A coupled cavity travelling wave tube comprising: a plurality of core segments (34, 36, 40, 42) arranged in spaced-apart fashion to form an electron beam tunnel; a first electrically conductive longitudinal member adjacent the plurality of core segments (34, 36, 40, 42) alternately extending toward and receding from successive core segments; a second electrically conductive longitudinal member adjacent to the plurality of electrically conductive core segments (34, 36, 40, 42) alternately extending toward and receding from successive core segments, wherein the first and second longitudinal members are offset to extend toward different core segments, wherein the first and second longitudinal members (14, 16) each comprise a body and a plurality of protrusions (24, 26, 30, 32) which extend from the bodies toward each corresponding core segment (34, 36, 40, 42), wherein the pluralities of protrusions (24, 26, 30, 32) and the corresponding core segments (34, 36, 40, 42) comprise mating surfaces, wherein the mating surfaces of the pluralities of protrusions are placed in contact with the mating surfaces of the corresponding core segments, and wherein the mating surfaces are substantially flat; characterised in that said coupled cavity travelling wave tube further comprises a cylindrical electrically conductive housing, the plurality of core segments (34, 36, 40, 42) and the first and second longitudinal members (14, 16) being substantially contained within the housing, wherein the plurality of core segments (34, 36, 40, 42) comprise electrically conductive wings (106, 110) extending perpendicularly to the plurality of protrusions (24, 26, 30, 32) from each of the plurality of core segments to inner side walls of the housing, wherein the pluralities of protrusions in combination with said wings form a series of coupled cavities (44, 46, 50, 52), wherein the series of cavities (44, 46, 50, 52) are coupled together via gaps between successive core segments, and wherein an inner cross-section of the housing is substantially rectangular. a plurality of core segments (34, 36, 40, 42) arranged in spaced-apart fashion to form an electron beam tunnel; a first electrically conductive longitudinal member adjacent the plurality of core segments (34, 36, 40, 42) alternately extending toward and receding from successive core segments; a second electrically conductive longitudinal member adjacent to the plurality of electrically conductive core segments (34, 36, 40, 42) alternately extending toward and receding from successive core segments, wherein the first and second longitudinal members are offset to extend toward different core segments, wherein the first and second longitudinal members (14, 16) each comprise a body and a plurality of protrusions (24, 26, 30, 32) which extend from the bodies toward each corresponding core segment (34, 36, 40, 42), wherein the pluralities of protrusions (24, 26, 30, 32) and the corresponding core segments (34, 36, 40, 42) comprise mating surfaces, wherein the mating surfaces of the pluralities of protrusions are placed in contact with the mating surfaces of the corresponding core segments, and wherein the mating surfaces are substantially flat; characterised in that said coupled cavity travelling wave tube further comprises a cylindrical electrically conductive housing, the plurality of core segments (34, 36, 40, 42) and the first and second longitudinal members (14, 16) being substantially contained within the housing, wherein the plurality of core segments (34, 36, 40, 42) comprise electrically conductive wings (106, 110) extending perpendicularly to the plurality of protrusions (24, 26, 30, 32) from each of the plurality of core segments to inner side walls of the housing, wherein the pluralities of protrusions in combination with said wings form a series of coupled cavities (44, 46, 50, 52), wherein the series of cavities (44, 46, 50, 52) are coupled together via gaps between successive core segments, and wherein an inner cross-section of the housing is substantially rectangular.