Abstract: Technology is described for vacuum electron device (e.g., sheet beam klystron) that includes a hollow tube structure. In one example, the hollow tube structure includes at least three resonant cavities and at least two drift tube sections. Each resonant cavity includes a cavity width along a major axis and a cavity height along a minor axis. Each drift tube section includes a drift tube section width and a drift tube section height, and the cavity height is greater than the drift tube section height. A first drift tube section is disposed between a first resonant cavity and a second resonant cavity. A second drift tube section is disposed between the second resonant cavity and a third resonant cavity. A drift tube section width of the first drift tube section is substantially different from a drift tube section width of the second drift tube section.