Axial RF Power Input into Photocathode Electron Guns

The RF power input into an accelerating cavity is usually performed with special couplers that deliver the power in an asymmetric manner. Further, for superconducting cavities, the delivery point is typically into the beam tube beyond the cavity cells. In photocathode electron guns, where the cathode is isolated from the surrounding cavity by a coaxial vacuum gap, another power coupling possibility exists, since the cathode gap forms a coaxial line which can be used for RF power input. In the present paper, we show the advantages of this input coupling method. We then discuss a particular coupler design and present numerical results for normal and superconducting cavities.