Linac Coherent Light Source Research Articles

Subthreshold Erosion of an Organic Polymer Induced by Multiple Shots of an X-Ray Free-Electron Laser

Solids irradiated by energetic photons can be eroded in two modes, depending on the radiation intensity. High average, low-peak power sources, e.g., synchrotron radiation and high-order harmonics, induce desorption of the material at a low etch rate. In contrast, high-peak-power radiation from extreme ultraviolet and x-ray lasers usually causes a massive removal of the material even by a single shot. In this contribution, an effective material erosion is reported in PMMA exposed to multiple accumulated pulses generated by the free-electron x-ray-laser Linac Coherent Light Source (LCLS, tuned at a photon energy of 830 eV in this study, operated in Menlo Park at Stanford, CA, USA) at a fluence below the single-pulse ablation threshold. The effect is caused by polymer-chain scissions initiated by single photons carrying enough energy to break the $\mathrm{C}---\mathrm{C}$ bounds. High efficiency of the erosion is supposed to occur due to a correlation of the single-photon effects. The subthreshold damage exhibits a nonlinear dose dependence resulting from a competition between chain scissions and cross-linking processes. The cross-linking is proven by Raman spectroscopy of the irradiated polymer. Two theoretical models of the x-ray free-electron-laser-induced erosion are suggested, which provide an excellent agreement with the experimental results.

Ronchi shearing interferometry for wavefronts with circular symmetry.

Ronchi testing of a focused electromagnetic wave has in the last few years been used extensively at X-ray free-electron laser (FEL) facilities to qualitatively evaluate the wavefront of the beam. It is a quick and straightforward test, is easy to interpret on the fly, and can be used to align phase plates that correct the focus of aberrated beams. In general, a single Ronchigram is not sufficient to gain complete quantitative knowledge of the wavefront. However the compound refractive lenses that are commonly used at X-ray FELs exhibit a strong circular symmetry in their aberration, and this can be exploited. Here, a simple algorithm that uses a single recorded Ronchigram to recover the full wavefront of a nano-focused beam, assuming circular symmetry, is presented, and applied to experimental measurements at the Matter in Extreme Conditions instrument at the Linac Coherent Light Source.

Ptychography at the Linac Coherent Light Source in a parasitic geometry

X-ray free-electron lasers (FELs) are being recognized as a powerful tool in an ever-increasing number of research fields, but are very limited as to the number of experiments that they can support. This work shows that more beamtime could be made available by using `parasitic' geometries, where a secondary experiment uses the X-ray beam that the primary experiment does not utilize. The first successful ptychography experiment, a scanning coherent diffractive imaging technique, in a parasitic geometry at an X-ray FEL is demonstrated. Utilizing the CXI hutch at the Linac Coherent Light Source (LCLS), it is shown that the obtained data are of high quality and that characterizing the beam using ptychography can be much faster than traditional imprinting methods.

Cats and llamas could offer a path to coronavirus therapies

Computation and experimentation also yield possible therapeutic compounds for COVID-19 [ABSTRACT FROM AUTHOR] Copyright of Physics Today is the property of American Institute of Physics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission However, users may print, download, or email articles for individual use This abstract may be abridged No warranty is given about the accuracy of the copy Users should refer to the original published version of the material for the full abstract (Copyright applies to all Abstracts )

Beam-loss detection for the high-rate superconducting upgrade to the SLAC Linac Coherent Light Source

