Experimental studies of local and global emission uniformity for a magnetron injection gun

We report measurements of the azimuthal emission nonuniformity of a gyrotron´s thermionic cathode as determined from two experiments; one which determines the total emission, and one which scans the emitted beam using a rotating probe. In the total emission experiment, which measured the total collector current over a range of voltages, the cathode´s total work function spread was found to be 0.033 eV at T=1000°C, centered around 1.88 eV. In the subsequent experiment, which used a mechanically rotatable current probe, global, and local effects were investigated. Voltage was varied to examine emission nonuniformities in different regions of cathode operation. While emission became almost completely uniform in the low-voltage space-charge limited regime, emission varied as much as 50% at high voltages. In another part of the current probe experiment, current--voltage curves were measured at azimuthal locations in 30° increments for several cathode temperatures. From this extensive set of data the work function distribution parameters were identified over small sections of the cathode for the entire cathode surface.