Development of a 1.3 GHz, 10 MW, 65% efficiency multi-beam klystron for XFEL

CPI's second generation, 10 MW, 1300 MHz multiple beam klystron (MBK), designated the VKL-8301B, has been developed and is being built for the European X-ray free electron laser (XFEL), which is under construction at DESY in Hamburg (Germany). At 10 MW peak rf output the MBK is required to provide at least 65% efficiency and more than 3 MHz instantaneous -1 dB bandwidth. Average power and rf pulse length are 150 kW and 1.5 ms, respectively. MBKs have the advantage of generating high rf output powers at moderate electron beam energies. For our design, six off-axis electron beams enable low cathode current density for longer cathode life. Compared to our first- generation design, where higher-order-mode cylindrical cavities were used, we are utilizing ring resonators operating in the fundamental-mode. This ensures sufficient beam separation while still keeping the overall diameter of the device small in order to reduce cost. Our in-house 1D large-signal rf code (LSCEX) was used to come up with the initial rf design. The 2.5D rf code TESLA enabled us to verify and refine the design. For the cavity design we utilized the commercially available 3D codes MAFIA and HFSS. The baseline beam optics design was created using our in-house 2.5D code XGun. With state-of- the-art 3D modeling software for beam optics (MICHELLE) and magnetics (MagNet) we then validated and optimized the design. Most of the mechanical design is completed. Material for the prototype is currently being procured and we will start building the klystron by the end of February 2008. We are going to present simulated data showing the expected performance of the device. The simulations exceed the requirements by a healthy margin. Measured data will be presented as they become available throughout the assembly process.