Longitudinal photon and virtual diagnostics workshops

6 Nov 2024, 08:30 → 8 Nov 2024, 18:00 Europe/Rome

Aula Touschek and Aula Salvini (INFN-LNF)

We are excited to announce two consecutive workshops, bringing together experts from both the laser and FEL communities to foster an exchange of ideas on photon beam diagnostics:

 

1. Longitudinal Diagnostics for Photon Beams – November 6-7, 2024

2. Virtual Diagnostics Workshop – November 7-8, 2024

 

The first workshop, Longitudinal Diagnostics for Photon Beams, will cover diagnostic techniques for photon beams from FELs, synchrotrons, betatron sources, inverse Compton scattering, HHG, and other sources within the EUV to hard X-ray range. We encourage contributions from all participants and aim to facilitate cross-community discussions, with a focus on the following topics:

 

- Nonlinear optics techniques (e.g., transient reflectivity, cross-correlation methods)

- Gas ionization streaking (e.g., longitudinal or transverse streaking using visible or THz pulses)

- Indirect source reconstruction (e.g., analyzing spent electrons from FELs using deflecting cavities, THz generation)

- Advanced longitudinal diagnostic techniques (e.g., wavefront reconstruction, interferometry, speckle-based methods)

- Data analysis with AI and virtual diagnostics (e.g., using AI to enhance diagnostics, virtual diagnostics for extracting new data)

- Characterization of large bandwidth or partially coherent photon beams (e.g., synchrotron pink beams, plasma accelerator betatron sources, inverse Compton scattering)

 

The second workshop, Virtual Diagnostics, will focus on AI-driven data analysis and innovative virtual diagnostics techniques. We will focus on how these tools can extract more detailed or alternative diagnostics information from existing photon diagnostics.

 

In addition to the scientific sessions, we are pleased to offer two social events, sponsored by LEAPS, on the evenings of 6th and 7th November.

 

Both workshops encourage open dialogue, collaboration, and discussion between the laser and FEL communities. 

 

The event will be held at the INFN National Laboratories of Frascati, from November 6th to 8th, 2024, and participation is free of charge.

 

mappa_LNF.pdf

Privacy_Policy_EN.pdf

SCHEDA-COMPORTAMENTALE-EN.pdf

WiFi SSID INFN instructions_Longitudinal pd.pdf

Wednesday, 6 November

08:30 Registration

Introduction

Streaking instrumentation 1

1 5 years of experience with the THz streaking beamline FL21 at FLASH

The intense ultra-short XUV pulses of the free-electron laser FLASH@DESY fluctuate from pulse to pulse due to the underlying SASE-FEL operating principle and thus demand single-shot diagnostics. To cope with this, a beamline (FL21) for temporal diagnostics was designed, built and put into operation at FLASH2 in 2019.
The beamline has been equipped with a permanently installed terahertz field-driven streaking setup that enables the determination of single shot pulse duration and arrival time.
In numerous beamtimes we had the chance to collect a large number of THz streaking measurements. Here we present some lessons learned and the variety of different options offered by the streaking setup. Looking at gain curve measurements, two-color FEL operation, harmonic content of the FEL radiation, frequency chirp of the XUV pulse as well as on correlations between the pulse duration and other parameters like pulse energy or spectral distribution, we can learn more about the performance of FLASH. Having all the experimental data at hand, we also used it to compare to different theoretical models and to find/verify scaling laws.
Finally, an outlook will be given on the plans for pulse duration measurements at FLASH2 in the next years.

