Pulsed Induction Joining of Sapphire to Metal for Gyrotron Windows

41291 November 19, 1996 FM Technologies, Inc. The window is currently the single most serious mode of failure of high power gyrotrons, a device for heating magnetic fusion plasmas. This project will develop a new technique for joining ceramic materials, used for these windows, to the metals that hold the windows in place. A pulsed induction coil system will be used to set up an induced current that causes the temperature of the material to rise rapidly to the joining temperature. Because currents are induced only within the portion of the specimen that is enclosed by the coil, rapid heating of just the interlayer can be achieved. The pulsed method uses a peak power that is many times greater than conventional induction heating, while greatly reducing the total heating time. In previous work, a first generation pulsed induction system was built and tested by heating a copper ring 1000oC in less than one second. In Phase I, test specimens consisting of disks of sapphire and tubes of stainless steel will be joined. The microstructure of the test specimens will be examined using Scanning Electron Microscopy and their mechanical integrity will be evaluated. In Phase II, test specimens representing "racetrack" shaped windows will be joined and evaluated. The windows will be large enough for testing with high power gyrotrons currently under development. Commercial Applications and Other Benefits as described by the awardee: High power gyrotrons are a critical component of Electron Cyclotron Resonance Heating (ECRH), a preferred and promising heating method for magnetic fusion plasmas. This project, if successful, will provide a method for attachment of low-loss windows in gyrotron oscillators with minimal residual stress, which will greatly increase window design flexibility.