X-ray beam diagnostics at the MID instrument of the European X-ray Free-Electron Laser Facility
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-04-08 |
| Journal | Journal of Synchrotron Radiation |
| Authors | Ulrike Boesenberg, G. Ansaldi, Alexander Bartmann, L E Batchelor, Felix BrauĂe |
| Institutions | European X-Ray Free-Electron Laser |
| Citations | 4 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThe Materials Imaging and Dynamics (MID) instrument at the European X-ray Free-Electron Laser Facility (EuXFEL) has implemented a Multipurpose Diagnostic End-Station (DES) to enable critical, pulse-resolved beam characterization.
- High-Rate Diagnostics: The DES provides essential diagnostics for X-ray pulses at repetition rates up to 4.5 MHz, crucial for EuXFEL operation.
- Spectral Characterization: A bent-diamond-crystal spectrometer allows pulse-resolved measurement of the X-ray spectrum, providing energy calibration and characterization of special accelerator modes (e.g., HXRSS).
- Ultrafast Pulse Measurement: Spectroscopic analysis using the g2 correlation function enables estimation of ultra-short pulse durations, achieving Fourier-transform-limited estimates down to 0.73 fs.
- Intensity Monitoring: Diamond-based solid-state ionization chambers provide highly transmissive, pulse-resolved intensity monitoring with a fast response time (approximately 150 ns FWHM).
- Beam Alignment and Imaging: An imager unit, utilizing B-doped diamond screens, ensures reproducible beam alignment, monitors pointing stability, and aids in the setup of complex optics like the X-ray split-and-delay line (SDL).
- High Heat Load Management: The system incorporates a layered, heavy-duty beamstop consisting of 40 mm Boron Carbide (B4C) and 10 mm Aluminum (Al) to safely absorb high-intensity, high-repetition-rate X-ray beams.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum Repetition Rate | 4.5 | MHz | X-ray pulse train bursts (10 Hz repetition rate) |
| Photon Energy Range | 5 to 25 | keV | Operational range for diagnostics |
| Spectrometer Crystal | Diamond C(220) or C(440) | N/A | Bent crystal radius R ~ 90 mm |
| Spectrometer Energy Resolution (Measured) | 0.35 | eV | At 9 keV using C(220) reflection |
| Minimum Estimated Pulse Duration | 0.73 | fs | Derived from g2 correlation function (short pulse setting) |
| Spectrometer Piezo Stage Accuracy | 0.5 | ”rad | Positioning precision for crystal rotation |
| 1D Detector Pixel Pitch | 50 | ”m | Gotthard-I linear detector (1280 pixels) |
| 1D Detector Max Trigger Rate | 556 | kHz | Measures every fourth pulse at 2.25 MHz operation |
| Diamond Detector Thickness | 40 | ”m | Solid-state ionization chamber |
| Diamond Detector Response Time | 150 | ns | Approximate FWHM signal duration |
| Low-Resolution Imager Screen | 20 | ”m | B-doped diamond screen |
| Low-Resolution Imager Pixel Size | 12 x 8.5 | ”m2 | Achieved with KOWA LM50JC10M objective |
| Beamstop Material 1 | 40 | mm | Boron Carbide (B4C) |
| Beamstop Material 2 | 10 | mm | Aluminum (Al) |
| Beamstop Transmission (25 keV) | < 0.1 | % | Combined B4C and Al layers |
| SDL Beam Separation Angle (Measured) | 100 | nrad | Example measurement during alignment |
Key Methodologies
Section titled âKey MethodologiesâThe DES integrates multiple diagnostic components within two connected vacuum chambers, ensuring mutual alignment and high-vacuum compatibility.
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System Mounting and Alignment:
- The entire DES unit is mounted on a vertically motorized table to accommodate variable beam heights resulting from upstream optics (monochromators, lenses).
- Horizontal adjustments are achieved by sliding the vacuum chambers on rails.
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Spectrometry Setup:
- Bent diamond crystals are mounted on a high-precision piezo stage, allowing rotation around the beam axis with 0.5 ”rad accuracy.
- The dispersed spectrum is collected by a Gotthard-I 1D linear detector (pulse-resolved) and a 2D YAG screen imager (integrated over the bunch train).
- The 1D detector is synchronized to the X-ray bunch train, measuring every fourth pulse when operating at 2.25 MHz.
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Intensity Monitoring:
- Diamond-based solid-state ionization chambers (40 ”m thick) are inserted directly into the beam.
- A moderate bias voltage (not exceeding 100 V) is applied, and the signal is read out by a fast analog-to-digital converter (StruckSIS8300) to achieve MHz-rate pulse resolution.
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Beam Imaging:
- Low-Resolution: A 20 ”m B-doped diamond screen, angled at 45°, is used for high-intensity, high-repetition-rate imaging (10 Hz frame rate).
- High-Resolution (Commissioning): A 25 ”m YAG:Ce screen is used for finer spatial characterization (target pixel size ~2 ”m).
- Both screens are mounted on SmarAct translation positioners for precise switching and alignment (x, y, z directions).
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Beam Attenuation and Safety:
- A filter wheel provides beam attenuation or thin metal foils for relative photon energy calibration (e.g., using absorption edges).
- The layered beamstop (B4C followed by Al) is designed to handle high peak intensity and repetition rates, positioning ablation-hard materials first.
Commercial Applications
Section titled âCommercial ApplicationsâThe technologies and methodologies developed for the EuXFEL MID DES are critical for industries requiring ultra-high-speed, high-power beam diagnostics and precision instrumentation.
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Accelerator Science and Synchrotron Facilities:
- Design and implementation of next-generation diagnostic end-stations for XFELs and high-repetition-rate synchrotrons.
- Development of pulse-resolved spectral and intensity monitors for beam tuning and optimization.
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Advanced Detector Technology:
- Manufacturing and integration of radiation-hard, solid-state diamond detectors for high-flux environments (e.g., intensity monitors, BPMs).
- Development of ultra-fast readout electronics (ADCs) capable of handling MHz signal rates.
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Ultrafast Science and Dynamics:
- Instrumentation for measuring and controlling ultra-short X-ray pulses (sub-femtosecond regime) used in time-resolved materials science and chemistry experiments.
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Precision Motion and Vacuum Systems:
- Application of high-precision piezo stages (sub-microradian accuracy) for crystal alignment and optical element positioning in complex vacuum environments.
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Radiation Shielding and Materials Engineering:
- Design and testing of layered, high-power beamstops using materials like B4C and Al, optimized for thermal management and ablation resistance under extreme X-ray flux.
View Original Abstract
The Materials Imaging and Dynamics (MID) instrument at the European X-ray Free-Electron Laser Facility (EuXFEL) is equipped with a multipurpose diagnostic end-station (DES) at the end of the instrument. The imager unit in DES is a key tool for aligning the beam to a standard trajectory and for adjusting optical elements such as focusing lenses or the split-and-delay line. Furthermore, the DES features a bent-diamond-crystal spectrometer to disperse the spectrum of the direct beam to a line detector. This enables pulse-resolved characterization of the EuXFEL spectrum to provide X-ray energy calibration, and the spectrometer is particularly useful in commissioning special modes of the accelerator. Together with diamond-based intensity monitors, the imager and spectrometer form the DES unit which also contains a heavy-duty beamstop at the end of the MID instrument. Here, we describe the setup in detail and provide exemplary beam diagnostic results.