Dielectric loss measurements of CVD diamond disks for ITER windows

Diamond disks manufactured by chemical vapor deposition (CVD) are
essential elements of windows of the Electron Cyclotron Heating and
Current Drive systems of fusion reactors like ITER. Diamond is selected
as window material because of its high mechanical stability, high
thermal conductivity and low dielectric loss. Only diamond disks with
a low loss tangent guarantee a high transmission, i.e. a low absorption
of microwave power in the disk. The latter results in moderate window
temperatures and therefore in low thermal stresses. Hence, the
measurement of the loss tangent is essential for the qualification of diamond
disks for high-power windows. Dedicated measurement facilities
(Fabry-Perot resonators) at KIT allow a high resolution measurement
of the loss tangent at the disk centre (spherical set-up) as well as a
mapping over the disk area to estimate its homogeneity (hemispherical
set-up). Within a contract between F4E and KIT more than 60 diamond
disks (D=70mm, t=1.11mm) produced similarly by MPA-CVD
need to be qualified for their application in the ITER EC-system. The
development of a dedicated test plan as well as initial results for the
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first disks delivered to KIT will be presented.