Activation Process of Impregnated Dispenser Cathode Viewed in the Large-screen Emission Microscope†

Abstract

On the 21 in. diameter screen of a magnetic omission microscope the mechanism of thermionic omission from an impregnated dispenser cathode is shown in a sequence of photographs. Emission phases clearly reveal the difference between the first and subsequent heat activations. Correlation to empirical processing data is established by displaying the corresponding physical phenomena, thus minimizing theoretical uncertainties. Forced diffusion of barium at 1160°c is found to be necessary for starting full and patchless emission by continuous barium dispensation as well as an operating temperature somewhat below 1160°c for maximum life, stability, minimum evaporation and sublimation of barium The probability of retaining barium polylayers on the tungsten surface is evidenced by barium pool formations of long life which is well supported by theories on vapour and contact ionization in barium Matrix porosity, average pore diameter and pore separation are determined from the screen for future theoretical efforts in explaining the integrated functioning of dispenser cathodes The solid tungsten land, covered by migrating barium, is found to provide the main emission current and not the pores, the latter operating in the space-charge-limited region because of progressive deepening of pore channels by filler depletion Four consecutive reactivations by heating are demonstrated, each induced by the appearance of a barium flood after which total exhaustion follows.