Timothy Maxwell

Few-femtosecond time-resolved measurements of X-ray free-electron lasers

X-ray free-electron lasers, with pulse durations ranging from a few to several hundred femtoseconds, are uniquely suited for studying atomic, molecular, chemical and biological systems. Characterizing the temporal profiles of these femtosecond X-ray pulses that vary from shot to shot is not only challenging but also important for data interpretation. Here we report the time-resolved measurements of X-ray free-electron lasers by using an X-band radiofrequency transverse deflector at the Linac Coherent Light Source. We demonstrate this method to be a simple, non-invasive technique with a large dynamic range for single-shot electron and X-ray temporal characterization. A resolution of less than 1 fs root mean square has been achieved for soft X-ray pulses. The lasing evolution along the undulator has been studied with the electron trapping being observed as the X-ray peak power approaches 100 GW.

High-intensity double-pulse X-ray free-electron laser.

The X-ray free-electron laser has opened a new era for photon science, improving the X-ray brightness by ten orders of magnitude over previously available sources. Similar to an optical laser, the spectral and temporal structure of the radiation pulses can be tailored to the specific needs of many experiments by accurately manipulating the lasing medium, that is, the electron beam. Here we report the generation of mJ-level two-colour hard X-ray pulses of few femtoseconds duration with an XFEL driven by twin electron bunches at the Linac Coherent Light Source. This performance represents an improvement of over an order of magnitude in peak power over state-of-the-art two-colour XFELs. The unprecedented intensity and temporal coherence of this new two-colour X-ray free-electron laser enable an entirely new set of scientific applications, ranging from X-ray pump/X-ray probe experiments to the imaging of complex biological samples with multiple wavelength anomalous dispersion.

Generating femtosecond X-ray pulses using an emittance-spoiling foil in free-electron lasers

Generation of femtosecond to sub-femtosecond pulses is attracting much attention in X-ray free-electron laser user community. One method is to use a slotted, emittance-spoiling foil which was proposed before (P. Emma et al., Phys. Rev. Lett. 92, 074801 (2004)) and has been widely used at the Linac Coherent Light Source. Direct experimental characterization of the slotted-foil performance was previously unfeasible due to a lack of appropriate diagnostics. With a recently installed X-band radio-frequency transverse deflector, we are able to characterize the electron bunch spoiling effect and X-ray pulse when using the slotted foil. We show that few-femtosecond X-ray pulses are generated with flexible control of the single-pulse duration or double-pulse separation with comparison to the theoretical model.

Optical Shaping of X-Ray Free-Electron Lasers

In this Letter we report the experimental demonstration of a new temporal shaping technique for x-ray free-electron lasers (FELs). This technique is based on the use of a spectrally shaped infrared (IR) laser and allows optical control of the x-ray generation process. By accurately manipulating the spectral amplitude and phase of the IR laser, we can selectively modify the electron bunch longitudinal emittance thus controlling the duration of the resulting x-ray pulse down to the femtosecond time scale. Unlike other methods currently in use, optical shaping is directly applicable to the next generation of high-average power x-ray FELs such as the Linac Coherent Light Source-II or the European X-FEL, and it enables pulse shaping of FELs at the highest repetition rates. Furthermore, this laser-shaping technique paves the way for flexible tailoring of complex multicolor FEL pulse patterns required for nonlinear multidimensional x-ray spectroscopy as well as novel multicolor diffraction imaging schemes.

Measurements of wake-induced electron beam deflection in a dechirper at the Linac Coherent Light Source

The RadiaBeam/SLAC dechirper, a structure consisting of pairs of flat, metallic, corrugated plates, %a corrugated structure in flat geometry, has been installed just upstream of the undulators in the Linac Coherent Light Source (LCLS). As a dechirper, with the beam passing between the plates on axis, longitudinal wakefields are induced that can remove unwanted energy chirp in the beam. However, with the beam passing off axis, strong transverse wakes are also induced. This mode of operation has already been used for the production of intense, multi-color photon beams using the Fresh-Slice technique, and is being used to develop a diagnostic for attosecond bunch length measurements. Here we measure, as function of offset, the strength of the transverse wakefields that are excited between the two plates, and also for the case of the beam passing near to a single plate. We compare with analytical formulas from the literature, and find good agreement. This report presents the first systematic measurements of the transverse wake strength in a dechirper, one that has been excited by a bunch with the short pulse duration and high energy found in an X-ray free electron laser.

Auger electron and photoabsorption spectra of glycine in the vicinity of the oxygen K-edge measured with an X-FEL

We report the first measurement of the near oxygen K-edge auger spectrum of the glycine molecule. Our work employed an x-ray free electron laser as the photon source operated with input photon energies tunable between 527 and 547 eV. Complete electron spectra were recorded at each photon energy in the tuning range, revealing resonant and non-resonant auger structures. Finally ab initio theoretical predictions are compared with the measured above the edge auger spectrum and an assignment of auger decay channels is performed.