Speaker: Stefan Duesterer (DESY)

2 Towards attosecond streaking at X-ray free-electron lasers

Since the generation of the first sub-femtosecond extreme-ultraviolet (XUV) pulses in 2001 [1], the frontiers of ultrafast science have been rapidly expanded by attosecond time-resolved techniques. Notably, attosecond streaking spectroscopy has been used to reconstruct the electric field of light pulses, clock core-hole decay lifetimes, and even measure photoemission delays as short as tens of attoseconds [2-5]. Whilst the majority of these measurements have been made using table-top extreme-ultraviolet (XUV) sources based on laser-driven high-harmonic generation (HHG), there have been contemporaneous advances in large-scale sources of X-ray radiation. Across the natural sciences, X-ray free-electron lasers (XFELs) have become established tools for pioneering experiments that exploit their intense, ultrafast X-ray pulses. In addition to the extreme brilliance and high photon energies accessible at these large-scale facilities, XFELs are now able to generate X-ray pulses with attosecond duration [6]. This could herald a new era in ultrafast science, with experimentalists able to simultaneously capitalise upon the extreme time resolution offered by attosecond pulses without the restrictions on photon energy tunability suffered by HHG sources.

However, applying established attosecond streaking techniques at XFELs is challenging. Unlike XUV sources, XFELs cannot inherently synchronise their pulses with those of an external probe laser, leading to timing jitter on the scale of tens or hundreds of femtoseconds. Furthermore, most XFEL facilities do not currently have access to carrier-envelope-phase-stable laser pulses, which are typically a prerequisite for attosecond streaking spectroscopy. Despite the increasing availability of attosecond XFEL pulses, these two challenges have generally kept attosecond-resolution experiments out of reach for XFELs.

Here, we present a novel technique to overcome these limitations and unlock the full potential of attosecond XFEL pulses for ultrafast science. Dubbed ‘self-referenced attosecond streaking’, the method takes advantage of inherent fixed properties of the experimental sample or geometry, using these to reconstruct timing and phase information. In this way, the technique sidesteps the usual strict timing and phase requirements for attosecond streaking spectroscopy. It is anticipated that this will render a hitherto challenging class of experiments – those that demand extreme time resolution and short-wavelength X-rays – to be realised at XFELs.

[1] Hentschel, M. et al. Attosecond metrology. Nature 414, 509–513 (2001).
[2] Itatani, J. et al. Attosecond streak camera. Physical Review Letters 88 (17) (2002).
[3] Drescher, M. et al. Time-resolved atomic inner-shell spectroscopy. Nature 419 (6909), 803–807 (2002).
[4] Goulielmakis, E. et al. Direct measurement of light waves. Science 305 (5688), 1267–1269 (2004).
[5] Cavalieri, A. L. et al. Attosecond spectroscopy in condensed matter. Nature 449 (7165), 1029–1032 (2007).
[6] Duris, J. et al. Tunable isolated attosecond x-ray pulses with gigawatt peak power from a free-electron laser. Nature Photonics 14 (1), 30–36 (2019).

Speaker: Daniel Haynes

AttoStreakingXFELs.pptx

3 Operation of Photo Electron Spectrometers for Photon Diagnostics at the European X-ray Free Electron Laser

Angle resolved photo-electron spectrometers with micro-channel plate detectors and combined with fast digitizer electronics are versatile and powerful devices for providing both soft and hard X-ray non-invasive single shot photon diagnostics at MHz repetition rate X-ray free-electron lasers.
Hard X-ray beamlines imposes specific design challenges due to poor photo-ionization cross-section and very high photo-electron velocities.
Furthermore, recent advancements in machine learning enables resolution enhancement by training the photo-electron spectrometer together with an invasive high resolution spectrometer which generates a response function model.

Speaker: Joakim Laksman (European XFEL)

PES_Laksman.pptx

11:30 Coffee break

Streaking instrumentation 2

4 Experiences with online streakng diagnostics at SwissFEL

Speaker: Christopher Arrell

5 Experimental Demonstration of Two-Color Attosecond Pump–Probe Spectroscopy with an X-ray Free-Electron Laser

Pump–probe experiments with attosecond resolution are the key to understanding electronic dynamics in quantum systems. Isolated attosecond soft X-ray pulses produced with a Free-Electron Laser (FEL) have pulse energies up to hundreds of microjoules [1]. The intense pulse energies of attosecond soft X-ray FEL pulses are sufficient for nonlinear X-ray spectroscopy and enable attosecond pump-probe experiments. In this talk, I will present the generation and control of sub-femtosecond pulse pairs from a two-color X-ray FEL [2]. By measuring the delay between the two pulses using an angular streaking diagnostic, we characterize the group velocity of the X-ray FEL and demonstrate the control of the pulse delay down to 270 as. We demonstrate the applicability of this technique to a pump–probe measurement in core-ionized para-aminophenol. These results reveal the ability to perform pump–probe experiments with sub-femtosecond resolution and atomic site specificity.

