Simulation of distributed-emission and injected-beam crossed-field amplifiers. Part 2: The crossed-field electron gun

Numerical models developed for the crossed-field electron gun predict the current and the beam shape and stability for varied voltages or magnetic field. These programs use a deformable triangular mesh to reproduce the realistic electrode boundaries. A static iterative analysis predicts beam currents about 5 percent higher than measured for a short Kino gun, but is not stable for long cathodes with returning electrodes. However, the alternative time-dependent method predicts a stable, well-defined beam in both cases. Excess shot noise in a long-cathode crossed-field gun is believed to be due to cycloiding electrons which return to the cathode from well beyond the potential minimum. Of three theories compared here, only the Ho and Van Duzer model includes these electrons and predicts an instability. With added shot noise the computer simulation may be capable of reproducing the effect. Experimental measurements have been obtained by subcontract for a Northrop gridded crossed-field gun. The two dimensional time-dependent analysis, which ignores the grids, predicts the beam current with good order-of-magnitude agreement.