A Diplexer to Operate Two Cavity Eigenmodes in Parallel

To fulfil the demand of future high power and high luminosity FEL and storage ring sources, an intensive electron beam with short bunch length, small emittance and large bunch charge is required. Normal conducting (NC) laser driven radio frequency (RF) photocathode guns can deliver 1 nC bunches with an emittance of 1 π μrad. But to realize the demand on high average currents, a superconducting (SC) RF gun appears to be the best solution. First long term operation has been demonstrated at FZD [1]. In difference to the NCRF guns, the application of static magnetic fields near the cathode is not possible. Instead, the use of the magnetic field of a transverse electric (TE) mode in parallel to the accelerating mode was proposed. Numerical simulations have shown that this RF focusing can be applied to compensate the emittance growth [2]. This contribution will introduce a possibility to use the existing coaxial RF coupler of TESLA like cavities, as a RF power input for TE modes in parallel to its normal operation. The additional coupler component outside the module accomplishes the task of combining two different frequencies from different sources to one load. Thus, it corresponds to the working principle of a high power RF diplexer. Based on the 3 cell Rossendorf SRF-Gun [3], a concrete technical implementation and results of its operation at the cold SRF-Gun cavity will be presented. Input Port 1 (f1) (from Klystron) Input Port 2 (f2) Output Port 3 (f1+ f2) (to Cavity) Coaxial FPC Waveguide Input Cavity (f2) • Coupling Slot • Tuning Stamp • Inner Conductor • Outer Conductor Choke Cavity (f2) • Coupling Slot λ 4 Doorknob Transition (f1) Figure 1: Cross section of the coaxial diplexer consisting of an input cavity and a reflecting choke cavity. The picture shows the simulated magnitude of the electric field @ 2.5 GHz for a random phase.