The highly flexible design of the Athos soft X-ray beamline at SwissFEL enables the generation of isolated attosecond pulses [3] and holds the potential for two-color X-ray attosecond pump-probe techniques. We are currently working on designing an angular streaking instrument at the Maloja endstation to diagnose isolated attosecond X-ray pulses and to further enable attosecond soft X-ray pump-probe measurements at SwissFEL.

REFERENCES
[1] Duris, J., et al. Nature Photonics 14.1 (2020): 30-36.
[2] Guo, Z., et al. Nature Photonics (2024): 1-7.
[3] Prat, E., et al. APL Photonics 8.11 (2023).

Speaker: Zhaoheng Guo (Paul Scherrer Institute)

LPVDW_2024_Z_Guo.pptx

13:00 discussion

13:30 Lunch

Experimental methods with short pulses

6 X-ray chronoscopy for investigation of attosecond dynamics in matter

The X-ray free electron lasers (XFELs) [1] are nowadays used to study the structure and dynamics in matter with unprecedented temporal and spatial resolutions. However, some fundamental electron processes in the matter are too fast to be followed, even with the shortest available XFEL pulses. In this work we report on a new experimental approach to study sub-femtosecond processes in matter. Based on the X-ray chronoscopy concept [2], it explores the time distribution of X-ray pulses before and after interaction with a sample, relaxing the strict requirement of the ultra short pulse duration but needing precise time structure measurement. The pulse time structure can be measured using the state-of-the-art terahertz streaking cameras at XFELs [3] arranged in the camera-sample-camera sequence.
In this presentation, the behavior of X-ray pulse time distribution will be shown for two scenarios: (1) the nonlinear process of saturable absorption at ultra-short time scales [4] and (2) an optical pump-X-ray probe scenario [5], using a simulation based on a rate equation model. In study (1), time distribution of a 7 fs-short (FWHM) pulse of 7130 eV-photons was simulated as it is before and after interaction with 20 μm-thick iron foil. The high photon flux lead to deviation of the transmitted pulse time envelope from that of the incident pulse, including loss of the original Gaussian shape and shift in time phase (i.e. shift of the pulse temporal center of mass). Other effects will be reported as well. The results tend to confirm that X-ray-induced dynamics leading to the target X-ray transparency can be probed through measurement of X-ray pulses time structure. In study (2), ZnSe quantum dots were subjected to interaction with intense optical pump pulses with near‑UV photon energy (3.1 eV) and 5 fs (FWHM) pulse duration to excite valence band electrons of the material, and were probed at a delay with an X-ray pulse. The average energy of the X-ray probe pulse was set to 0.1 µJ, the spot on the sample to 100 µm$^{2}$ and pulse duration to 20 fs (FWHM). The results showed sensitivity of the transmitted X-ray pulse time distribution to the excited state lifetime and to the pump-probe delay which will be presented in detail.
Part of this work has been published [4] and attracted the EurekAlert!’s attention [6].
Acknowledgements: This work was supported by the National Science Centre (Poland) under Grant No. 2017/27/B/ST2/01890.
References:
[1] B. W. J. McNeil, N. R. Thompson, Nature Photonics 4 (2010) 814.
[2] D. J. Bradley et al., Optics Communications 15 (1975) 231.
[3] U. Frühling et al., Nature Photonics 3 (2009) 523.
[4] W. Błachucki et al., Applied Sciences 12 (2022) 1721.
[5] R. Fanselow, W. Błachucki, J. Szlachetko, submitted to The European Physical Journal D (2024).
[6] https://www.eurekalert.org/news-releases/952040 .

