Characterization of cryo-cooled silicon crystal monochromators via measurement of flux versus power
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-05-23 |
| Journal | Journal of Synchrotron Radiation |
| Authors | Lucia Alianelli, H. Khosroabadi, John P. Sutter, A. C. Walters, Pierpaolo Romano |
| Institutions | Diamond Light Source |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study characterizes the thermal resilience of cryo-cooled silicon Double-Crystal Monochromators (DCMs) at Diamond Light Source (DLS) to validate their performance for the upcoming Diamond-II upgrade.
- DCM Resilience Confirmed: Existing cryo-cooled silicon DCMs demonstrated high thermal stability, successfully coping with absorbed power levels up to 380 W and power densities up to 13.7 W/mm2.
- Model Validation: Experimental flux measurements confirmed the accuracy of an analytical thermo-mechanical model used to predict the critical power threshold (Pc) where crystal deformation becomes significant.
- Flux Loss Mechanism: Significant flux losses (leveling off) were observed only in the two experimental scenarios (104 beamline, full acceptance) where the incident power (P) exceeded the predicted critical threshold (Pc).
- Cooling Efficiency: Analysis of crystal temperature rise confirmed excellent thermal contact conductance (KSi-Cu ~ 2000 W m-2 K-1) between the silicon crystal and the copper cooling plates.
- Diamond-II Readiness: The validated analytical model provides a reliable, fast tool for initial performance analysis of DCMs under the significantly increased flux density expected from the 3.5 GeV, 160 pm rad Diamond-II machine.
- Non-DCM Flux Issues: For beamlines where flux was lower than theoretical expectations but power was below Pc, the flux deficit was confirmed not to be caused by monochromator thermal deformation.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum Absorbed Power (Tested) | 380 | W | 104 Beamline, Setting 1 (highest load) |
| Maximum Power Density (Tested) | 13.7 | W/mm2 | 104 Beamline, Setting 1 & 2 |
| Thermal Contact Conductance (KSi-Cu) | ~2000 | W m-2 K-1 | Silicon-Copper interface (Indium foil) |
| Baseline Crystal Temperature (T) | ~77 | K | Temperature during beam pauses/LN2 cooling |
| Maximum Temperature Excursion (ÎT) | 13 | K | Observed at 380 W power load |
| Silicon Null Thermal Expansion Temp (T0) | ~125 | K | Used in the Pc analytical model |
| Diamond-II Electron Beam Energy | 3.5 | GeV | Upgrade specification |
| Diamond-II Horizontal Emittance | 160 | pm rad | Reduced from 2.7 nm rad (current DLS) |
| Acceptable Slope Error (Guideline) | 1 to 2 | ”rad | Required for optimal diffraction efficiency |
| DCM Cooling Method | Indirect | N/A | Liquid Nitrogen (LN2) via copper plates |
Key Methodologies
Section titled âKey MethodologiesâThe study utilized a combined experimental and analytical approach to characterize the thermal performance of the cryo-cooled DCMs.
- Experimental Setup: Measurements were conducted on four hard X-ray undulator beamlines (I04, I07, I18, I19) equipped with cryo-cooled silicon DCMs.
- Power Variation: Incident power (P) and power density (PD) were systematically varied by ramping the storage ring current (50 mA to 300 mA) and adjusting primary slit openings and Bragg angles.
- Thermal Stabilization: Sufficient wait time was enforced between current ramps and flux measurements to ensure the crystal temperature reached a stable state, minimizing temporal thermal response lag.
- Flux Measurement: Monochromatic photon flux was measured using calibrated diagnostics, including X-ray beam position monitors (XBPMs), diodes, and ionization chambers, positioned before and after the focusing optics.
- Temperature Monitoring: Crystal temperature (T) was continuously monitored using a calibrated PT100 sensor placed in the center of the non-cooled side of the crystal mount.
- Analytical Modeling: An existing analytical thermo-mechanical model was used to calculate the critical power threshold (Pc) based on material properties (Si), cooling design, and power density (PD).
- Validation: Experimental flux response (linearity vs. power) was compared directly against the predicted Pc values. Flux leveling off was expected and observed only when P > Pc.
Commercial Applications
Section titled âCommercial ApplicationsâThe findings and validated methodology are critical for the design and operation of high-performance X-ray optics in advanced scientific facilities.
- Synchrotron Light Source Engineering: Essential for the design and validation of optical components (monochromators, mirrors) for high-brightness, high-power density fourth-generation synchrotron upgrades (e.g., Diamond-II, ESRF-EBS).
- Cryogenic Thermal Management: Provides validated data on the performance and reliability of indirect LN2 cooling systems for precision silicon optics operating under extreme heat loads (up to 380 W).
- Advanced X-ray Optics Metrology: Establishes a validated analytical tool for predicting thermal deformation and subsequent loss of diffraction efficiency, reducing the reliance on complex and time-consuming Finite Element Analysis (FEA).
- High-Flux Beamline Operation: Ensures optimal operational regimes for hard X-ray beamlines, guaranteeing the successful delivery of high-flux and high-brightness beams for micro- and nano-focus experiments.
- Materials Science Research: Supports high-throughput experiments in macromolecular crystallography and surface/interface diffraction by maintaining stable, high-quality monochromatic beams.
View Original Abstract
A study on the thermal load of cryogenically cooled silicon in synchrotron double-crystal monochromators is presented, based on experimental data from four different beamlines at Diamond Light Source. Different amounts of power are deposited on the first monochromator crystal by varying the storage ring current. The resulting crystal deformation causes a decline in the diffraction efficiency when power and power density are above threshold values. The results are compatible with an analytical model of thermo-mechanical deformation. Acceptable monochromator heat load values are determined with this model, to ensure optimal function of the monochromator. This model, previously tested against finite element analyses, is now validated against measured data and it will be used as a tool for initial analysis of monochromator performance on upgraded photon sources.