Special Issue "Latest Trends in Free Electron Lasers"

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Optics and Lasers".

Deadline for manuscript submissions: closed (31 January 2022) | Viewed by 15243

Special Issue Editor

Dr. Emiliano Principi
E-Mail Website
Guest Editor
Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
Interests: condensed matter under nonequilibrium conditions; electronic processes in light-activated functional materials; matter in extreme conditions; solid-density plasma; nonlinear interactions between extreme ultraviolet light and matter; ultrafast chemical reactions

Special Issue Information

Dear Colleague,

I invite you to submit an original research paper, a review, or a perspective article to this Special Issue on “Trends in Free Electron Lasers” in Applied Sciences.

After about 10 years of intense development and necessary commissioning, the free electron laser (FEL) technology is reaching maturity and can provide ultrashort intense coherent X-ray and extreme ultraviolet pulses for innovative investigations on matter. Following a natural adaptation period, an interdisciplinary scientific community routinely working on FEL facilities has grown, incorporating synchrotron and table-top fs-laser know-how. Stimulated by experimental findings, theoretical scientists have begun to simulate FEL-driven ultrafast dynamics in matter and develop new nonequilibrium theoretical methods. Meanwhile, a novel class of FEL sources experts, the “FEL physicists”, have become crucial to the proper operation and improvement of FELs.

Today, people involved in the amazing rise of FELs can be proud of the work done and look to future advances. FEL users, aware of both the limits and the potential of FEL experimental methods and techniques, can now formulate ideas for breakthrough experiments and identify future scientific challenges with the assistance of advanced numerical calculations perfected by theoreticians. FEL physicists have the crucial role of further developing FEL sources, improving their performance and flexibility, and pursuing original FEL light generation schemes.

It is important to understand in what new directions the young FEL community is moving and inform both present and potential future members about promising opportunities.

The aim of this Special Issue is to anticipate and highlight scientific and technological trends involving FELs, facilitate the dissemination of new ideas in the field, and contribute to the consolidation of the FEL community. Topics of interest include:

  1. frontier FEL science;
  2. challenging FEL experiments to be attempted;
  3. novel experimental methods, instrumentation, and techniques;
  4. numerical calculations on nonequilibrium matter;
  5. simulation of ultrafast dynamics;
  6. breakthrough FEL technologies and devices;
  7. new FEL light generation schemes; and
  8. future developments of FEL facilities.

Your contribution to this Special Issue is welcome.

Dr. Emiliano Principi
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Applied Sciences is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2300 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • free electron lasers
  • FEL sources
  • FEL light generation schemes
  • relativistic linear accelerators
  • FEL experimental methods
  • ultrafast phenomena
  • nonequilibrium matter
  • sub-picosecond dynamics
  • nonlinear x-ray/matter interaction
  • high-energy and density matter
  • coherent control
  • ab initio theoretical calculations

Published Papers (12 papers)

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Editorial

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Editorial
Special Issue on Latest Trends in Free Electron Lasers
Appl. Sci. 2022, 12(21), 11215; https://doi.org/10.3390/app122111215 - 05 Nov 2022
Viewed by 325
Abstract
In the last decade, free electron laser (FEL) sources operating from the extreme ultraviolet (EUV) up to the hard X-ray photon energy range [...] Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)

