Abstracts

MATERIAL ISSUES OF WINDOWS FOR ELECTRON CYCLOTRON WAVE SYSTEMS

R. Heidinger^1a), M. Rohde^1a), G. Dammertz^1b), M. Thumm^1b),2)

1) Forschungszentrum Karlsruhe, Association EURATOM-FZK Institut für Materialforschung I^(a) and Institut für Hochleistungsimpuls- und Mikrowellentechnik^(b), D-76021 Karlsruhe, Germany

^{2) also, Universität Karlsruhe, Institut für Höchstfrequenztechnik und Elektronik,

D-76128 Karlsruhe, Germany.

The performance profile of electron cyclotron (EC) wave systems for next step fusion devices is governed by the transmission characteristics of electromagnetic waves in the mm-wave range through specially designed window structures which fulfil vacuum operation and tritium retention requirements. Transmission over a wide frequency range and low power absorption are two performance characteristics of an ideal window. Window design aspects are discussed for two distinct cases which are firstly windows serving for high power transmission typical for EC resonance heating and non-inductive current drive and for EC wave diagnostics (transport studies and collective Thomson scattering) and secondly windows offering especially broadband pass characteristics typical for low power diagnostics, like EC emission spectroscopy.
In low power systems, broadband windows are typically set up using monocrystalline ('Quartz') and amorphous ('Silica glass') SiO₂ grades. Material performance analysis requires a data base of mechanical strength as well as of dielectric properties including studies dedicated to the onset of radiation-induced material degradation. For Megawatt power transmission, severe demands are imposed on the mechanical, thermophysical and mm-wave properties of dielectric materials to overcome window failure by thermal crack formation. 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 (simple edge cooling by water). Material studies are summarised for CVD diamond and SiO₂ grades (including the neutron irradiations up to 10²² n/m²) based on post-irradiation examination covering mechanical bending strength, thermal conductivity and dielectric measurements in the millimeter wave range.
Special material issues of high power windows are addressed such as the occurrence of additional surface losses in brazed components. Their contribution to enhanced dielectric absorption differ characteristically between existing brazing techniques. Further intense light emission from diamond output windows of high power gyrotrons is discussed. Specially designed in-beam experiments with various diamond disks in a transmission cell prove that stationary light emission phenomena require vacuum conditions. Thus local hot spots in the bulk material can be ruled out as the origin of this effect.}