The Double-Disk Diamond Window as Backup Broadband Window Solution for the DEMO Electron Cyclotron System

At a Glance

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

Technical Documentation & Analysis: Double-Disk Diamond Windows for DEMO EC Systems

This document analyzes the research concerning the double-disk CVD diamond window for the DEMO Electron Cyclotron (EC) system, translating key findings into actionable technical specifications and demonstrating how 6CCVD’s advanced MPCVD diamond materials and customization capabilities meet and exceed the requirements for high-power fusion applications.

Executive Summary

Application Validation: The double-disk CVD diamond window is validated as a feasible, broadband backup solution for the DEMO EC H&CD system, capable of handling 2 MW Continuous Wave (CW) microwave power.
Worst-Case Performance: CFD analysis confirmed operation at the worst-case scenario (2 MW at 204 GHz) resulted in a maximum disk temperature of 238 °C, critically close to the 250 °C thermal limit for CVD diamond.
Material Criticality: The feasibility hinges on using high-quality, optical grade CVD diamond with an extremely low loss tangent (tanδ) of 3.5 x 10^-5 or better to minimize absorbed power (P_abs = 1847 W).
Thermal Management: A conceptual design change focusing on increasing fluid turbulence and flow rate (20 L/min) successfully reduced the maximum disk temperature significantly, achieving 186 °C and increasing safety margins.
Structural Integrity: Maximum thermal stresses in the diamond disk (99 MPa) were found to be well below the assumed allowable limit of 150 MPa, confirming structural viability under high thermal load.
6CCVD Value Proposition: 6CCVD provides the necessary large-area, high-purity MPCVD diamond (SCD/PCD) wafers up to 125 mm diameter, along with custom thickness control and metalization services required for robust brazing and high-power RF transmission.

Technical Specifications

The following table summarizes the critical operational and material parameters extracted from the CFD and structural analyses of the double-disk window design.

Parameter	Value	Unit	Context
Operating Power	2	MW	Continuous Wave (CW)
Worst-Case Frequency	204	GHz	Highest absorbed power
Disk Diameter	106	mm	Required aperture 80 mm
Disk Thickness (t)	1.85	mm	Resonant condition for 136, 170, 204 GHz
Reference Loss Tangent (tanδ)	3.5 x 10^-5	-	Assumed value for brazed disk
Optimistic Loss Tangent (tanδ)	2.0 x 10^-5	-	Results in 127 °C max temperature
Absorbed Power (P_abs)	1847	W	Single disk, reference case
Max Disk Temperature (Reference)	238	°C	10 L/min flow rate, close to 250 °C limit
Max Disk Temperature (Optimized)	186	°C	Modified design, 20 L/min flow rate
CVD Diamond Thermal Limit	250	°C	General assumption (onset of thermal conductivity degradation)
Max First Principal Stress (Diamond)	99	MPa	Located at the copper brazing interface
CVD Diamond Allowable Stress Limit	150	MPa	Generally assumed limit
Cooling Water Inlet Temperature	20	°C	Reference condition
Pressure Drop (Optimized Cooling)	1.52	bar	Modified design, 20 L/min flow rate

Key Methodologies

The feasibility of the double-disk window was confirmed through rigorous computational modeling, focusing on thermal and structural performance under extreme RF loading.

CFD-Conjugated Heat Transfer Analysis: Steady-state simulations were performed using ANSYS CFX 2021 R1. This method coupled fluid dynamics (water coolant) with heat transfer in the solid components (diamond, copper, steel).
Turbulence Modeling: The k-omega Shear Stress Transport (SST) model was employed, utilizing a very fine mesh (inflation layer with 10 µm first element size) at the cooling interface to accurately model near-wall heat transfer interactions.
Heat Load Application: The 1847 W absorbed power (P_abs) was applied to the 1.85 mm thick diamond disk as a volumetric power density (q'''(r)) following a Gaussian distribution (w₀ = 20 mm).
Parametric Sensitivity Study: The design robustness was tested against variations in critical parameters:
- Inlet mass flow rate (5 L/min to 20 L/min).
- Beam frequency (136, 170, 204 GHz).
- Loss tangent (tanδ: 2.0 x 10^-5 to 5.0 x 10^-5).
Conceptual Design Change: The intermediate cuff geometry was modified by increasing thickness and introducing 2 mm diameter holes to increase fluid velocity and turbulence, enhancing heat removal effectiveness.
Structural Analysis (FEM): Thermal results were transferred to ANSYS Workbench 2021 R1 for structural validation. Temperature-dependent properties were used, and a multilinear isotropic hardening plasticity model was applied to the copper cuffs to account for plastic deformation near the brazing interface.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-specification MPCVD diamond required for fusion energy applications, ensuring the necessary thermal, structural, and RF performance for DEMO-scale systems.

Applicable Materials

To replicate or extend this high-power, broadband window research, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): Required for the lowest possible loss tangent (tanδ < 1.0 x 10^-5 in bare disk) and highest thermal conductivity, providing the maximum safety margin against the 250 °C limit.
High-Purity Polycrystalline Diamond (PCD): Suitable for large-area applications (up to 125 mm diameter) where the required 106 mm diameter exceeds typical SCD limits, provided the grain structure is optimized for minimal RF loss.

Customization Potential

The research highlights several critical manufacturing challenges, particularly concerning large dimensions, precise thickness, and robust metal-diamond interfaces. 6CCVD addresses these directly:

Research Requirement	6CCVD Capability	Specification Match
Large Diameter Disks	Custom PCD Plates	Wafers available up to 125 mm diameter, exceeding the 106 mm requirement.
Precise Resonant Thickness	Advanced Thickness Control	SCD/PCD thickness controlled from 0.1 µm up to 500 µm (and substrates up to 10 mm), ensuring the 1.85 mm thickness is met with high uniformity.
High-Integrity Brazing Interface	Custom Metalization Services	In-house deposition of critical metal layers (e.g., Ti/Pt/Au, W, Cu) required for robust, low-stress brazing to the copper cuffs.
Optimal Thermal Contact	Ultra-Precision Polishing	Polishing capabilities to achieve Ra < 1 nm (SCD) and Ra < 5 nm (PCD), minimizing interface resistance during brazing and maximizing heat transfer efficiency.
Turbulence Optimization	Custom Laser Cutting/Machining	Ability to laser cut and machine complex geometries into diamond plates to facilitate advanced cooling designs (e.g., integrating features to increase fluid turbulence, as proposed in the modified design).

Engineering Support

6CCVD’s in-house PhD team specializes in the thermomechanical and RF properties of diamond. We offer comprehensive engineering support to assist researchers and engineers in similar High-Power Electron Cyclotron (EC) Window projects:

Material Selection: Guidance on selecting the optimal diamond grade (SCD vs. PCD) based on required tanδ, power density, and operating frequency.
Thermal Management Consultation: Assistance in designing cooling geometries and predicting thermal performance based on specific power loads and flow conditions.
Stress Mitigation: Expertise in designing metalization layers and interface geometries to minimize residual stresses caused by the brazing process, ensuring long-term structural reliability.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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

Original Source

DOI: https://doi.org/10.3390/jne3040021

References

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]