Low power mm-wave transmission characteristics of a frequency tuneable double disk CVD-diamond window

The torus window, which was manufactured for injection of up to 1 MW mm-wave power at 4 selected frequencies between 105 - 140 GHz into the plasma of ASDEX Upgrade, works on the principle of a double disk arrangement of two large area CVD diamond disks. The thickness of the two disks was 1.8 mm as this fulfils the resonant disc condition and thus provides low reflection at the frequency range limits. At the two intermediate frequency operation points minimum reflection is achieved by adjusting the Fabry Perot resonance condition by precise adjustment of the disk interspace with in the range of 4.1 to 6.0 mm. With measurements of the power reflection ratio performed with a Gaussian telescope quasi-optical scalar network analyser set-up operating between 130-160 GHz, it could be shown at the intermediate resonant frequency of 125 GHz, that a bandwidth of more than 0.5 GHz (at - 20 dB reflection level) is achieved with the double disk window and that thus typical frequency drifts during gyrotron operation of some 100 MHz can be tolerated without additional intervention at the window.