Cooling concepts for the CVD diamond brewster-angle window

At a Glance

Metadata	Details
Publication Date	2017-08-01
Authors	G. Aiello, T. Scherer, D. Strauß, Konstantinos A. Avramidis, John Jelonnek
Institutions	Karlsruhe Institute of Technology, École Polytechnique Fédérale de Lausanne
Citations	8
Analysis	Full AI Review Included

Technical Documentation & Analysis: CVD Diamond Brewster-Angle Windows for Fusion Applications

Source Paper: Cooling concepts for the CVD diamond Brewster-angle window (Aiello et al.)

Executive Summary

The necessity of high-ppower, frequency-tunable gyrotrons for plasma stability in nuclear fusion devices (DEMO) mandates highly robust output windows. This research confirms the viability of MPCVD diamond as the superior material, provided advanced cooling strategies are integrated.

Core Achievement: FEM thermal and structural analyses demonstrate that large-area, high-purity CVD diamond windows (140 mm major axis, 1.7 mm thick) can safely handle 2 MW beam power at 240 GHz for long-pulse operation.
Thermal Management: Optimized, geometry-conforming cooling channels (elliptical) are mandatory. Maximum temperatures were kept below 112 °C—well under the material safety limit of 250-300 °C.
Material Specification: Success relies on diamond exhibiting an extremely low loss tangent (tan δ) of 3.5 x 10^-5 to minimize heat absorption (P_absorbed max: 1545 W).
Structural Integrity: Maximum principal stresses remained below 130 MPa, maintaining a substantial safety margin relative to the diamond’s permissible stress limit (150 MPa) and ultimate strength (450-500 MPa).
6CCVD Advantage: Replication and advancement of this research require large-format, optical-grade CVD diamond and precision metalization services, which are core 6CCVD capabilities.

Technical Specifications

The following hard data points were extracted from the analysis, defining the operational environment and material requirements for the high-power Brewster window.

Parameter	Value	Unit	Context
Beam Power (Max Tested)	2	MW	Worst-case operational scenario for DEMO gyrotrons
Operating Frequency (Max)	240	GHz	Frequency step-tunable performance
Diamond Disc Major Axis	140	mm	Required elliptical window dimension
Diamond Disc Thickness	1.7	mm	Critical dimension for RF transmission
Loss Tangent (tan δ)	3.5 x 10^-5	N/A	Measured material property for low-loss transmission
Absorbed Power (Max)	1545	W	Thermal load on disc (2 MW @ 240 GHz scenario)
Max Temperature (Successful Cooling)	112	°C	Result for worst-case scenario (Outer elliptical channels)
Material Safety Temperature Limit	250 - 300	°C	Maximum allowable temperature for operation
Max Principal Stress (Result)	130	MPa	Measured at upper tip of the disc (2 MW @ 240 GHz)
Permissible Stress Limit (Design)	150	MPa	Required safety margin for long pulse operation
Coolant Mass Flow Rate	20	l/min	Water inlet flow rate used in FEM thermal analysis
Max Heat Exchange Coefficient	8946	W m^-2 K^-1	Calculated for inner elliptical channels (high efficiency)

Key Methodologies

The research focused on simulating thermal and structural performance across various operating scenarios and cooling geometries using Finite Element Methods (FEM).

Window Geometry Definition: Modeled a standard Brewster-angle geometry featuring a CVD diamond disc brazed to two copper waveguides (WGs) at a 67.2° angle.
- Disc dimensions: 140 mm major axis, 75 mm minor axis, 1.7 mm thickness.
Cooling Layout Comparison: Three indirect cooling concepts were analyzed to prevent coolant contact with the gyrotron interior:
- Cylindrical Channels (Simplest, proved infeasible).
- Outer Elliptical Channels (Conforming to disc geometry).
- Inner Elliptical Channels (Inside the WGs, required increasing WG thickness from 1 mm to 5 mm).
Beam Scenarios Applied (HE₁₁ Mode): FEM analysis used three specific scenarios for thermal loading:
- 2 MW @ 170 GHz (P_absorbed = 1094 W)
- 1.5 MW @ 240 GHz (P_absorbed = 1159 W)
- 2 MW @ 240 GHz (P_absorbed = 1545 W, defining the worst thermal load)
Heat Load Application: The absorbed power was modeled using the Bessel function of order zero, which describes the power pattern of the HE₁₁ beam inside the waveguide.
Coupled Analysis: Thermal analysis results (temperature distributions) were used as input for subsequent structural analyses to calculate corresponding thermal stress distributions in the diamond disc.

6CCVD Solutions & Capabilities

6CCVD provides the specialized, large-area CVD diamond materials and precision finishing services required to successfully fabricate, replicate, and advance the high-power Brewster window concept detailed in this research.

Applicable Materials

To meet the stringent thermal and optical requirements of high-power RF transmission windows, Optical Grade Single Crystal Diamond (SCD) or high-purity Polycrystalline Diamond (PCD) are required.

6CCVD Material Grade	Technical Requirement Fulfillment	Feature & Benefit
Optical Grade SCD	Ultra-low loss tangent (tan δ < 5 x 10^-5)	Minimizes mm-wave absorption, reducing thermal load and maximizing power throughput (crucial for 2 MW scenarios).
High-Purity MPCVD PCD	Large area, mechanical stability	Wafers up to 125 mm available, allowing production of the required 140 mm elliptical plates, providing superior intrinsic strength compared to other RF window materials.
Polishing Service	Minimal surface scattering	SCD can be polished to Ra < 1 nm, critical for minimizing scattering and maximizing transmission efficiency in optical/RF applications.

Customization Potential & Manufacturing Support

Replicating the complex geometry and multi-material integration described in the paper leverages 6CCVD’s unique production strengths.

Custom Dimensions: The required 140 mm elliptical major axis exceeds standard square wafer sizes. 6CCVD offers custom laser cutting services to precisely machine large PCD plates (up to 125 mm source material) into complex elliptical or Brewster-angle geometries required for optimal integration.
Metalization for Brazing: The successful integration relies on brazing diamond to copper WGs. 6CCVD offers in-house custom metalization capabilities, providing optimized adhesion layers (e.g., Ti/Pt/Au or Ti/W/Cu) necessary for high-integrity, stress-resistant joints capable of withstanding the thermal cycling of MW-class gyrotrons.
Thickness Control: The 1.7 mm thickness is critical for tuning reflective properties. 6CCVD ensures high precision control over material thickness (0.1 µm - 500 µm) through precise growth and advanced post-growth grinding.
Engineering Support: 6CCVD’s in-house PhD team can assist with material selection, optimizing SCD vs. PCD based on power level and frequency, and providing crucial inputs (thermal conductivity, tan δ, Young’s Modulus) required for similar Heating and Current Drive (HCD) or high-power RF transmission projects.

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

View Original Abstract

The chemical vapor deposition (CVD) diamond Brewster-angle window is a very promising broadband radio-frequency (RF) output window solution for frequency step-tunable high power gyrotrons foreseen in nuclear fusion devices like DEMO. Since gyrotrons operate in the megawatt-class power range, active cooling of the output window during operation is mandatory for long pulse operation. In this paper, different indirect cooling layouts were investigated and compared by finite element method (FEM) thermal and structural analyses. Scenarios with different power and frequency beam were taken into account in the analyses.

Tech Support

Original Source

DOI: https://doi.org/10.1109/irmmw-thz.2017.8067059

References

2010 - State of the art of high power gyro-devices and free electron masers
2017 - H&CD designs and their impact of the configurations on the performances for the EU DEMO fusion power plant reactor