The double-disk diamond window as backup broadband window solution for the DEMO Electron Cyclotron System

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
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maximum temperature in the disks can be reduced from 238 to 186 °C, leading, therefore, to lower
thermal gradients and stresses in the window.