High-power windows at microwave frequencies

During the past ten years the Microwave Laboratory at Stanford University has been engaged in development work on high-power klystrons. One of these is used in the linear accelerator and is operated at 20 MW (peak) and a few kilowatts average. The others, now produced commercially, operate between 1 and 3 MW (peak) in the X-, S- and L-bands. The characteristics of the output windows of these tubes are briefly reviewed, and the information obtained on life and the mechanism of failure of the windows used for the linear-accelerator klystron is considered in detail. From the operation of approximately 120 windows used with the accelerator tubes, an average window life of nearly 2000 hours has been observed. Most of the windows used with the accelerator consist of a ceramic disc in a circular waveguide. Most failures appear to start as a series of small holes in the ceramic, one of which eventually punctures the disc. The general direction of the puncture is normal to the direction of the E-field in the guide. The reasons for these failures are not yet clearly understood. Experimental and theoretical investigations are continuing on the effects of the impurities of the material, particle bombardment, X-ray bombardment, and geometry of the window. We expect, when a better understanding of the mechanism of failure has been achieved, that the average window life can be increased to a minimum of 5 000 hours.