ITER ECRF window development - CVD-diamond window. Final report

In accordance with the goals of this ITER Task experimental and theoretical investigations on 1 MW, 170 GHz, CVD-diamond gyrotron and torus windows for continuous wave (CW) operation have been continued. Two gyrotron window units were tested in collaboration with the Russian Hometeam. Unfortunately, both window disks cracked after operation at mm-wave power levels around 1 MW and pulse lengths up to 1.5 s. The reasons are only known for the second unit where a sag of the window disk (120-140 μm), introduced by its brazing, led to a static stress of approximately 0.7-0.8 MPa. A third CVD-diamond disk unit does not show any sag. The high power tests are scheduled for January 2002. A CVD-diamond window disk was irradiated with a neutron fluence of almost 1021 n/m2 and then brazed to two Inconel waveguide cuffs. The window unit has been mounted into a test housing in an evacuated corrugated HE11 waveguide (I.D. = 63.5 mm) and successfully tested in collaboration with the Japanese Hometeam at mm-wave power levels of 0.58 MW (0.2 MW) and pulse length 15 s (132 s). The temperature rises of the disk at a diameter of 65 mm and at the disk water cooling rim were 27 C and 2 C, respectively. Simulation calculations show agreement with the measured temperature rise curves for tanδ = 4.8 . 10-5 and k = 1200 W/mK which reveals that the reduction of the thermal conductivity by a factor of 2 after neutron irradiation was partially removed during the brazing at temperatures above 600 C. A 118 GHz CVD-diamond torus window unit was successfully in collaboration with CES Cadarache at a mm-wave power level of 0.3 MW and pulse durations up to 111 s. (orig.)