AbstractA hemispherical Fabry–Perot resonator was investigated for the determination of the mm-wave dielectric properties of CVD-diamond disks at elevated temperatures (300–750 K) with the plane mirror locally heated and the specimen fixed to it. It was found that the uncertainties in the dielectric loss measurements are strongly enhanced by convection in ambient atmosphere. Measurements under vacuum conditions have to account for systematic deviations in the apparent permittivity data arising from unbalanced thermal profiles. Measurements on large disks (4 inch diameter) for gyrotrons show only small steady increases for temperatures up to 750 K. Post-irradiation studies on small disks of advanced CVD-diamond grades (‘scale-up’ and ‘window’ grade) have been finished with the dielectric and thermophysical measurements performed on the specimen set irradiated to the final fast neutron fluence of 1022 n/m2 (E>0.1 MeV). Whereas after the previous irradiations to 1020 n/m2 and 1021 n/m2, no apparent indications for the degradation of dielectric properties were observed, substantial radiation-induced changes were identified at the fluence level of 1022 n/m2. Independent of the diamond grade, the permittivity was shifted from 5.67 to 5.73 and dielectric loss levels were increased by a factor of 2–3 at mm waves (90 GHz, 145 GHz). A very pronounced effect was found for the thermal conductivity, where after the final irradiation values of 200 W/mK were identified which have to be compared to the initially very high levels of 1700–1800 W/mK. For the test of a 170 GHz torus window unit for ITER which uses a CVD-diamond disk irradiated with fast neutrons to a fluence of 1021 n/m2 (E>0.1 MeV), the pre-irradiation characterisation of the window material was performed with respect to the dielectric parameters at millimeter wavelengths. The 170 GHz CVD-diamond window unit of the Forschungszentrum Karlsruhe (DeBeers, disk diameter 119 mm, thickness 2.22 mm, aperture 100 mm) has been welded to the new GYCOM-M 170 GHz ITER gyrotron with single-stage depressed collector. Up to now the experimental parameters are very promising: 0.7 MW, 0.5 s, 45% efficiency, without any problems with the window.