Accurate prediction of X-ray pulse properties from a free-electron laser using machine learning

A. Sanchez-Gonzalez,1 P. Micaelli,1 C. Olivier,1 T. R. Barillot,1 M. Ilchen,2, 3 A. A. Lutman,4 A. Marinelli,4 T. Maxwell,4 A. Achner,3 M. Agåker,5 N. Berrah,6 C. Bostedt,4, 7 J. Buck,8 P. H. Bucksbaum,2, 9 S. Carron Montero,4, 10 B. Cooper,1 J. P. Cryan,2 M. Dong,5 R. Feifel,11 L. J. Frasinski,1 H. Fukuzawa,12 A. Galler,3 G. Hartmann,8, 13 N. Hartmann,4 W. Helml,4, 14 A. S. Johnson,1 A. Knie,13 A. O. Lindahl,2, 11 J. Liu,3 K. Motomura,12 M. Mucke,5 C. O’Grady,4 J-E. Rubensson,5 E. R. Simpson,1 R. J. Squibb,11 C. Såthe,15 K. Ueda,12 M. Vacher,16, 17 D. J. Walke,1 V. Zhaunerchyk,11 R. N. Coffee,4 and J. P. Marangos1 1Department of Physics, Imperial College, London, SW7 2AZ, United Kingdom 2Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 3European XFEL GmbH, Holzkoppel 4, 22869 Schenefeld, Germany 4Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 5Department of Physics and Astronomy, Uppsala University, Uppsala, 75120, Sweden 6Department of Physics, University of Connecticut, 2152 Hillside Road, U-3046, Storrs, CT 06269, USA 7Argonne National Laboratory, Lemont, IL 60439, USA 8Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, Hamburg, 22607, Germany 9Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305, USA 10California Lutheran University, 60 W Olsen Rd, Thousand Oaks, CA 91360, USA 11Department of Physics, University of Gothenburg, Origovägen 6B, 41296 Gothenburg, Sweden 12Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan 13Institut für Physik und CINSaT, Universität Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany 14Physics Department, TU Munich, James-Franck-Str. 1, 85748 Garching, Germany 15MAX IV Laboratory, Lund University, Box 118, SE-221 00 Lund, Sweden 16Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom 17Department of Chemistry Ångtröm, Uppsala University, Uppsala, 75120, SwedenFree-electron lasers providing ultra-short high-brightness pulses of X-ray radiation have great potential for a wide impact on science, and are a critical element for unravelling the structural dynamics of matter. To fully harness this potential, we must accurately know the X-ray properties: intensity, spectrum and temporal profile. Owing to the inherent fluctuations in free-electron lasers, this mandates a full characterization of the properties for each and every pulse. While diagnostics of these properties exist, they are often invasive and many cannot operate at a high-repetition rate. Here, we present a technique for circumventing this limitation. Employing a machine learning strategy, we can accurately predict X-ray properties for every shot using only parameters that are easily recorded at high-repetition rate, by training a model on a small set of fully diagnosed pulses. This opens the door to fully realizing the promise of next-generation high-repetition rate X-ray lasers.

Stimulated X-Ray Raman Scattering with Free-Electron Laser Sources

Stimulated electronic x-ray Raman scattering is the building block for several proposed x-ray pump probe techniques, that would allow the study of electron dynamics at unprecedented timescales. We present high spectral resolution data on stimulated electronic x-ray Raman scattering in a gas sample of neon using a self-amplified spontaneous emission x-ray free-electron laser. Despite the limited spectral coherence and broad bandwidth of these sources, high-resolution spectra can be obtained by statistical methods, opening the path to coherent stimulated x-ray Raman spectroscopy. An extension of these ideas to molecules and the results of a recent experiment in CO are discussed.

Intensity interferometry measurements with hard x-ray FEL pulses at the Linac Coherent Light Source

Intensity interferometry measurements were carried out to study the spatial coherence properties of a Free-Electron Laser (FEL) in the Self-Amplified Spontaneous Emission (SASE) mode in the hard X-ray regime. Statistical analyses based on ensemble averages of the spatial intensity correlation function were performed on a large number of pulses, overcoming challenges associated with the FEL beam being non-stationary in time and highly collimated. The second-order intensity correlation functions consistently show deviations from unity, reminiscent of the classical Hanbury-Brown and Twiss effect. They also exhibit a slow decaying spatial dependence at length-scales larger than the width of the beam, indicating a high degree of spatial coherence. These measurements are consistent with the behavior of a highly brilliant but chaotic source obeying Gaussian statistics as expected for a SASE FEL. Our study could be used to devise an in-line diagnostic capable of providing quasi real-time feedback for understanding and tuning the FEL process.

Femtosecond-scale x-ray FEL diagnostics with the LCLS X-band transverse deflector

Analysis of single-shot, lasing-induced changes of the longitudinal electron bunch properties has proven invaluable for fs-scale reconstruction of otherwise difficult to measure x-ray FEL pulse profiles. In this talk, we report on measurements following the recent installation of an X-band transverse deflecting mode cavity at the LCLS. Limitations of the FEL pulse profiling technique employed are discussed. An unprecedented 1 to 3 fs RMS time resolution of x-ray and electron bunch profiles is demonstrated. Phenomena impacting x-ray FEL performance are also observed. The new tool is proven as a powerful diagnostic in support of user experiments and machine improvement studies.