Experimental results from an 8-beam, 5-cavity multiple-beam klystron with 6% bandwidth

We describe the results of recent experiments with an eight- beam, five-cavity multiple-beam klystron (MBK). The electron gun is of the same design as used in our previous MBK and operates at a nominal cathode voltage of -45 kV and a total current of 32 A. The electrodynamic circuit is comprised of a two-gap input cavity, a two-gap idler cavity, two additional single-gap idler cavities, and a two-gap output cavity and has a total length of 22 cm. All of the multi-gap cavities operate in the pi-mode. Some of the cavities were loaded with a lossy dielectric to reduce the Q. Output power is extracted from both of the output resonator gaps in four waveguide arms (two on each gap). The tube produces ~ 600 kW of peak output power at saturation with a corresponding electronic efficiency of 40%. Beam transmission is excellent: > 99% in the small-signal regime and > 93% at saturation. Despite a mismatch in the input circuit that reduces the gain in the upper portion of the band, the MBK has a measured 3-dB bandwidth of ~ 6%. We will describe efforts to improve the input circuit match and will compare measured data with the results of numerical modeling using the MAGIC 3D particle-in-cell code and the 2.5D large-signal code, TESLA.