X-ray optics for the cavity-based X-ray free-electron laser
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-06-21 |
| Journal | Journal of Synchrotron Radiation |
| Authors | Peifan Liu, P. C. Pradhan, Xianbo Shi, D. Shu, Keshab Kauchha |
| Institutions | Argonne National Laboratory, SPring-8 |
| Citations | 4 |
| Analysis | Full AI Review Included |
Technical Documentation: X-ray Optics for Cavity-Based XFEL (CBXFEL)
Section titled âTechnical Documentation: X-ray Optics for Cavity-Based XFEL (CBXFEL)â6CCVD Analysis of J. Synchrotron Rad. (2024). 31, 751-762
Executive Summary
Section titled âExecutive SummaryâThis research validates the critical role of ultra-high-quality Single Crystal Diamond (SCD) optics in achieving fully coherent, high-brilliance X-ray sources via the Cavity-Based X-ray Free-Electron Laser (CBXFEL) scheme.
- Core Achievement: Successful design, manufacturing, and characterization of diamond Bragg mirrors (C2-C4) achieving near-100% reflectivity at 9.831 keV (400 reflection).
- Material Validation: Type-IIa diamond crystals were confirmed as the ideal material due to their superlative properties, including high thermal conductivity, low thermal expansion, and exceptional wavefront preservation.
- Ultra-Precision Requirements: Optical components met stringent specifications, including a Bragg-plane slope error (BPSE) of $\le 0.2$ ”rad $\cdot$ mm-2 and an RMS wavefront phase error of $\le \lambda/70$ over the 100 ”m x 100 ”m beam footprint.
- Advanced Fabrication: Demonstrated successful fabrication of thin SCD drumhead membranes (15-20 ”m thick) for X-ray output coupling (C1), ensuring mechanical stability and strain-free mounting.
- 6CCVD Relevance: The extreme quality and precise dimensional control required for these SCD components align perfectly with 6CCVDâs core capabilities in custom MPCVD diamond growth, ultra-polishing (Ra < 1 nm), and precision machining.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the research paper detailing the requirements and performance of the diamond optical components.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Photon Energy (E0) | 9.83102 | keV | CBXFEL Operating Energy |
| Diamond Reflection | 400 | Bragg | Cavity Mirrors (C1-C4) |
| Bragg Angle ($\Theta_{H}$) | 45.0 | ° | Nominal Reflection Angle |
| SCD Plate Thickness (C2-C4) | 500 $\pm$ 100 | ”m | High-Reflectivity Mirrors |
| SCD Membrane Thickness (C1) | 15 - 20 | ”m | X-ray Outcoupling Drumhead |
| Required Reflectivity (C2-C4) | $\ge 99$ | % | Intra-Cavity Mirrors |
| Outcoupling Transmissivity (C1) | 2 - 7 | % | Via thin membrane |
| Surface Roughness (Ra) | $\lt 5$ | nm | Polished SCD Surfaces |
| Bragg-Plane Slope Error (BPSE) | $\le 0.2$ | ”rad $\cdot$ mm-2 | Required over 2 mm x 2 mm working area |
| RMS Wavefront Phase Error | $\le \lambda/70$ | r.m.s. | Required over 100 ”m x 100 ”m footprint |
| Crystal Miscut Angle ($\eta$) | $\le 0.3$ | ° | Angle between (400) planes and surface |
| Cavity Round-Trip Length | 65.50 | m | Total X-ray path |
| Si Monochromator Bandwidth (660) | 20 | meV | Narrowest bandwidth for diagnostics |
Key Methodologies
Section titled âKey MethodologiesâThe fabrication and characterization of the ultra-high-quality diamond optics involved a multi-step process focused on minimizing strain and maximizing crystalline perfection.
- Material Selection: High-quality Type-IIa diamond crystals (HPHT grown) were selected, characterized by an almost flawless 2 mm x 2 mm working area.
- Pre-Screening: Initial quality assessment utilized quasi-plane wave X-ray topography (SPring-8) to identify almost-defect-free regions.
- Precision Machining: Selected plates ($\sim 7$ mm x 7 mm) were cut into rectangular plates ($\sim 4$ mm x 5 mm) using laser cutting, incorporating two strain-relief cuts to protect the working area.
- Drumhead Fabrication (C1): Laser ablation was used to create the thin (15-20 ”m) membranes for output coupling, forming a monolithic drumhead structure for stable, strain-free mounting.
- Strain Removal: All laser-machined diamond plates were subjected to annealing at $\sim 630$ °C in air to efficiently remove crystal strain induced by the cutting process.
- Quality Characterization:
- Rocking-Curve Imaging (RCI): Used to map Full-Width Half-Maximum (FWHM) and Center-of-Mass (COM) values, confirming BPSE $\le 0.2$ ”rad $\cdot$ mm-2 in the working area.
