This thesis investigated the design decisions and associated optimisation methods of a 1.3 GHz Multiple Beam Klystron (MBK) for use in the Compact Linear Collider (CLIC). In this regard refinements have been made both to the MBK design, and investigation and optimisation methods used. The high desired efficiency of 80% requires low perveance beams, to achieve the specification output power 20 beams are needed. The choice of cavity used in the interaction structure of a klystron has a large impact on its size and efficiency. To optimise this a number of possible cavity designs were produced and compared to confirm selection of the most appropriate. The fundamental mode (TM0, 1, 0) coaxial cavity was selected due to its superior R/Q of 130-210 W and suitability as a 2nd harmonic cavity. Although the dipole mode proved to be close in frequency to the operating mode (within ~ 50 MHz), raising concerns of stability issues in an MBK. A novel model was developed using standard wake field theory to investigate the effects of this mode the klystron’s stability. A strategy for shifting this mode using a coupled shifting gap was proposed and achieves a shift of 125 MHz, although the models findings suggest it is not a significant problem. Existing methods of calculating dipole and higher order modes proved time consuming thus impeded a fully investigation of stability issues. An extended method of moments model allows efficient calculation of monopole and higher order modes. The model’s basis functions are altered to represent a range of TM and TE modes with azimuthal variation, allowing their rapid and accurate calculation. Optimising the klystron interaction structure by hand to find a viable configuration revealed shortcomings in this standard approach, although the target efficiency was achieved. An algorithmic approach was deemed necessary to allow a full investigation within reasonable time limits. The field of evolutionary algorithms is presented and an evolutionary algorithm to automate the optimisation of klystron interaction structures was developed. A number of important related issues were dealt with and suitable interaction structures (optimised for efficiency, bandwidth, length and electron exit velocity) produced. Finally a design was proposed for both the input and output couplers which is inspired by a coupler used in a gyrotron. Unconventionally, the latter exits the tube axially avoiding the focusing solenoid, but excessive heating may preclude its use.
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