Speaker: Wojciech Błachucki (Institute of Nuclear Physics, Polish Academy of Sciences, 31-342 Kraków, Poland)

Wojciech Blachucki_X-ray chronoscopy.pptx

7 COSP: compact wide band spectrometer for free electron laser monitoring

In recent years generation and control, both in amplitude and in phase, of multiple harmonics at the seeded Free Electron Lasers (FELs) FERMI have opened the way to a new class of experiments based on the wave mixing paradigm. For these experiments, the simultaneous acquisition of the whole spectrum of harmonics is required to monitor the stability of the emission, the relative intensity ratio between harmonics and the amount of possible spurious harmonics. A grating spectrometer designed to provide medium resolution in a wide spectral range can be used as a real-time monitor of the FEL operation, and the information gathered can be used to normalize and validate data in a multi messenger (electrons/photons) approach.
At FERMI, the online spectrometer PRESTO used to monitor the FEL emission is conceived for the acquisition of one harmonic at a time with high resolution. The request to extend the spectral range for single-shot acquisitions has driven the realization of a compact spectrometer capable of resolving a set of consecutive harmonics in an extended spectral range, and easily portable to the different end-stations of FERMI. It consists of a flat-field concave grating (Shimadzu) having a groove density of 300 gr/mm. The detector is a 40-mm-diameter MCP (Micro Channel Plate) with MgF2 photocathode and phosphor screen (PHOTEK VID140) read by a CMOS camera (Basler acA1300-75gm, 1280 X 1024 px) on a single-shot basis (50 Hz repetition rate). The use of a large-area MCP detector allows to acquire at-once a spectrum spanning the 10-100 nm region (120-12 eV). The detector can be manually displaced along the flat focal curve to cover wavelengths down to 5 nm (250 eV) or up to 124 nm (10 eV). The spectrometer is a useful tool both in the preparatory phase, when the machine parameters have to be optimized, and in the experiment phase, when absolute and relative stability between the different harmonics has to be monitored.

Speaker: Dr Luca Poletto (CNR-IFN)

Poletto_presentation INFN.pdf

8 TBD

Speaker: Martin Huppert

WorkshopFrascati.pptx

16:00 Coffee break

Round table: Funding opportunities and future requirements

Welcome Dinner

Thursday, 7 November

Longitudinal profile measurements

9 Photon diagnostics based on electron beam longitudinal phase space measurements

The electron beam is the lasing medium of a Free-Electron Laser (FEL). When driven to saturation, the high-gain FEL process leaves a measurable imprint in the electron beam longitudinal phase space (LPS, time-energy). In many FEL operation modes, there is an excellent time correlation between the FEL pulse and its imprint. After mapping time and energy coordinates to transverse coordinates by employing both electron streaking and an energy spectrometer, the LPS is measured with a scintillating screen. By comparing the LPS at FEL-enabled and FEL-disabled configurations, the FEL power profile is retrieved. This contribution provides an overview of electron streaking methods, in particular streaking from passive wakefield structures and and from time-synchronous rf fields. Furthermore, current and possible future applications of this concept and their limitations are discussed.

Speaker: Philipp Dijkstal (Paul Scherrer Institut)

10 Temporal photon diagnostics at the European XFEL

Pump and probe techniques utilizing high peak brightness X-ray free electron lasers (XFELs) are powerful tools for studying X-ray nonlinear optics, ultrafast X-ray matter interactions and subsequent ultrafast dynamics. Owning to the stochastical properties of the SASE X-ray pulses, ultrafast pump and probe experiment normally requires a precise characterization of the temporal properties of X-ray pulses, i.e., X-ray pulse duration, and profile, and its arrival time relative to external optical laser pulses, preferably on a shot-to-shot basis.
In this presentation we will give an overview of X-ray temporal diagnostic techniques and introduce the photon arrival time measurement campaigns at the European XFEL based on X-ray/optical cross-correlation and x-ray induced secondary source emission that reliably operates at repetition rate up to 1.13 MHz with fs measurement accuracy. In addition, we will present the current and future plans towards complete characterization of attosecond x-ray pulses that have been demonstrated at both the soft and hard X-ray beamlines at the European XFEL.

