  A progogype 5-GHz 500-kW CW klystron (model E3762 provided by Toshiba
Electron Tubes & Devices Co. Ltd.) has been operating as the RF source for the
lower hybrid current drive (LHCD) system in the KSTAR tokamak. In order to
investigate how the efficiency of the 5-GHz 500-kW CW klystron prototype could
be enhanced, the cavity design study is being carried out with simulation code
based on the main klystron operation parameters. This is being done by
simulating klystron performances for various cavity parameters including the
number of cavities, inter-cavity distance, and cavity tuning frequencies. The
simulation has been done with the FCI (field charge interaction) code aided by
a matlab script for scanning input parameters. Initial set of scan parameters
was obtained by benchmarking the E3762 klystron. It was possible to obtain
optimized design parameters with better efficiency for a cavity system adopting
a multi-cell output cavity. However, the multi-cell output cavity is prone to
produce a self-oscillation due to the prolonged (several half RF periods)
beam-field interaction along its multiple gaps. We have checked the feasibility
of the optimization by evaluating the stability of output cavity system. The
stability is given by the ratio of a beam-loading conductance to the circuit
conductance.

Accelerator Physics;Instrumentation and Detectors