Weld techniques for reservoir cathodes

We report improvements in weld technology in support of our efforts to produce a commercial, long-life, miniature reservoir cathode. We have found that the laser weld between sintered tungsten pellet and reservoir sleeve can be made reliably hermetic if we apply a subsequent coating of molybdenum-ruthenium brazing alloy to the sides of the pellet and especially to the weld region. The emissive material is then inserted in the open end of the reservoir sleeve, which is then closed by a secondary weld. This secondary weld is reliably hermetic without the Mo-Ru coating, because it does not involve the porous tungsten matrix. Moreover, we have discovered an inspection procedure that allows us to check weld hermeticity at each assembly step. Thermal cycling tests indicate good weld integrity. Emission life testing is proceeding.