A comparative study of high power, multistage, X-band TWT amplifiers

Our previous work on high-power efficient X-band TWT amplifiers has used a two stage device with bunching produced in a greater than light phase velocity region, immediately followed by a short low phase velocity output structure. The device is driven by a 7 mm diameter 750 kV, 450 A pencil electron beam. The structure, which has a 4 GHz bandwidth in the bunching section, produces an amplified output with a power in the range 20-60 MW. At higher output powers pulse shortening develops. A serious candidate for the pulse shortening is excitation of the HEM/sub 11/ mode in the structure. This mode overlaps the frequency domain of the desired TM/sub 01/ mode. We have designed and tested new amplifier structures in which the separation of these modes is substantially increased. The performance of the new amplifier(s) will be compared with that of the older device, and the relevance of the hybrid modes to pulse shortening assessed.