M. Thumm*+, G. Dammertz*, I. Danilov**, R. Heidinger**, A. Meier**
Forschungszentrum Karlsruhe, Association EURATOM-FZK
*Institut fuer Hochleistungsimpuls- und Mikrowellentechnik
**Institut fuer Materialforschung I
Postfach 3640, 76021 Karlsruhe
+Universitaet Karlsruhe. Institut fuer Hoechstfrequenztechnik und Elektronik (IHE),
CVD diamond windows are world-wide the
first ("only") choice to access the long pulse regime for Megawatt gyrotrons and
ECRH torus windows. New record values in transmitted energy are being achieved
in the test-runs of the 'Prototype'-tube for
W7-X (0.85 MW for 3 minutes).
Large area CVD diamond disks are now well-available "ultralow loss dielectrics". The presently achieved levels of mm-wave losses are determined by 'non-diamond like phases' in microcracks and open grain boundaries and by surface effects from various carbon compounds or contaminants.
Substantial progress has been achieved towards introducing a non-degrading brazing technology. Yet a comprehensive follow-up of the window integration steps is still mandatory to keep evidence for potential increases above the ultralow bulk absorption levels and to identify efficient strategies for remedies.
Light emission phenomena, as observed in high power operation, have without any doubt their origin at the vacuum face of the CVD-diamond window. They are created by (amorphous or nanostructured?) carbon-based surface contaminants. They are concomitant phenomena and not a pre-failure alarm of high power gyrotron operation.