good repeatability. Using high-resolution x-ray spectroscopy, we studied highly resolved spectra of H-like and He-like Cl lines emitted from polyvinyl chloride targets and demonstrated that the plasma parameters (electron temperature and density) derived from the measured spectra are consistent with hydro-simulations. These results demonstrate the fulfillment of the designed platform performance which should become one of the most important assets for the HEDP community in the following decades.

Wavefront Correction in Vacuum of SULF-1PW Laser Beamline

The focusing quality of high peak power lasers plays a crucial role in laser wakefield electron acceleration investigations. We report here an improvement in the focusing quality of the SULF-1PW laser beamline, planning to drive and generate 5~10 GeV electron beams. After the wavefront correction in vacuum with an adaptive optical system and the focusing with an f/56 off-axis parabolic mirror, near-diffraction-limited focal spots with a size of 52 × 54 μm2 at full width at half maximum are achieved, and the enclosed energy inside this size is ~36.6%. Consequently, the focused intensity of ~1.66 × 1019 W/cm2 can be achieved at 1 PW peak power. Moreover, we also examine the wavefront stability in air and vacuum, respectively. From the statistical analysis of 1900 shots of successive laser pulses at 1 Hz, we identify the wavefront fluctuation resulting from air turbulence and the better correction capacity in vacuum. This work demonstrates the importance and necessity of wavefront correction in vacuum for high peak power lasers.

Estimating and monitoring laser-induced damage size on glass windows with a deep-learning-based pipeline.

Laser-induced damage is a major issue in high power laser facilities such as the Laser MégaJoule (LMJ) and National Ignition Facility (NIF) since they lower the efficiency of optical components and may even require their replacement. This problem occurs mainly in the final stages of the laser beamlines and in particular in the glass windows through which laser beams enter the central vacuum chamber. Monitoring such damage sites in high energy laser facilities is, therefore, of major importance. However, the automatic monitoring of damage sites is challenging due to the small size of damage sites and to the low-resolution images provided by the onsite camera used to monitor their occurrence. A systematic approach based on a deep learning computer vision pipeline is introduced to estimate the dimensions of damage sites of the glass windows of the LMJ facility. The ability of the pipeline to specialize in the estimation of damage sites of a size less than the repair threshold is demonstrated by showing its higher efficiency than classical machine learning approaches in the specific case of damage site images. In addition, its performances on three datasets are evaluated to show both robustness and accuracy.

Acceleration of 60 MeV proton beams in the commissioning experiment of SULF-10 PW laser

We report the experimental results of the commissioning phase in the 10 PW laser beamline of the Shanghai Superintense Ultrafast Laser Facility (SULF). The peak power reaches 2.4 PW on target without the last amplifying during the experiment. The laser energy of 72 ± 9 J is directed to a focal spot of approximately 6 μm diameter (full width at half maximum) in 30 fs pulse duration, yielding a focused peak intensity around 2.0 × 1021 W/cm2. The first laser-proton acceleration experiment is performed using plain copper and plastic targets. High-energy proton beams with maximum cut-off energy up to 62.5 MeV are achieved using copper foils at the optimum target thickness of 4 μm via target normal sheath acceleration. For plastic targets of tens of nanometers thick, the proton cut-off energy is approximately 20 MeV, showing ring-like or filamented density distributions. These experimental results reflect the capabilities of the SULF-10 PW beamline, for example, both ultrahigh intensity and relatively good beam contrast. Further optimization for these key parameters is underway, where peak laser intensities of 1022–1023 W/cm2 are anticipated to support various experiments on extreme field physics.

Cation-π Interactions between a Noble Metal and a Polyfunctional Aromatic Ligand: Ag+ (benzylamine).

The structure of an isolated Ag+(benzylamine) complex is investigated by infrared multiple photon dissociation (IRMPD) spectroscopy complemented with quantum chemical calculations of candidate geometries and their vibrational spectra, aiming to ascertain the role of competing cation‐N and cation‐π interactions potentially offered by the polyfunctional ligand. The IRMPD spectrum has been recorded in the 800–1800 cm−1 fingerprint range using the IR free electron laser beamline coupled with an FT‐ICR mass spectrometer at the Centre Laser Infrarouge d'Orsay (CLIO). The resulting IRMPD pattern points toward a chelate coordination (N‐Ag+‐π) involving both the amino nitrogen atom and the aromatic π‐system of the phenyl ring. The gas‐phase reactivity of Ag+(benzylamine) with a neutral molecular ligand (L) possessing either an amino/aza functionality or an aryl group confirms N‐ and π‐binding affinity and suggests an augmented silver coordination in the product adduct ion Ag+(benzylamine)(L) .

Liquid-cooled Nd:phosphate split-slab large aperture amplifier pumped by flash-lamps

High power laser facilities have been developed to achieve novel standards in light-matter interactions. The repetition rate is severely limited by the heating of the large size components, and in particular the large-aperture amplifiers. Cooling these optical components turns out to be a major challenge for increasing the repetition rate of high power laser beam lines. Here we report a practical implementation of a large aperture Nd:phosphate split-slab based amplifier using a liquid cooling system. The amplification of a 90 × 90 mm2 beam with a gain coefficient of 1.135 per pass and wave-front deformation lower than λ/3 per pass is achieved with a repetition rate of 1 shot per minute. The cooling system is sufficient to maintain the gain as well as the beam quality over a long time operation. Additionally, the pressure inside the amplifier can be controlled to minimize the optical deformation. This single liquid-cooled amplifier is a promising step towards future beam-lines able to deliver hundreds of shots per day with beam quality compatible with target focusing applications.

Evaluation of the effect of filler metal interlayer on the weld joint quality in fiber laser welding of alloy steel (A516)

Laser welding of A516 steel was performed using the filler metal to serve as the interlayer. The effects of laser welding parameters on the melt pool microstructure, weld bead shape, dimensions of the melt pool, and temperature field around the melt pool were investigated. The results indicated that the nozzle distance had the most significant influence on the temperature around the molten pool. As the maximum energy of the laser beam was irradiated on the center of the filler, the highest value of the temperature near the fusion zone was about 360°C. The welding speed, determining the overlap factor, had a considerable effect on the weld bead shape. The laser power, therefore, could determine laser beam line energy to melt the filler volume effectively. Enhancing the laser power up to 450 W increased the temperature to about 300 °C; also, the melting rate of the filler used was almost completed and the weld bead appearance was clearly improved; therefore, the width of the melt pool was increased by about 10%. Increasing the nozzle distance from 3 to 7 mm also reduced the width of the molten pool by about 75% and 40% with and without the filler, respectively. By using the filler metal, the geometry of the welding bead changed from a concave one to a convex one. The major fusion zone microstructure of the fusion zone was an acicular ferrite characterized by needle-shaped ferrite crystallites.