A study of the inter-diffusion of tungsten and osmium-ruthenium in M-cathodes and the effects on performance and life

M type cathodes have become the most widely used electron sources for microwave vacuum electron devices such as klystrons and travelling wave tubes. These cathodes are barium calcium aluminate impregnated porous tungsten over coated with 5000 Å of osmium ruthenium. The advantages of this type of cathode are the ability to achieve over two times higher current densities at the same temperature as an uncoated, impregnated porous tungsten cathode or achieving the same current at between 75 and 100°C lower temperatures. The latter can result in eight times longer life expectancy. A concern among users is the permanency of the osmium ruthenium coating. This study reveals that from the very beginning of life that tungsten does diffuse to the emitting surface. However instead of a deterioration of performance with time as a result of this activity an improvement is actually achieved. Data is presented showing the effects of the diffusion on the tungsten surface concentration. Auger depth profiling is used to demonstrate the phenomenon. Data is presented for the initial status after cathode activation, after 33,517 hours of aging at 1000°C and after 53,142 aging at 1000°C. Data will also be presented showing surface tungsten concentration versus aging time at different temperatures.