The Linac Coherent Light Source (LCLS) x-ray free-electron laser is driven by the third kilometer of the 3-km SLAC linac, which accelerates electrons in normal-conducting copper cavities pulsed at 120 Hz. The first kilometer is being replaced by LCLS-II, a superconducting (SC) electron linac driven by continuous rf at 1.3 GHz and with a normal-conducting photocathode gun using continuous rf at a subharmonic, 186 MHz. Its 4-GeV, 120-kW beam has a 1-MHz maximum rate, with an upgrade to 8 GeV in planning. The beam from either linac can be switched pulse by pulse to either of two new undulators, to generate hard and soft x rays. Control of beam loss is critical for machine and personnel safety. Previous SLAC protection systems have depended on ionization chambers, including both local devices at expected loss sites and long gas-dielectric coaxial cables providing distributed coverage. These devices are unsuited to the SC-linac beam, because their ion collection time, over 1 ms, may allow the space charge of accumulated ions to null the electric field inside the detector, blinding it to an increase in loss. Instead, both the local and the distributed detectors have been replaced with faster devices. The full 4 km will be spanned by multiple radiation-hard optical fibers in lengths of up to 200 m, each coupled to a photomultiplier tube, to capture Cherenkov light from loss showers. These are supplemented by single-crystal diamond detectors at expected loss sites. Signals are integrated with a 500-ms time constant; the beam is stopped within $\mathrm{200}\mathrm{\text{ }}\mathrm{\text{ }}\ensuremath{\mu}\mathrm{s}$ if a threshold is exceeded. We report on our extensive tests of the detectors and the new signal processing.

Electronic Population Transfer via Impulsive Stimulated X-Ray Raman Scattering with Attosecond Soft-X-Ray Pulses.

Free-electron lasers provide a source of x-ray pulses short enough and intense enough to drive nonlinearities in molecular systems. Impulsive interactions driven by these x-ray pulses provide a way to create and probe valence electron motions with high temporal and spatial resolution. Observing these electronic motions is crucial to understand the role of electronic coherence in chemical processes. A simple nonlinear technique for probing electronic motion, impulsive stimulated x-ray Raman scattering (ISXRS), involves a single impulsive interaction to produce a coherent superposition of electronic states. We demonstrate electronic population transfer via ISXRS using broad bandwidth (5.5eV full width at half maximum) attosecond x-ray pulses produced by the Linac Coherent Light Source. The impulsive excitation is resonantly enhanced by the oxygen 1s→2π^{*} resonance of nitric oxide (NO), and excited state neutral molecules are probed with a time-delayed UV laser pulse.

Development of a Platform at the Matter in Extreme Conditions End Station for Characterization of Matter Heated by Intense Laser-Accelerated Protons

High-intensity short-pulse lasers have made possible the generation of energetic proton beams, unlocking numerous applications in high energy density science. One such application is uniform and isochoric heating of materials to the warm dense matter (WDM) state. We have developed a new experimental platform to simultaneously create and probe WDM at the matter in extreme conditions (MEC) end station at the Linac Coherent Light Source (LCLS). The short pulse optical laser (delivering up to 1 J in 45 fs) and the ultrabright LCLS X-ray laser with tunable frequency, respectively, deliver high power required to heat materials to WDM and precision-timed high-resolution X-rays to probe them. The laser-accelerated proton beam driven from a flat 1.5- $\mu \text{m}$ Cu foil was first measured then directed to a secondary sample of Al or polypropylene (PP), typically 300– $400~\mu \text{m}$ away. The time evolution of the sample electron temperature was measured using streaked optical pyrometry, where we observed a peak temperature of 0.9 ± 0.15 eV on the rear surface of an Al sample heated by the proton beam. Simulations using the hybrid-PIC code LSP and the rad-hydro code HELIOS show that a measured proton beam can heat Al to approximately 4 eV and PP to 1 eV if instead focused by a hemispherical Cu target. Through additional LSP simulations, we anticipate creating hotter WDM states (~20 eV) by increasing the laser energy to 10 J and keeping the other laser parameters fixed.

