Testing of CVD diamond disks for the torus windows of the ITER EC H&CD system

Testing of CVD diamond disks for the torus windows of the ITER EC H&CD system

Sabine Schreck1, Gaetano Aiello1, Pablo Estebanez2, Jürgen Gafert1, Andreas Meier1, Theo Scherer1, Dirk Strauss1

1Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1,
76344 Eggenstein-Leopoldshafen, Germany;
2Fusion for Energy (F4E), Josep Pla 2, Torres Diagonal Litoral B3, 08019 Barcelona, Spain
*Corresponding author: sabine.schreck@kit.edu
The ITER electron cyclotron (EC) system with its four upper launchers (UL) and one equatorial launcher (EL) will be used for central heating and current drive as well as for the control of magneto-hydrodynamic instabilities of the plasma. The diamond windows are part of the EC transmission line system, allow the transmission of high mm-wave power (at 170 GHz) from the gyrotrons to the launchers and serve as vacuum and tritium barriers to the ITER vacuum vessel. A polycrystalline diamond disk produced by MPA-CVD (microwave plasma assisted chemical vapour deposition) process is the functional element of such a torus window and acts as part of the first confinement barrier (SIC-1). Diamond is selected as window material because of its high mechanical stability, high thermal conductivity and very low dielectric loss. ... mehrThe latter guarantees a minimal absorption of the mm-wave power and results in moderate temperatures and consequent low thermal stresses.
The determination of the losses and the verification of the required low value is therefore essential for the qualification of diamond disks for high-power windows. Dedicated measuring set-ups (Fabry-Perot resonators) at Karlsruhe Institute of Technology (KIT) enable high resolution measurements of the loss tangent at the disk centre (spherical set-up) as well as a mapping over the disk area to estimate its homogeneity (hemispherical set-up). The disk characterization is then completed by an inspection with a digital microscope.
Prior to their integration to the window assemblies, more than 60 diamond disks (D=70mm, t=1.11mm) need to be qualified with respect to the requirements. The work is performed within a contract between F4E and KIT. A dedicated test plan has been provided and the first batches of disks produced by Diamond Material GmbH have been delivered to KIT. First results of loss tangent measurements and microscopy investigations will be presented.
The work leading to this publication has been partially funded by Fusion for Energy under the contract No. F4E-OFC-842-SC03. The views and opinions expressed herein reflect only the author’s views. Fusion for Energy and the ITER Organisation are not liable for any use that may be made of the information contained therein.