Search

Nuclear fusion reactors require electron cyclotron heating and current drive (EC H&CD) systems for plasma heating and stabilization. Chemical vapor deposition (CVD) polycrystalline diamond windows on both the torus and gyrotron sides of the reactors act as confinement and/or vacuum boundaries allowing the transmission of high-power microwave beams. For example, the beam power scenarios of 1.5 MW and 2 MW are the current targets considered respectively in Wendelstein 7-X and European DEMO fusion machines. In this work, with reference to both reactors, the numerical analyses required to verify the thermal and structural performance of the windows are discussed. Experimental measurements of loss tangent in diamond provided inputs for the numerical analyses. Sensitivity studies of the windows with respect to loss tangent and other parameters were also carried out to check the temperature reserve margins of the design.

In nuclear fusion, Microwave Plasma Assisted (MPA) Chemical Vapour Deposition (CVD) polycrystalline diamond is the only material allowing for transmission of high power microwave beams (1-2 MW) in long-pulse gyrotron operations. The reason lies in the combination of extraordinary thermal, mechanical and optical properties of diamond, which is used in the shape of disks having thickness of 1 to 2 mm for windows. Being diamond a brittle material, failure to fracture is the main failure mode. Accordingly, an appropriate mechanical characterization is required as diamond plays a major safety role in fusion machines. Due to limited body of work in literature, fracture toughness measurements have to be first carried out for this material and then a design criterion for structural integrity assessment has to be applied. In this work, the preliminary activities aiming to define the optimum experimental measurement method of fracture toughness for thin diamond samples are shown and discussed. An outlook to the next steps is also given.

Diamond disks manufactured by chemical vapor deposition (CVD) are essential elements of windows of the Electron Cyclotron Heating and Current Drive systems of fusion reactors like ITER. Diamond is selected as window material because of its high mechanical stability, high thermal conductivity and low dielectric loss. Only diamond disks with a low loss tangent guarantee a high transmission, i.e. a low absorption of microwave power in the disk. The latter results in moderate window temperatures and therefore in low thermal stresses. Hence, the measurement of the loss tangent is essential for the qualification of diamond disks for high-power windows. Dedicated measurement facilities (Fabry-Perot resonators) at KIT allow a high resolution measurement of the loss tangent at the disk centre (spherical set-up) as well as a mapping over the disk area to estimate its homogeneity (hemispherical set-up). Within a contract between F4E and KIT more than 60 diamond disks (D=70mm, t=1.11mm) produced similarly by MPA-CVD need to be qualified for their application in the ITER EC-system. The development of a dedicated test plan as well as initial results for the first disks delivered to KIT will be presented.