A NEW METHOD FOR MULTIPLE BRAZING OF CERAMIC WINDOWS TO A SINGLE FLANGE

The lower hybrid experiment in FTU has been designed to inject up to 8 MW,8 GHz pulsed power into the plasma. The main technical problems to overcome in the design of the launching structures for injecting lower hybrid power into the plasma is how to reduce the probability of breakdowns inside the waveguides. The phylosophy we have adopted is to pressurize the waveguides inserting ceramic windows close to the mouth of the grill. In FTU this separation has been made by an array of 12 × 4 waveguides brazing 48 ceramic windows onto a single flange which achieves the vacuum-atmosphere transition. Prototypes of this flange have already been realized by modifying the method used for brazing single windows. The results were not completely satisfactory: the vacuum tightness was obtained but several microcracks were observed as a consequence of high local stresses due to the formation of intermetallic compounds. This paper describes the new method that has been successfully developed by changing the brazing alloy and inserting a ductile cushion between the ceramic and the metal in order to reduce the loads acting on the ceramic.