Research

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Article
Approaching the Attosecond Frontier of Dynamics in Matter with the Concept of X-ray Chronoscopy
Appl. Sci. 2022, 12(3), 1721; https://doi.org/10.3390/app12031721 - 08 Feb 2022
Cited by 1 | Viewed by 1530
Abstract
X-ray free electron lasers (XFELs) have provided scientists opportunities to study matter with unprecedented temporal and spatial resolutions. However, access to the attosecond domain (i.e., below 1 femtosecond) remains elusive. Herein, a time-dependent experimental concept is theorized, allowing us to track ultrafast processes [...] Read more.
X-ray free electron lasers (XFELs) have provided scientists opportunities to study matter with unprecedented temporal and spatial resolutions. However, access to the attosecond domain (i.e., below 1 femtosecond) remains elusive. Herein, a time-dependent experimental concept is theorized, allowing us to track ultrafast processes in matter with sub-fs resolution. The proposed X-ray chronoscopy approach exploits the state-of-the-art developments in terahertz streaking to measure the time structure of X-ray pulses with ultrahigh temporal resolution. The sub-femtosecond dynamics of the saturable X-ray absorption process is simulated. The employed rate equation model confirms that the X-ray-induced mechanisms leading to X-ray transparency can be probed via measurement of an X-ray pulse time structure. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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Article
Investigating Coherent Magnetization Control with Ultrashort THz Pulses
Appl. Sci. 2022, 12(3), 1323; https://doi.org/10.3390/app12031323 - 26 Jan 2022
Cited by 3 | Viewed by 1607
Abstract
Coherent terahertz control of magnetization dynamics is an area of current interest due to its great potential for the realization of magnetization control on ultrafast timescales in commercial devices. Here we report on an experiment realized at the THz beamline of the free [...] Read more.
Coherent terahertz control of magnetization dynamics is an area of current interest due to its great potential for the realization of magnetization control on ultrafast timescales in commercial devices. Here we report on an experiment realized at the THz beamline of the free electron laser FLASH at DESY which offers a tunable terahertz radiation source and spontaneously synchronized free-electron laser X-ray pulses to resonantly probe the magnetization state of a ferromagnetic film. In this proof-of-principle experiment, we have excited a thin Permalloy film at different THz wavelengths and recorded the induced magnetization dynamics with photons resonantly tuned to the Ni M2,3 absorption edge. For THz pump pulses including higher orders of the undulator source we observed demagnetization dynamics, which precise shape depended on the employed fundamental wavelength of the undulator source. Analyzing the shape in detail, we can reconstruct the temporal profile of the electric field of the THz pump pulse. This offers a new method for the realization of an in-situ terahertz beamline diagnostic which will help researchers to adjust the pulse characteristics as needed, for example, for future studies of THz induced coherent control of magnetization dynamics. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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Article
Perspectives towards Sub-Ångström Working Regime of the European X-ray Free-Electron Laser with Low-Emittance Electron Beams
Appl. Sci. 2021, 11(22), 10768; https://doi.org/10.3390/app112210768 - 15 Nov 2021
Cited by 2 | Viewed by 930
Abstract
Sub-ångström working regime refers to a working state of free-electron lasers which allows the generation of hard X-rays at a photon wavelength of 1 ångström and below, that is, a photon energy of 12.5 keV and above. It is demonstrated that the accelerators [...] Read more.
Sub-ångström working regime refers to a working state of free-electron lasers which allows the generation of hard X-rays at a photon wavelength of 1 ångström and below, that is, a photon energy of 12.5 keV and above. It is demonstrated that the accelerators of the European X-ray Free-Electron Laser can provide highly energetic electron beams of up to 17.5 GeV. Along with long variable-gap undulators, the facility offers superior conditions for exploring self-amplified spontaneous emission (SASE) in the sub-ångström regime. However, the overall FEL performance relies quantitatively on achievable electron beam qualities through a kilometers-long accelerator beamline. Low-emittance electron beam production and the associated start-to-end beam physics thus becomes a prerequisite to dig in the potentials of SASE performance towards higher photon energies. In this article, we present the obtained results on electron beam qualities produced with different accelerating gradients of 40 MV/m–56 MV/m at the cathode, as well as the final beam qualities in front of the undulators via start-to-end simulations considering realistic conditions. SASE studies in the sub-ångström regime, using optimized electron beams, are carried out at varied energy levels according to the present state of the facility, that is, a pulsed mode operating with a 10 Hz-repetition 0.65 ms-long bunch train energized to 14 GeV and 17.5 GeV. Millijoule-level SASE intensity is obtained at a photon energy of 25 keV at 14 GeV electron beam energy using a gain length of about 7 m. At 17.5 GeV, half-millijoule lasing is achieved at 40 keV. Lasing at up to 50 keV is demonstrated with pulse energies in the range of a few hundreds and tens of microjoules with existing undulators and currently achievable electron beam qualities. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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Article
Complex Attosecond Waveform Synthesis at FEL FERMI
Appl. Sci. 2021, 11(21), 9791; https://doi.org/10.3390/app11219791 - 20 Oct 2021
Cited by 2 | Viewed by 1197
Abstract
Free-electron lasers (FELs) can produce radiation in the short wavelength range extending from the extreme ultraviolet (XUV) to the X-rays with a few to a few tens of femtoseconds pulse duration. These facilities have enabled significant breakthroughs in the field of atomic, molecular, [...] Read more.
Free-electron lasers (FELs) can produce radiation in the short wavelength range extending from the extreme ultraviolet (XUV) to the X-rays with a few to a few tens of femtoseconds pulse duration. These facilities have enabled significant breakthroughs in the field of atomic, molecular, and optical physics, implementing different schemes based on two-color photoionization mechanisms. In this article, we present the generation of attosecond pulse trains (APTs) at the seeded FEL FERMI using the beating of multiple phase-locked harmonics. We demonstrate the complex attosecond waveform shaping of the generated APTs, exploiting the ability to manipulate independently the amplitudes and the phases of the harmonics. The described generalized attosecond waveform synthesis technique with an arbitrary number of phase-locked harmonics will allow the generation of sub-100 as pulses with programmable electric fields. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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Article
