The Double-Disk Diamond Window as Backup Broadband Window Solution for the DEMO Electron Cyclotron System
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-11-15 |
| Journal | Journal of Nuclear Engineering |
| Authors | G. Aiello, G. Gantenbein, John Jelonnek, Andreas Meier, T. Scherer |
| Institutions | Karlsruhe Institute of Technology |
| Citations | 3 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Feasibility Confirmed: The double-disk CVD diamond window is validated as a feasible backup broadband solution for the 2 MW, 204 GHz (worst-case) DEMO Electron Cyclotron (EC) H&CD system.
- Thermal Margin Concern: The reference design (10 L/min flow rate) resulted in a maximum disk temperature of 238 °C, which is critically close to the generally accepted CVD diamond limit of 250 °C.
- Structural Integrity: Finite Element Method (FEM) analysis showed maximum first principal stresses in the diamond disk (up to 99 MPa) remain safely below the assumed allowable limit of 150 MPa.
- Sensitivity to Loss Tangent (tanδ): The window performance is highly sensitive to the diamondâs loss tangent; a conservative increase in tanδ (5.0 x 10-5) would push the maximum temperature far beyond the 250 °C limit (to 384 °C).
- Design Optimization: A conceptual design change was proposed, introducing features to increase fluid turbulence along the cooling path to enhance heat exchange effectiveness.
- Improved Performance: Combining the modified design with a higher flow rate (20 L/min) successfully reduced the maximum disk temperature to 186 °C, establishing a robust safety margin.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Beam Power (Pbeam) | 2 | MW | Continuous Wave (CW) operation scenario. |
| Worst-Case Frequency (f) | 204 | GHz | Highest frequency tested, leading to maximum absorption. |
| Disk Thickness (t) | 1.85 | mm | Resonant thickness for DEMO frequencies (136, 170, 204 GHz). |
| Disk Diameter | 106 | mm | Physical dimension of the CVD diamond disk. |
| Beam Radius (w0) | 20 | mm | Assumed Gaussian beam radius (upper boundary limit). |
| Assumed Loss Tangent (tanδ) | 3.5 x 10-5 | N/A | Reference case assumption for brazed CVD diamond. |
| Absorbed Power (Pabs) | 1847 | W | Calculated absorption in one disk (Reference Case). |
| Coolant Inlet Temperature | 20 | °C | Water coolant reference temperature. |
| Coolant Flow Rate (Reference) | 10 (0.167) | L/min (kg s-1) | Reference case flow rate. |
| Max T (Reference Design) | 238 | °C | Maximum temperature in the disk center (10 L/min). |
| Max T (Modified Design) | 186 | °C | Maximum temperature achieved with 20 L/min flow rate. |
| Diamond T Limit (Assumed) | 250 | °C | General operational limit for CVD diamond. |
| Allowable Diamond Stress | 150 | MPa | Generally assumed ultimate limit for CVD diamond. |
| Max Diamond Stress (Reference) | 99 | MPa | First principal stress maximum in the brazing area. |
| Pressure Drop (Reference, 10 L/min) | 0.38 | bar | Pressure drop across the cooling circuit. |
| Pressure Drop (Modified, 20 L/min) | 1.52 | bar | Pressure drop across the cooling circuit (increased turbulence). |
Key Methodologies
Section titled âKey Methodologiesâ-
CFD Conjugated Heat Transfer Analysis:
- Code: ANSYS CFX 2021 R1 was used for steady-state analysis.
- Material Properties: Temperature-dependent properties were used for pure copper, CVD diamond, and steel.
- Turbulence Modeling: The k-omega Shear Stress Transport (SST) model was selected to accurately model near-wall interactions and heat transfer at the cooling interface.
- Heat Load Application: The absorbed power (Pabs) was applied as a volumetric power density (q'''(r)) following a Gaussian distribution along the radial coordinate.
- Mesh Refinement: A fine mesh (13.3 x 106 elements) was generated, including a very fine inflation layer (10 Âľm first element size) at the fluid-solid boundary to capture boundary layer effects accurately.
-
FEM Structural Analysis:
- Code: ANSYS Workbench 2021 R1 was used to determine thermal stresses.
- Analysis Type: Plastic steady-state structural analysis was performed, necessary because stresses in the copper cuffs approached the yield strength.
- Material Modeling: Multilinear isotropic hardening was used as the plasticity model for copper, based on stress-strain curves up to 250 °C.
- Loading: The temperature distribution derived from the CFD results was applied as the thermal load.
- Stress Evaluation: Stresses were evaluated using first principal stresses for diamond and equivalent von Mises stresses for metallic components.
-
Design Optimization and Parametric Study:
- Conceptual Change: The intermediate cuff was modified by increasing its thickness and introducing a series of 2 mm diameter holes to intentionally increase fluid velocity and turbulence along the cooling path.
- Sensitivity Testing: Analyses were conducted to check the windowâs sensitivity to variations in mass flow rate (5 to 20 L/min), beam frequency (136, 170, 204 GHz), beam radius (15, 20 mm), and loss tangent (2.0 x 10-5 to 5.0 x 10-5).
Commercial Applications
Section titled âCommercial Applicationsâ- Fusion Energy Systems: Essential component (window) for high-power Electron Cyclotron Heating and Current Drive (EC H&CD) systems in next-generation fusion reactors (e.g., DEMO, ITER).
- High-Power RF/Microwave Technology: Output windows for high-frequency, high-power vacuum electronic devices (VETs) such as gyrotrons and klystrons, requiring robust, low-loss dielectric interfaces.
- Broadband Communication and Radar: Applications requiring frequency-step tunable transmission windows capable of handling multi-megawatt continuous wave (CW) power loads.
- Advanced Thermal Management: Utilization of brazed CVD diamond components in high heat flux environments where efficient, single-sided cooling is required, leveraging diamondâs exceptional thermal conductivity.
- Industrial Laser Optics: High-power windows and optical components where thermal lensing and absorption must be minimized under intense continuous operation.
View Original Abstract
The second variant of the electron cyclotron heating and current drive system in DEMO considers the deployment of 2 MW power Gaussian microwave beams to the plasma by frequency steering. Broadband optical grade chemical vapor deposition diamond windows are thus required. The Brewster-angle window represents the primary choice. However, in the case of showstoppers, the double-disk window is the backup solution. This window concept was used at ASDEX Upgrade for injection of up to 1 MW at four frequencies between 105 and 140 GHz. This paper shows computational fluid dynamics conjugated heat transfer and structural analyses of such a circumferentially water-cooled window design aiming to check whether it might be used for DEMO microwave beam scenarios. This design was then characterized with respect to different parameters. Temperature and thermal stress results showed that it is a feasible window solution for DEMO, but safety margins against limits shall be increased by introducing design features able to make the fluid more turbulent. A first design change is proposed, showing that, in combination with a higher inlet flow rate, the maximum temperature in the disks can be reduced from 238 to 186 °C, leading, therefore, to lower thermal gradients and stresses in the window.
Tech Support
Section titled âTech SupportâOriginal Source
Section titled âOriginal SourceâReferences
Section titled âReferencesâ- 2021 - Integration concept of an Electron Cyclotron System in DEMO [Crossref]
- 2015 - Efficient frequency step-tunable megawatt-class D-band gyrotron [Crossref]
- 2019 - Overview of recent gyrotron R&D towards DEMO within EUROfusion work package heating and current drive [Crossref]
- 2020 - Towards large area CVD diamond disks for Brewster-angle windows [Crossref]
- 2021 - Large area diamond disk growth experiments and thermomechanical investigations for the broadband Brewster window in DEMO [Crossref]