Speaker: Jia Liu (European XFEL)

11 Two-color delay study at SwissFEL

Recent years particular attention has been paid to X-ray pump-X-ray probe experiments enabling, for example, excitation of selective sites in a molecule or subshell in an atom with the pump pulse and monitoring its evolution with the probe. To mitigate the jitter of X-ray pulses produced independently (i.e. in two separate instruments), different X-ray free-electron lasers (XFELs) are developing the so called two color scheme, where from a single electron bunch two X-ray pulses are produced. The two color mode of SwissFEL is attained with a spoiler picosecond UV pulse overlapped to the photocathode drive laser increasing the electron bunch emittance at the selected time region [1]. This leads to an inhibited FEL process in this time region and effectively to production of two X-ray subpulses rather than a single X-ray pulse.
In this study the SwissFEL was operated in the 2 color mode, delivering X-ray pulses of the average intensity of 570 µJ composed of 6937 eV and 6978 eV components (energy separation of 41 eV). The time delay between the two energy components in each X-ray pulse was measured with the THz streaking setup and was confronted with the time structure of electron bunch probed with the passive streaker installed at the end of the undulator section. The delay obtained with the THz streaking was about 1.5 – 3.0 larger than the 60 fs implied from the passive streaker measurement.
Acknowledgements: W.B. acknowledges the National Science Centre (Poland) for financial support under the grant of number 2019/03/X/ST2/00949.

[1] C. Vicario et al., Phys. Rev. Accel. Beams 24, 060703 (2021).

Speaker: Wojciech Błachucki (Institute of Nuclear Physics, Polish Academy of Sciences)

Wojciech Blachucki_Two-color.pptx

11:00 discussion

11:30 Coffee break

Opportunities at EuPRAXIA facilities

12 EuPRAXIA facilities

Speaker: Massimo Ferrario (Istituto Nazionale di Fisica Nucleare)

EuPRAXIA_Long.pptx

13 EuPRAXIA beamlines and diagnostics

Speaker: Fabio Villa (Istituto Nazionale di Fisica Nucleare)

EuPRAXIA@SPARC_LAB beamlines.pptx

12:50 discussion

13:30 Lunch

Round table: lange bandwidth ultrafast sources and longitudinal measurements

Round_table_betatron.pdf

16:00 Coffee break

16:30 Laboratories visit

19:30 Dinner

Friday, 8 November

Virtual Diagnostics 1

Virtual-diagnostics.pptx

14 Virtual Diagnostics ?

General considerations about virtual diagnostics:
- what are the real beam diagnostics ?
- what is a virtual beam diagnostics ?
- what is NOT a virtual beam diagnostics ?
- what can we gain by virtual beam diagnostics ?
- what are the practical foundations ?
- how can we create virtual diagnostics ?
These slides are intended as starters for the workshop on virtual diagnostics.

Speaker: Dr Jan Grünert (European XFEL)

15 Virtual Spectroscopy and Surrogate Models at the European XFEL

This talk will cover a selective summary of recent activities related to virtual spectroscopy and diagnostics at the European XFEL. Virtual diagnostics can complement physical ones by combining information from several sources, thereby profiting from the advantages of each one. To this end, we present the Virtual Spectrometer, which maps data from a low-resolution time-of-flight spectrometer to a high-resolution one. While the low-resolution spectrometer is non-invasive, it can operate at 4.5 MHz and has complex calibration procedure. On the other hand, the high-resolution spectrometer is invasive, operates at 10 Hz, but has a simpler calibration procedure. Such a map, allows one to obtain a spectrometer with higher resolution than the time-of-flight spectrometer while maintaining its other benefits. For example, after a short (approx. 30 minutes) setup and training period with the invasive grating spectrometer, it is removed from the beamline. The resulting virtual spectra are obtained at 4.5 MHz non-invasively with an up to 40% increased
resolution.

