MATERIAL STUDIES ON DIAMOND WINDOWS FOR HIGH POWER EC WAVE TRANSMISSION
R. Heidinger1a), G. Dammertz1b), M. Thumm1b),2)
1)
Forschungszentrum Karlsruhe, Institut für Materialforschung I(a) and Institut für Hochleistungsimpuls- und Mikrowellentechnik(b), D-76021 Karlsruhe, Germany2)
also, Universität Karlsruhe, Institut für Höchstfrequenztechnik und Elektronik,
In next step thermonuclear fusion experiments, Electron Cyclotron Wave (ECW) systems are assigned for the essential part of plasma heating, non-inductive current drive and MHD stability control. This strategy relies on millimetre (mm)-wave sources that generate Megawatt output power in (quasi-) continuous wave (CW) operation. The gyrotron development has world-wide successfully responded to this challenge. One of the key components of the ECW systems are output and transmission windows. Dedicated material development tasks identified processes to produce large-area diamond disks for which the required outstanding thermo-physical and dielectric properties are given at and above room temperature. Consequently the water-cooled diamond window is presently the preferred option as it eliminates the restrictions in pulse length of ballistically operated BN windows and the complexity of cryogenically-cooled Sapphire windows.
The dielectric property measurements performed with low power open resonator studies validate the required specifications for mm-wave absorption of large area CVD diamond disks. In brazed components, additional surface losses are put to evidence. Their contribution to enhanced dielectric absorption differ characteristically between existing brazing techniques. In particular, it is shown that a brazing technology is available for which this adverse effect does not appear. Treatments to reduce surface losses for alternatively brazed windows, such as plasma ashing or chemical oxidation of the surfaces are discussed.
Under high power operation, intense light emission from diamond output windows of high power gyrotrons has been observed by various groups. Experiments are reported which were performed with bare and brazed diamond disks arranged in a transmission cell. This arrangement allows in-beam video and infrared inspections of single transmission windows under normal atmosphere as well as pairs of windows with one side each under vacuum. Evidence for stationary light emission phenomena from irregularly distributed spot-like centres are only observed with the evacuated transmission cell. Identical emission patterns registered from different angles relative to the beam rule out light emission from the gyrotron tube as the origin of the phenomenon. Strong suppression and even elimination of the light emission was achieved with discs which were treated by chemical or plasma ashing processes selected to remove organic sources for carbon contanimation (such as cellulose fibres).
As a consequence, the principle validity of the diamond window concept is asserted by clearly excluding local hot spots in the bulk material as the origin of the light emission from the high power diamond windows. Furthermore, there is strong evidence that light emission arises in vacuum by excitation of carbon structures which are formed by organic carbon compounds degenerated under the high power mm-wave beam. Careful sealing and handling of the diamond windows before their integration into the tube is proposed to avoid this surface contamination effect.
als Vortrag:
22nd SOFT, Helsinki, Finnland, 9-13 Sept. 2002