2Universität Karlsruhe, Institut für Höchstfrequenztechnik und Elektronik,

Kaiserstr. 12, D-76128 Karlsruhe, Germany

Intense light emission from the output windows of high power gyrotrons is a spectacular phenomenon that has been reported by various groups that use CVD diamond windows for generating long pulse Electron Cyclotron (EC) waves. Immediate concerns were raised due to the light emission events about the principle validity of the diamond window concept, as local hot spots from non-diamond phase inclusions might induce locally critical thermal stresses or even a progressive material degradation by transforming neighbouring ultralow loss diamond phases into highly absorbing graphitic phases. An important milestone in the high power gyrotron development was the ‘Maquette’ tube, by which a team from several EURATOM Associations and from Thales Electron Devices could overcome at 140 GHz the ‘minute barrier’ in pulse lengths fulfilling EC resonance heating requirements [1]. An important aspect during the installation of this tube were studies of the output window where infrared and video inspection was combined with pre- and post-operation dielectric and microscopic studies. Additional evidence was gathered from CVD diamond disks placed as individual transmission components into the high power mm-wave beam at atmospheric pressure [2]. In contrast to these components exposed on both sides to air, the light emission was apparently only stationary for the special conditions of the gyrotron window. Because the evident difference is seen in the vacuum conditions at the inner surface of the output window, the major contribution to light emission was expected at this surface [3].

The present paper reports on studies with a specially designed transmission cell in which a pair of CVD diamond disks can be studied in the bare, as-received state as well as brazed disks. The cell can be evacuated to 10^-5 mbar so that the individual diamond phases could be exposed alternatively to air or to vacuum conditions. This cell was installed in the beam line of the reconditioned Maquette tube and exposed to short mm-wave pulses ( typically 100 ms ) of about 600-800 kW. Video inspection could be performed for the three diamond windows (including the gyrotron window) simultaneously, alternatively one transmission window could be studied with two video cameras under different angles. An optical spectrometer was installed to analyse the wavelength range from 350 to 1000 nm. Detailed studies were performed with three recently delivered bare CVD diamond disks for the EC wave system of the W7-X stellarator at IPP Greifswald (Germany). Stable light emission was observed only in transmission windows which had one face operated in vacuum. Spots with the highest intensity of light emission could be related with noticeable contamination areas clearly formed during operation. Turning the related disk, such that the sides originally exposed to vacuum and to air were interchanged, proved to result in a quenching of the light emission effect, in an extreme case related with some initial arcing around a contamination spot. This strongly supports the previously proposed model that the surface contamination by particles is the origin of the light emission. Microscopic analysis of the surfaces is discussed to narrow down the potential sources of the contamination.