Status report on CVD-diamond window development for high power ECRH

M. Thumm et al., Status report on CVD-diamond window development for high power ECRH, FUSION ENG, 53, 2001, pp. 517-524
Citations number
Categorie Soggetti
Nuclear Emgineering
Journal title
ISSN journal
0920-3796 → ACNP
Year of publication
517 - 524
SICI code
A hemispherical Fabry-Perot resonator was investigated for the determinatio n of the mm-wave dielectric properties of CVD-diamond disks at elevated tem peratures (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 me asurements are strongly enhanced by convection in ambient atmosphere. Measu rements under vacuum conditions have to account for systematic deviations i n 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 per formed on the specimen set irradiated to the final fast neutron fluence of 10(22) n/m(2) (E > 0.1 MeV). Whereas after the previous irradiations to 10( 20) n/m(2) and 10(21) n/m(2), no apparent indications for the degradation o f dielectric properties were observed, substantial radiation-induced change s were identified at the fluence level of 10(22) n/m(2). Independent of the diamond grade, the permittivity was shifted from 5.67 to 5.73 and dielectr ic loss levels were increased by a factor of 2-3 at mm waves (90 GHz, 145 G Hz). 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 10(21) n/m(2) (E > 0.1 MeV), the pre-irradiation characterisation of the window material was performed with respect to the dielectric parameters at millimeter wavelengths. The 17 0 GHz CVD-diamond window unit of the Forschungszentrum Karlsruhe (DeBeers, disk diameter 119 mm, thickness 2.22 mm, aperture 100 mm) has been welded t o the new GYCOM-M 170 GHz ITER gyrotron with single-stage depressed collect or. Up to now the experimental parameters are very promising: 0.7 MW, 0.5 s , 45% efficiency, without any problems with the window. (C) 2001 Elsevier S cience B.V. All rights reserved.