- At-Wavelength Wavefront Sensing (WS): Used in Bragg diffraction geometry to measure the RMS wavefront phase error, confirming performance better than $\lambda/65$.
- Alignment Reference: A separate diamond crystal (Cx) was used in 440 exact Bragg backscattering geometry for accurate intra-cavity photon energy calibration and initial angular alignment of the cavity crystals.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe CBXFEL project demands diamond optics with unparalleled crystalline quality, dimensional precision, and surface finish. 6CCVDâs advanced MPCVD capabilities are uniquely positioned to meet and exceed these stringent requirements for next-generation X-ray optics and FEL systems.
Applicable Materials
Section titled âApplicable MaterialsâTo replicate or extend this research, 6CCVD recommends the following materials:
- Optical Grade Single Crystal Diamond (SCD): Required for the high-reflectivity Bragg mirrors (C2-C4) and the drumhead outcoupler (C1). 6CCVDâs MPCVD SCD offers Type-IIa purity equivalent or superior to the HPHT material used, ensuring minimal absorption and maximum thermal stability under high radiation flux.
- Custom Thin SCD Wafers: Essential for the drumhead membrane (C1). 6CCVD offers SCD thickness control from 0.1 ”m up to 500 ”m, allowing precise tuning of outcoupling transmissivity (2-7% achieved here with 15-20 ”m thickness).
Customization Potential
Section titled âCustomization Potentialâ6CCVD provides the necessary engineering and fabrication services to produce the complex optical components described:
| Requirement from Paper | 6CCVD Capability | Technical Advantage |
|---|---|---|
| Ultra-Low Surface Roughness (Ra $\lt 5$ nm required) | Precision Polishing: Guaranteed Ra < 1 nm (SCD). | Superior wavefront preservation and reduced scattering losses, exceeding the paperâs requirement. |
| Custom Dimensions (7 mm x 7 mm plates, 2 mm x 2 mm working area) | Custom Dimensions: Plates/wafers up to 125 mm (PCD) and custom SCD sizes. | Ability to supply larger source material and precision-cut final components with strain-relief geometry. |
| Drumhead Structure / Strain Relief | Precision Laser Machining: In-house laser cutting and ablation services. | Capability to fabricate complex geometries, including thin membranes and strain-relief cuts, essential for maintaining the required BPSE ($\le 0.2$ ”rad $\cdot$ mm-2). |
| Substrate Thickness (500 ”m plates, 15-20 ”m membranes) | Thickness Control: SCD available from 0.1 ”m to 500 ”m (wafers) and substrates up to 10 mm. | Direct control over Bragg reflection properties and outcoupling efficiency. |
| Metalization for Mounting/Diagnostics | Internal Metalization: Capability to deposit Au, Pt, Pd, Ti, W, Cu. | Although bare diamond was used for the mirrors, 6CCVD can apply custom metal layers for stable mounting, thermal contact, or integrated diagnostics. |
Engineering Support
Section titled âEngineering SupportâThe successful implementation of CBXFEL optics relies on precise material selection and detailed knowledge of X-ray diffraction physics.
- 6CCVDâs in-house PhD engineering team specializes in advanced X-ray optics and high-power thermal management. We can assist researchers in material selection, optimizing crystal orientation (e.g., 400 Bragg reflection at 45°), and determining optimal thickness for specific photon energies (e.g., 9.831 keV) for similar X-ray Free-Electron Laser (XFEL) and Synchrotron Optics projects.
- We offer consultation on achieving ultra-low strain mounting solutions necessary to maintain the required Bragg-plane slope error and wavefront quality.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
A cavity-based X-ray free-electron laser (CBXFEL) is a possible future direction in the development of fully coherent X-ray sources. CBXFELs consist of a low-emittance electron source, a magnet system with several undulators and chicanes, and an X-ray cavity. The X-ray cavity stores and circulates X-ray pulses for repeated FEL interactions with electron pulses until the FEL reaches saturation. CBXFEL cavities require low-loss wavefront-preserving optical components: near-100%-reflectivity X-ray diamond Bragg-reflecting crystals, outcoupling devices such as thin diamond membranes or X-ray gratings, and aberration-free focusing elements. In the framework of the collaborative CBXFEL research and development project of Argonne National Laboratory, SLAC National Accelerator Laboratory and SPring-8, we report here the design, manufacturing and characterization of X-ray optical components for the CBXFEL cavity, which include high-reflectivity diamond crystal mirrors, a diamond drumhead crystal with thin membranes, beryllium refractive lenses and channel-cut Si monochromators. All the designed optical components have been fully characterized at the Advanced Photon Source to demonstrate their suitability for the CBXFEL cavity application.