Design and Operation of an L-Band MBK for TESLA

Summary form only given. The design and operation of a 1.3 GHz higher-order-mode multiple beam klystron (MBK) that has been developed for the TESLA facility will be described. The klystron contains six off-axis electron beams set on a 26.7 cm bolt circle diameter that interact with a combination of TM₀₂ and TM₀₁ mode cavities to efficiently produce high power RF. The off-axis electron beams are the salient feature of the device which results in a low cathode current density loading and a long product lifetime. Extensive two and three dimensional simulations were conducted to design the novel magnetic focusing and RF circuits. Initial testing proved the basic design concepts and has validated the simulation techniques used. However, an arcing problem developed in one of the windows due to a poor contact in a high RF current carrying joint. This necessitated a repair and re-processing of the klystron. New test results show improved performance. The goals of producing 10 MW of peak power and 150 kW of average power have been achieved, although at a lower efficiency of 59% than had been originally expected. The klystron has also demonstrated stability into a 1.2:1 VSWR mismatch at six equally varying phases over 180 electrical degrees. Approximately 11 MW of power was generated, corresponding to an efficiency of 64.5%, at the mismatch phase corresponding to the highest interaction impedance in the output cavity gaps. Additional refinements to the RF circuit will be implemented in a future klystron to reach the 65% efficiency goal level while maintaining stability into the 1.2:1 mismatch