a microcavity forming device is provided for making microcavities in a tungsten wire . the microcavity forming device includes a source of particles ; a housing for receiving a heated tungsten wire ; and a plurality of jet nozzles disposed in the housing for spraying the particles toward the heated tungsten wire . the particles are 0 . 35 – 0 . 75 micron in diameter . the heated tungsten wire is received in the housing and the jet nozzles spray the particles toward the tungsten wire to form the microcavities in the tungsten wire .