Sheet-beam klystron RF cavities

A high-frequency sheet-beam klystron operating at a low perveance per square can produce high peak power at high efficiency. In order to provide beam stability and to maximize power extraction efficiency for a flat beam with a finite width, we have designed RF cavities in which the electric field is nearly constant across the width of the beam (on the order of several wavelengths). However, in such cavities, the electric field in the fundamental mode can couple to the TE propagating mode in the drift section if any cavity or beam asymmetry is present. The dipole field can couple between adjacent cavities even in the absence of these asymmetries as long as the frequency is above cutoff of the drift tube. The effects of coupling between RF cavities are calculated using an equivalent circuit model. Threshold parameters for onset of RF oscillation in the fundamental mode as a result of cavity coupling are obtained. We have designed choke cavities which effectively prevent such possible oscillations