A progogype 5-GHz 500-kW CW klystron (model E3762 provided by ToshibaElectron Tubes & Devices Co. Ltd.) has been operating as the RF source for thelower hybrid current drive (LHCD) system in the KSTAR tokamak. In order toinvestigate how the efficiency of the 5-GHz 500-kW CW klystron prototype couldbe enhanced, the cavity design study is being carried out with simulation codebased on the main klystron operation parameters. This is being done bysimulating klystron performances for various cavity parameters including thenumber of cavities, inter-cavity distance, and cavity tuning frequencies. Thesimulation has been done with the FCI (field charge interaction) code aided bya matlab script for scanning input parameters. Initial set of scan parameterswas obtained by benchmarking the E3762 klystron. It was possible to obtainoptimized design parameters with better efficiency for a cavity system adoptinga multi-cell output cavity. However, the multi-cell output cavity is prone toproduce a self-oscillation due to the prolonged (several half RF periods)beam-field interaction along its multiple gaps. We have checked the feasibilityof the optimization by evaluating the stability of output cavity system. Thestability is given by the ratio of a beam-loading conductance to the circuitconductance.
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