This invention relates to a triode type cathode structure comprising, in superposition, an electrode forming a cathode (13) and supporting means made of an electron emitting material in the form of a layer (14), an electrical insulation layer (11) and a grid electrode (15), an opening (12) formed in the grid electrode and in the electrical insulation layer exposing the means made of an electron emitting material. The means made of an electron emitting material (14) are located in the central part of the opening of the grid electrode (15), this opening being in the form of a slit and the means made of an electron emitting material exposed by the slit being composed of elements aligned along the longitudinal axis of the slit.