Projection ablation lithography cathodes for a high current relativistic magnetron

Summary form only given. Experiments have been performed on a relativistic magnetron using innovative metal cathodes (Al) rather than the cotton fiber cathode previously used. The new cathode is fabricated by projecting a pattern on the cathode and ablating the metal by a KrF laser; defined here as projection ablation lithography (PAL). The PAL cathode utilizes microtexturing of a solid metal cathode substrate to provide electric field enhancement. The potential advantages of this cathode are that, unlike microtips, which can overheat, explode and create plasma, the PAL cathode emission regions are heat-sinked to the base cathode material. Furthermore, the PAL cathode consists of pure metal, so after an initial bakeout or plasma cleaning (or a few conditioning shots), there is low gas emission (unlike fabric cathodes). The relativistic, 6-vane, (Titan) magnetron is powered by the MELBA-C accelerator with parameters: 0.3 MV, 1-10 kA and 0.5 /spl mu/s pulselength. The microwave startup time for PAL cathode has decreased to an average of 118 ns compared to 233 ns for the cotton cathode. Microwave pulselength average for the PAL cathode has increased to 217 ns, from 144 ns experimentally observed for the cotton cathode. With these improvements in microwave startup-time and pulselength, the average microwave power has remained approximately the same as conventional cloth cathodes (100-500 MW total, peak microwave power extracted). A new, triple-azimuthal-emission region has been tested as means of pre-bunching the electrons into the 3 spokes desired for pi-mode operation in a 6-cavity magnetron. This Tri-PAL cathode has demonstrated the lowest average microwave starting-times (109 ns). Cathode plasma closure for some shots has been dramatically decreased, from 3 /spl mu/s/cm to less than 0.5 /spl mu/s/cm; certain shots have even shown zero plasma closure.