The talk will also discuss an on-going effort that, leveraging the Virtual Spectrometer's success, aims to predict the x-ray pulse properties from the machine settings and available diagnostics by creating a surrogate model of the machine. While still at an early stage, preliminary results are shown. The fulls scope involves creating a mathematical model of the injector, LINAC, undulators, and potentially also optical components. The goal of the program is not only to provide a surrogate model of the machine, but also to allow for its inversion; i.e. providing a systematic method to obtain machine setting ranges that produce the desired photon beam properties.

Speaker: Fady Bishara (European XFEL)

virtual_diagnostics_workshop_bishara.pdf

16 Deciphering FEL pulse profiles with autoencoder networks

Free electron lasers (FEL) play an important role across diverse scientific disciplines. Many experiments can benefit from non-destructive photon diagnostics of provided FEL photon pulses. One method to obtain information about the pulse profile involves analyzing not the photons directly, but rather the energy distribution of the electrons downstream of a undulator [1]. While measuring the longitudinal phase space with a Transverse Deflection Structure (TDS) is relatively straightforward, obtaining the XUV pulse profiles can be challenging, especially with non-uniform current distributions, an energy chirp, or weak lasing pulses.

In recent times, artificial neural networks (ANNs) have gained widespread recognition as powerful analytical tools spanning various scientific domains, highlighted by this year's Nobel Prize in Physics awarded for contributions to ANNs. This potential suggests that these networks could assist in assigning a lasing off reference for each lasing on data point and therefore help to isolate the FEL lasing process in the data.
We present two case studies in which ANNs were applied to enhance the diagnostics of FEL photon pulse lengths with a TDS. The first case involves the use of beta-variational autoencoder (β-VAE) networks to characterize SASE X-ray pulses produced by the FLASH facility in Hamburg. We will discuss how data obtained from a Polarizable X-Band Transverse Deflection Structure (PolariX TDS) [2,3] allowed the β-VAE to identify SASE strength, a significant parameter in real-world data from FLASH. The second is a cooperation between SLAC and DESY where we used a U-net autoencoder network to enable the analysis of weak self-seeded X-ray pulses, where the weak lasing signal would make a classical analysis infeasible.
We will demonstrate both ANN approaches for pulse diagnostics using real-world data.

References:
[1] Behrens C. et al., Nat. Com. 5, 3762 (2014), https://doi.org/10.1038/ncomms4762 [2] Craievich P. et al., Phys. Rev. Accel. Beams 23, 112001 (2020), https://doi.org/10.1103/PhysRevAccelBeams.23.112001 [3] Christie F. et al. , Proceedings of FEL2019 (2019) https://doi.org/10.3204/PUBDB-2019-03833

Speaker: Gesa Goetzke (DESY - FLASH)

20241108Frescati.pdf

11:15 discussion

11:30 coffee break

Virtual Diagnostics 2

17 Virtual Photon Diagnostics at SwissFEL for Real-Time Spectral Analysis Using Convolutional Neural Networks

The analysis of SASE X-ray FEL spectra plays a critical role in supporting user experiments and advancing machine development. The number and width of SASE spikes in a spectrum serve as key indicators of FEL pulse duration and electron beam energy chirp. In this study, we present a method for real-time spectral analysis using convolutional neural networks (CNNs). A significant aspect of this work involves training the model to detect the number of SASE spikes, even when individual spike widths fall below the spectrometer’s resolution. We trained the CNNs on extensive experimental datasets (N>50000) from the hard and soft X-ray beamlines at SwissFEL, as well as on synthetically generated data. The results of our spectral analysis are compared with time-domain measurements based on electron streaking. This approach provides a practical solution for enhancing photon spectra diagnostics at SwissFEL.

Speaker: Christopher Arrell

12:30 discussion

13:30 Lunch

Round table: virtual diagnostics

16:00 coffee break

18 Summary and closing remarks