The field matching technique is applied to do the theoretical analysis and numerical simulation of the buncher cavity in gyroklystron.The influence of side apertures on resonance characteristic and the way that quality factor is decreased by adding absorber have been discussed.The simulation results of buncher cavity with low quality factor in a 34GHz

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mode gyroklystron are presented.The numerical calculation shows that the diameter of aperture has obvious influence on the resonance characteristic and field distribution of stationary wave in the cavity

and the quality factor decreases rapidly as the thickness of absorber increases.When the structure of cavity is correct

the absorbing medium and its thickness are selected properly;the designed parameters can be obtained.