an improved coupled cavity type traveling wave vacuum tube of the type containing serially spaced electromagnetically coupled cavities defined in spaces between serially spaced magnetic pole pieces , each of which contains magnetic ferrule portions projecting from the pole piece walls and bordering an electron beam passage through the pole piece , an electrically conductive nonmagnetic material of high thermal conductivity lines the inner walls of the ferrule , and in which a void formed in between the front and back side walls defining each pole piece is filled with a nonmagnetic material of a high thermal conductivity characteristic positioned in a thermal conducting relationship with said lining portion whereby the ability of the tube to dissipate heat generated therewithin during operation is enhanced and the tube may thereby be operated at higher duty cycles .