Development of a Forced Gas-Gooled Brewster window for High-Power, CW Millimeter Waves

Development of a vacuum barrier window for high power millimeter waves is one of the key technical issues to realize CW (Continuous Waves) ECH system. For this purpose, we have been developing a forced gas-cooled Brewster window with low loss silicon nitride composite (SN-287 Kyocera) for megawatt and CW power transmission. The thermal analysis of the window with forced gas-cooling on the basis of the finite element method shows a 0.5 MWCW, HE1 ' mode transmission is possible to the 100 x 300 mm2 silicon nitride disk. Since this material has very low thermal expansion coefficient, we first designed and assembled a prototype Brewster window using normal silicon nitride (SN-220 Kyocera) to check the structure of brazing and welding. To transmit millimeter waves the comrgated waveguide sections are prepared at the both sides of the disk. At the atmospheric side of the waveguide nozzles for gas-cooling are drilled on the wall. The number of the nozzle holes is optimized to be nine with the diameter of lmm, and air is blown on the disk unidirectionally. This structure is determined from the experimental results using a simulated heat source. On the basis of the experiments, we have assembled a real window and prepared to test it by high power CW power transmission. Keywords: brewster window, tunable gyrotron, electron cyclotron heating, vacuum window, silicon nitrid