Flexible and Coherent Soft X-ray Pulses at High Repetition Rate: Current Research and Perspectives
Appl. Sci. 2021, 11(20), 9729; https://doi.org/10.3390/app11209729 - 18 Oct 2021
Cited by 2 | Viewed by 1093
Abstract
The successful realization of high gain free-electron lasers has opened new possibilities to X-ray scientists for investigating matter in different states. The availability of unprecedented photon properties stimulated the development of new experimental techniques capable of taking full advantage of these options and [...] Read more.
The successful realization of high gain free-electron lasers has opened new possibilities to X-ray scientists for investigating matter in different states. The availability of unprecedented photon properties stimulated the development of new experimental techniques capable of taking full advantage of these options and has started a virtuous collaboration between machine experts and photon users to improve further and optimize the generated X-ray pulses. Over the recent years, this has led to the development of several advanced free-electron laser (FEL) schemes to tailor the photon properties to specific experimental demands. Presently, tunable wavelength X-ray pulses with extremely high brilliance and short pulse characteristics are a few of the many options available at FELs. Few facilities can offer options such as narrowband or extremely short pulses below one fs duration and simultaneous pulses of multiple colors enabling resonant X-ray pump—X-ray probe experiments with sub fs resolution. Fully coherent X-ray radiation (both spatial and temporal) can also be provided. This new option has stimulated the application of coherent control techniques to the X-ray world, allowing for experiments with few attoseconds resolution. FELs often operate at a relatively low repetition rate, typically on the order of tens of Hz. At FLASH and the European XFEL, however, the superconducting accelerators allow generating thousands of pulses per second. With the implementation of a new seeded FEL line and with an upgrade at FLASH linac, all the new features will become available in the soft X-ray spectral range down to the oxygen K edge with unprecedented average photon flux due to the high repetition rate of pulses. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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Article
Resonant X-ray Emission Spectroscopy with a SASE Beam
Appl. Sci. 2021, 11(18), 8775; https://doi.org/10.3390/app11188775 - 21 Sep 2021
Cited by 1 | Viewed by 1099
Abstract
Aqueous iron (III) oxide nanoparticles were irradiated with pure self-amplified spontaneous emission (SASE) X-ray free-electron laser (XFEL) pulses tuned to the energy around the Fe K-edge ionization threshold. For each XFEL shot, the incident X-ray pulse spectrum and Fe Kβ emission spectrum were [...] Read more.
Aqueous iron (III) oxide nanoparticles were irradiated with pure self-amplified spontaneous emission (SASE) X-ray free-electron laser (XFEL) pulses tuned to the energy around the Fe K-edge ionization threshold. For each XFEL shot, the incident X-ray pulse spectrum and Fe Kβ emission spectrum were measured synchronously with dedicated spectrometers and processed through a reconstruction algorithm allowing for the determination of Fe Kβ resonant X-ray emission spectroscopy (RXES) plane with high energy resolution. The influence of the number of X-ray shots employed in the experiment on the reconstructed data quality was evaluated, enabling the determination of thresholds for good data acquisition and experimental times essential for practical usage of scarce XFEL beam times. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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Article
Frequency-Mixing Lasing Mode at European XFEL
Appl. Sci. 2021, 11(18), 8495; https://doi.org/10.3390/app11188495 - 13 Sep 2021
Cited by 1 | Viewed by 869
Abstract
We demonstrate generation of X-ray Free-Electron Laser (XFEL) pulses in frequency mixing mode at the SASE3 line of the European XFEL. The majority of the SASE3 FEL segments are tuned at two frequencies ω1 and ω2 following an alternate pattern. Leveraging [...] Read more.
We demonstrate generation of X-ray Free-Electron Laser (XFEL) pulses in frequency mixing mode at the SASE3 line of the European XFEL. The majority of the SASE3 FEL segments are tuned at two frequencies ω1 and ω2 following an alternate pattern. Leveraging on non-linearities generated through longitudinal dispersion in the system, we obtain electron bunching at a frequency difference ωFM=ω2ω1. FEL amplification at ωFM follows in a few last radiator segments. We report on the generation of frequency mixing at photon energies between 500 eV and 1100 eV with pulse energies, depending on the length of the radiator, in the mJ level. This method allows generating low photon energies in cases where the FEL runs at high electron energy and the target photon energy cannot be reached in the main undulator, with the simple addition of a short, custom-made afterburner. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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Article
Limitations of Structural Insight into Ultrafast Melting of Solid Materials with X-ray Diffraction Imaging
Appl. Sci. 2021, 11(11), 5157; https://doi.org/10.3390/app11115157 - 02 Jun 2021
Cited by 3 | Viewed by 1507
Abstract
In this work, we analyze the application of X-ray diffraction imaging techniques to follow ultrafast structural transitions in solid materials using the example of an X-ray pump–X-ray probe experiment with a single-crystal silicon performed at a Linac Coherent Light Source. Due to the [...] Read more.
In this work, we analyze the application of X-ray diffraction imaging techniques to follow ultrafast structural transitions in solid materials using the example of an X-ray pump–X-ray probe experiment with a single-crystal silicon performed at a Linac Coherent Light Source. Due to the spatially non-uniform profile of the X-ray beam, the diffractive signal recorded in this experiment included contributions from crystal parts experiencing different fluences from the peak fluence down to zero. With our theoretical model, we could identify specific processes contributing to the silicon melting in those crystal regions, i.e., the non-thermal and thermal melting whose occurrences depended on the locally absorbed X-ray doses. We then constructed the total volume-integrated signal by summing up the coherent signal contributions (amplitudes) from the various crystal regions and found that this significantly differed from the signals obtained for a few selected uniform fluence values, including the peak fluence. This shows that the diffraction imaging signal obtained for a structurally damaged material after an impact of a non-uniform X-ray pump pulse cannot be always interpreted as the material’s response to a pulse of a specific (e.g., peak) fluence as it is sometimes believed. This observation has to be taken into account in planning and interpreting future experiments investigating structural changes in materials with X-ray diffraction imaging. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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Review

