The transmission of high-power microwaves via dielectric diamond windows: Design, qualification and first steps towards a broadband diamond window in the range of 30GHz to several THz for actual and future fusion devices
Theo Andreas Scherer and Dirk Strauss
Karlsruhe Institute of Technology, Assoc. KIT-EURATOM, CS
The development of artificial diamond disks fabricated by special RF CVD processes lead to a new generation of dielectric high power millimetre wave windows with extremely low absorption and scattering losses for high power transmission. The quality of diamond as a window material is further given by its well known excellent mechanical properties and extremely high thermal conductivity. The growth process of the diamond disks is based on chemical vapour deposition (CVD) with micro/nano diamond nuclei. The transmission losses of the disk are caused by graphite formation mainly at the surface of the disk but also on the grain boundaries. RAYLEIGH scattering limits the value of loss tangent in bulk. A high quality measurement of the disk surface and bulk properties using spherical and hemispherical resonators is presented for frequencies from 90 up to 170 GHz.
The state of the art windows used in high power electron cyclotron heating and current drive (ECRH&CD) for large fusion devices such as ITER consist of a disk perpendicular to the millimetre wave beam propagation. As reflection have to be kept on a minimal level, the window thickness limits the allowed frequencies to a limited set defined by multiples of l/2 in the dielectric matter.