Macroscopic emission non-uniformity in thermionic cathodes due to profilimetry variation

Summary form only given. Micron-scale ridges, introduced by the surface machining of thermionic dispenser cathodes, may lead to significant angular variations in the macroscopic current density on ring-shaped cathodes commonly used in gyrotrons or flat cathodes used in traveling wave tubes. Most gyrotrons utilize thermionic dispenser cathodes operating in the temperature limited regime. Non-uniform emission in gyrotrons leads to mode competition, mode hopping, efficiency degradation, and severe local heating in the collector region. Insofar as emission non-uniformity can be related to small-scale differences in surface geometry, a theoretical analysis of the angular dependence of field enhancement variation induced by machining processes is topical to electron source technologies that are not operated in the space charge limited regime. Recent studies of ring-shaped gyrotron cathodes show large scale angular variation in emission current from quadrant to quadrant, with adjacent quadrants differing by as much as a factor of two. Thermionic dispenser cathodes are generally machined to make their emission area smooth, but the machining process introduces parallel micron-scale ridges along the surface. Conical cathodes are, moreover, rotated about an axis as they are machined: if asymmetries are introduced by a slight wobble about the axis of symmetry plus a small ellipsoidal characteristic of the cathode cross-section, then the first three terms of a Fourier transform of the emission as a function of angle may be inferred to account for most of the variation that experimentally observed. It is unlikely that large scale quadrant variations can be caused by the random, microscopic variations in the low work function coatings or field enhancement factors due to protrusions on the cathode surface, or by variations in the heating coil used to elevate the cathode temperature