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Review
Recent Progress of the PAL-XFEL
Appl. Sci. 2022, 12(3), 1010; https://doi.org/10.3390/app12031010 - 19 Jan 2022
Cited by 4 | Viewed by 1032
Abstract
The X-ray free-electron laser of the Pohang Accelerator Laboratory (PAL-XFEL) was opened to users in 2017. Since then, significant progress has been made in PAL-XFEL operation and beamline experiments. This includes increasing the FEL pulse energy, increasing the FEL photon energy, generating self-seeding [...] Read more.
The X-ray free-electron laser of the Pohang Accelerator Laboratory (PAL-XFEL) was opened to users in 2017. Since then, significant progress has been made in PAL-XFEL operation and beamline experiments. This includes increasing the FEL pulse energy, increasing the FEL photon energy, generating self-seeding FEL, and trials of two-color operation. In the beamline, new instruments or endstations have been added or are being prepared. Overall, beamline operation has been stabilized since its initiation, which has enabled excellent scientific results through efficient user experiments. In this paper, we describe details of the recent progress of the PAL-XFEL. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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Other

Perspective
Potential of Time-Resolved Serial Femtosecond Crystallography Using High Repetition Rate XFEL Sources
Appl. Sci. 2022, 12(5), 2551; https://doi.org/10.3390/app12052551 - 28 Feb 2022
Cited by 3 | Viewed by 1087
Abstract
This perspective review describes emerging techniques and future opportunities for time-resolved serial femtosecond crystallography (TR-SFX) experiments using high repetition rate XFEL sources. High repetition rate sources are becoming more available with the European XFEL in operation and the recently upgraded LCLS-II will be [...] Read more.
This perspective review describes emerging techniques and future opportunities for time-resolved serial femtosecond crystallography (TR-SFX) experiments using high repetition rate XFEL sources. High repetition rate sources are becoming more available with the European XFEL in operation and the recently upgraded LCLS-II will be available in the near future. One efficient use of these facilities for TR-SFX relies on pump–probe experiments using a laser to trigger a reaction of light-responsive proteins or mix-and-inject experiments for light-unresponsive proteins. With the view to widen the application of TR-SFX, the promising field of photocaged compounds is under development, which allows the very fast laser triggering of reactions that is no longer limited to naturally light-responsive samples. In addition to reaction triggering, a key concern when performing an SFX experiment is efficient sample usage, which is a main focus of new high repetition rate-compatible sample delivery methods. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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Project Report
The SXFEL Upgrade: From Test Facility to User Facility
Appl. Sci. 2022, 12(1), 176; https://doi.org/10.3390/app12010176 - 24 Dec 2021
Cited by 8 | Viewed by 1507
Abstract
The Shanghai soft X-ray Free-Electron Laser facility (SXFEL), which is the first X-ray FEL facility in China, is being constructed in two phases: the test facility (SXFEL-TF) and the user facility (SXFEL-UF). The test facility was initiated in 2006 and funded in 2014. [...] Read more.
The Shanghai soft X-ray Free-Electron Laser facility (SXFEL), which is the first X-ray FEL facility in China, is being constructed in two phases: the test facility (SXFEL-TF) and the user facility (SXFEL-UF). The test facility was initiated in 2006 and funded in 2014. The commissioning of the test facility was finished in 2020. The user facility was funded in 2016 to upgrade the accelerator energy and build two undulator lines with five experimental end-stations. The output photon energy of the user facility will cover the whole water window range. This paper presents an overview of the SXFEL facility, including considerations of the upgrade, layout and design, construction status, commissioning progress and future plans. Full article
(This article belongs to the Special Issue Latest Trends in Free Electron Lasers)
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