Temporal X-ray Reconstruction using Temporal and Spectral Measurements at LCLS

Transverse deflecting structures (TDSs) are widely used in accelerator physics to measure the longitudinal density of particle bunches. When used in combination with a dispersive section, the whole longitudinal phase space density can be imaged. At the Linac Coherent Light Source (LCLS), the installation of such a device downstream of the undulators enables the reconstruction of the X-ray temporal intensity profile by comparing longitudinal phase space distributions with lasing on and lasing off. However, the resolution of this TDS is limited to around 1 fs rms (root mean square), and therefore, it is not possible to resolve single self-amplified spontaneous emission (SASE) spikes within an X-ray photon pulse. By combining the power spectrum from a high resolution photon spectrometer and the temporal structure from the TDS, the overall resolution is enhanced, thus allowing the observation of temporal, single SASE spikes. The combined data from the spectrometer and the TDS is analysed using an iterative algorithm to obtain the actual intensity profile. In this paper, we present some improvements to the reconstruction algorithm as well as real data taken at LCLS.

A high-power, high-repetition-rate THz source for pump-probe experiments at Linac Coherent Light Source II.

Experiments using a THz pump and an X-ray probe at an X-ray free-electron laser (XFEL) facility like the Linac Coherent Light Source II (LCLS II) require frequency-tunable (3 to 20 THz), narrow bandwidth (∼10%), carrier-envelope-phase-stable THz pulses that produce high fields (>1 MV cm-1) at the repetition rate of the X-rays and are well synchronized with them. In this paper, a two-bunch scheme to generate THz radiation at LCLS II is studied: the first bunch produces THz radiation in an electromagnet wiggler immediately following the LCLS II undulator that produces X-rays from the second bunch. The initial time delay between the two bunches is optimized to compensate for the path difference in THz transport. The two-bunch beam dynamics, the THz wiggler and radiation are described, as well as the transport system bringing the THz pulses from the wiggler to the experimental hall.

Using simultaneous x-ray diffraction and velocity interferometry to determine material strength in shock-compressed diamond

We determine the strength of laser shock-compressed polycrystalline diamond at stresses above the Hugoniot elastic limit using a technique combining x-ray diffraction from the Linac Coherent Light Source with velocity interferometry. X-ray diffraction is used to measure lattice strains, and velocity interferometry is used to infer shock and particle velocities. These measurements, combined with density-dependent elastic constants calculated using density functional theory, enable determination of material strength above the Hugoniot elastic limit. Our results indicate that diamond retains approximately 20 GPa of strength at longitudinal stresses of 150–300 GPa under shock compression.

Evaluation of Skyshine from an Accelerator Facility: Dependence on Distance and Angle.

Neutron skyshine from Linac Coherent Light Source II 4 GeV electron beam operation at SLAC National Accelerator Laboratory can contribute to prompt radiation exposure to the public at distances far beyond the accelerator tunnel housing. One of the shielding design requirements at SLAC is that the annual dose to a member of the public is no more than 0.05 mSv y. This study uses Monte Carlo code FLUKA to simulate the generation of neutrons from 4 GeV electron beam losses on a thick copper target inside a generalized geometry of the Linac Coherent Light Source II Beam Transport Hall accelerator tunnel section. The effective dose from neutron skyshine was characterized as a function of both distance from the tunnel wall (up to 1 km away) and angle relative to the beam direction (between 0° and 180°). This new methodology for evaluating neutron skyshine dose is applicable to high-energy GeV-range electron accelerator facilities both at SLAC and elsewhere.

Seventh User Workshop on High-Power Lasers at the Linac Coherent Light Source

We report on a seventh annual workshop in a series focused on science realized by the combination of hard X-ray free electron lasers with high power optical lasers, hosted at the SLAC National Accelerator Laboratory in Menlo Park, CA. Members from the user community of the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source (LCLS) and other scientists met with local scientists to discuss developments at LCLS and MEC and related facilities, including experimental results and future plans.

