A vacuum electron device with a hollow tube means for reducing transverse electric mode instability, comprising: at least two resonant cavity means for amplifying a signal using an electron beam, each resonant cavity means includes a cavity width along a major axis, a cavity height along a minor axis, and a cavity length (266A, 266D) along a propagation axis, and the major axis is substantially orthogonal to the minor axis; at least one drift tube section means for separating the at least two resonant cavity means, each drift tube section means includes a drift tube section width (282A-F) along the major axis, a drift tube section height along the minor axis, and a drift tube section length along the propagation axis, and the cavity height is greater than the drift tube section height; a first drift tube section means (280A-F) of the at least one drift tube section means disposed between a first resonant cavity means (260 A-E) and a second resonant cavity means (260 A-E) of the at least two resonant cavity means; when the hollow tube means includes at least three resonant cavity means and at least two drift tube section means, a second drift tube section means (280 A-F) of the at least two drift tube sections disposed between the second resonant cavity means (260 A-E) and a third resonant cavity means (260 A-E) of the at least three resonant cavities; and means for varying frequencies of a trapped mode in the at least one drift tube section means to dampen oscillations of the trapped mode in the at least one drift tube section means, or means for modifying reflection coefficient of the trapped mode of the at least two resonant cavity means to radiate out radio frequency, RF, fields of the trapped mode from the at least one drift tube section means, characterized in that the means for varying frequencies of the trapped mode in the at least one drift tube section means, or the means for modifying reflection coefficient of the trapped mode of the at least two resonant cavity means, comprises a first drift tube section width (282 A-F) of the at least one drift tube section means (280 A-F) that is substantially different from a second drift tube section width (282 A-F) of the at least one drift tube section means (280 A-F). at least two resonant cavity means for amplifying a signal using an electron beam, each resonant cavity means includes a cavity width along a major axis, a cavity height along a minor axis, and a cavity length (266A, 266D) along a propagation axis, and the major axis is substantially orthogonal to the minor axis; at least one drift tube section means for separating the at least two resonant cavity means, each drift tube section means includes a drift tube section width (282A-F) along the major axis, a drift tube section height along the minor axis, and a drift tube section length along the propagation axis, and the cavity height is greater than the drift tube section height; a first drift tube section means (280A-F) of the at least one drift tube section means disposed between a first resonant cavity means (260 A-E) and a second resonant cavity means (260 A-E) of the at least two resonant cavity means; when the hollow tube means includes at least three resonant cavity means and at least two drift tube section means, a second drift tube section means (280 A-F) of the at least two drift tube sections disposed between the second resonant cavity means (260 A-E) and a third resonant cavity means (260 A-E) of the at least three resonant cavities; and means for varying frequencies of a trapped mode in the at least one drift tube section means to dampen oscillations of the trapped mode in the at least one drift tube section means, or means for modifying reflection coefficient of the trapped mode of the at least two resonant cavity means to radiate out radio frequency, RF, fields of the trapped mode from the at least one drift tube section means, characterized in that the means for varying frequencies of the trapped mode in the at least one drift tube section means, or the means for modifying reflection coefficient of the trapped mode of the at least two resonant cavity means, comprises a first drift tube section width (282 A-F) of the at least one drift tube section means (280 A-F) that is substantially different from a second drift tube section width (282 A-F) of the at least one drift tube section means (280 A-F).