Submicropulse electron-beam dynamics correlated with short-range wakefields in Tesla-type superconducting rf cavities

We report direct observations of submicropulse beam centroid shifts (head-tail kicks) correlated with short-range wakefields generated by off-axis electron-beam steering in Tesla-type superconducting rf cavities. The experiments were performed at the Fermilab Accelerator Science and Technology (FAST) Facility using its unique configuration of a photocathode rf gun injecting beam into two separated nine-cell cavities. The cavities are in series with corrector magnets and beam position monitors (BPMs) located before, between, and after them. The off-axis steering in the cavity was guided by the rf BPM data and higher-order mode circuitry targeting the first and second dipole passbands. The centroid shifts of up to 300 μm from head to tail of the ∼10-ps-long micropulses at 500 pC/b in a 3-MHz pulse train were measured via optical transition radiation at a downstream screen with a Hamamatsu C5680 synchroscan streak camera. We also showed that we could compensate such kicks from the first cavity with the short-range wakefields (SRWs) in the second cavity, and we observed the dilution of the beam size in the tail of the pulses. A simple numerical model of the SRW effect in a single Tesla cavity is compared to the experiment successfully. In principle, these fundamental results may be scaled to cryomodule configurations of major free-electron laser (FEL) facilities such as the European XFEL, Linac Coherent Light Source or LCLS-II XFEL, and the conceptual international linear collider.

Observation of the molecular response to light upon photoexcitation

When a molecule interacts with light, its electrons can absorb energy from the electromagnetic field by rapidly rearranging their positions. This constitutes the first step of photochemical and photophysical processes that include primary events in human vision and photosynthesis. Here, we report the direct measurement of the initial redistribution of electron density when the molecule 1,3-cyclohexadiene (CHD) is optically excited. Our experiments exploit the intense, ultrashort hard x-ray pulses of the Linac Coherent Light Source (LCLS) to map the change in electron density using ultrafast x-ray scattering. The nature of the excited electronic state is identified with excellent spatial resolution and in good agreement with theoretical predictions. The excited state electron density distributions are thus amenable to direct experimental observation.

Bi-cross validation of spectral clustering hyperparameters

One challenge impeding the analysis of terabyte scale X-ray scattering data from the Linac Coherent Light Source (LCLS) is determining the number of clusters required for the execution of traditional clustering algorithms. Here, we demonstrate that the previous work using bi-cross validation to determine the number of singular vectors directly maps to the spectral clustering problem of estimating both the number of clusters and hyperparameter values. Applying this method to LCLS X-ray scattering data enables the identification of dropped shots without manually setting boundaries on detector fluence and provides a path toward identifying rare and anomalous events.

The ePix10k 2-megapixel hard X-ray detector at LCLS.

The ePix10ka2M (ePix10k) is a new large area detector specifically developed for X-ray free-electron laser (XFEL) applications. The hybrid pixel detector was developed at SLAC to provide a hard X-ray area detector with a high dynamic range, running at the 120 Hz repetition rate of the Linac Coherent Light Source (LCLS). The ePix10k consists of 16 modules, each with 352 × 384 pixels of 100 µm × 100 µm distributed on four ASICs, resulting in a 2.16 megapixel detector, with a 16.5 cm × 16.5 cm active area and ∼80% coverage. The high dynamic range is achieved with three distinct gain settings (low, medium, high) as well as two auto-ranging modes (high-to-low and medium-to-low). Here the three fixed gain modes are evaluated. The resulting dynamic range (from single photon counting to 10000 photons pixel-1 pulse-1 at 8 keV) makes it suitable for a large number of different XFEL experiments. The ePix10k replaces the large CSPAD in operation since 2011. The dimensions of the two detectors are similar, making the upgrade from CSPAD to ePix10k straightforward for most setups, with the ePix10k improving on experimental performance. The SLAC-developed ePix cameras all utilize a similar platform, are tailored to target different experimental conditions and are designed to provide an upgrade path for future high-repetition-rate XFELs. Here the first measurements on this new ePix10k detector are presented and the performance under typical XFEL conditions evaluated during an LCLS X-ray diffuse scattering experiment measuring the 9.5 keV X-ray photons scattered from a thin liquid jet.

Temporal power reconstruction for an x-ray free-electron laser using convolutional neural networks

The free-electron laser process extracts energy from a relativistic electron beam to create high-power, coherent x-ray radiation. The energy loss leaves an imprint of the radiation on the longitudinal energy-time phase-space of the electron beam. At the Linac Coherent Light Source, the X-band transverse deflecting mode cavity measures the longitudinal phase-space, and an x-ray temporal power profile is predicted according to time slices difference between lasing-off and lasing-on measurements. However, the algorithm cannot include physical effects such as slippage in deep saturation and introduces errors from mismatches between the lasing-off and lasing-on measurements. Instead, considering the two-dimensional phase-space as a spatial image, we use a computer vision algorithm to predict the power profile. We simulate thousands of pairs of electron beams and x-ray power profiles to train convolutional neural networks, and test models on both simulated and experimental data. We demonstrate significant improvement compared with the traditional algorithm for a range of conditions.

Attosecond Coherence Time Characterization in Hard X-Ray Free-Electron Laser

One of the key challenges in scientific researches based on free-electron lasers (FELs) is the characterization of the coherence time of the ultra-fast hard x-ray pulse, which fundamentally influences the interaction process between x-rays and materials. Conventional optical methods, based on autocorrelation, are very difficult to realize due to the lack of mirrors. Here, we experimentally demonstrate a novel method which yields a coherence time of 174.7 attoseconds for the 6.92 keV FEL pulses at the Linac Coherent Light Source. In our experiment, a phase shifter is adopted to control the cross-correlation between x-ray and microbunched electrons. This approach provides critical diagnostics for the temporal coherence of x-ray FELs and is universal for general machine parameters; applicable for wide range of photon energy, radiation brightness, repetition rate and FEL pulse duration.

Coherence time characterization method for hard X-ray free-electron lasers.

Coherence time is one of the fundamental characteristics of light sources. Methods based on autocorrelation have been widely applied from optical domain to soft X-rays to characterize the radiation coherence time. However, for the hard X-ray regime, due to the lack of proper mirrors, it is extremely difficult to implement such autocorrelation scheme. In this paper, a novel approach for characterizing the coherence time of a hard X-ray free-electron laser (FEL) is proposed and validated numerically. A phase shifter is adopted to control the correlation between X-ray and microbunched electrons. The coherence time of the FEL pulse can be extracted from the cross-correlation. Semi-analytical analysis and three-dimensional time-dependent numerical simulations are presented to elaborate the details. A coherence time of 218.2 attoseconds for 6.92 keV X-ray FEL pulses is obtained in our simulation based on the configuration of Linac Coherent Light Source. This approach provides critical temporal coherence diagnostics for X-ray FELs, and is decoupled from machine parameters, applicable for any photon energy, radiation brightness, repetition rate and FEL pulse duration.

Laguerre-Gaussian Mode Laser Heater for Microbunching Instability Suppression in Free-Electron Lasers.

Microbunching instability (MBI) driven by beam collective effects is known to be detrimental to high-brightness storage rings, linacs, and free-electron lasers (FELs). One known way to suppress this instability is to induce a small amount of energy spread to an electron beam by a laser heater. The distribution of the induced energy spread greatly affects MBI suppression and can be controlled by shaping the transverse profile of the heater laser. Here, we present the first experimental demonstration of effective MBI suppression using a LG_{01} transverse laser mode and compare the improved results with respect to traditional Gaussian transverse laser mode at the Linac Coherent Light Source. The effects on MBI suppression are characterized by multiple downstream measurements, including longitudinal phase space analysis and coherent radiation spectroscopy. We also discuss the role of LG_{01} shaping in soft x-ray self-seeded FEL emission, one of the most advanced operation modes of a FEL for which controlled suppression of MBI is critical.