The present invention relates to a slow wave structure (1) for transmitting an electromagnetic wave in a travelling wave tube (2), wherein the slow wave structure (1) comprises an electrical conductor (10) formed along a helix, wherein the helix twists around a longitudinal axis (11). In order to provide a slow wave structure and a traveling wave tube amplifier facilitating a simplified amplification of high frequency electromagnetic waves, the electrical conductor (10) comprises at least one coupling section (12), wherein the coupling section (12) is formed in order to generate a non-zero axial electric field outside a space confined by an envelope of the helix of the electrical conductor (10) in a direction parallel to the longitudinal axis (11), when during operation of the slow wave structure (1), the electromagnetic wave is transmitted along the electrical conductor (10), wherein the axial electric field is generated between two ends of the coupling section (20, 20'), wherein the ends of the coupling section (20, 20') are distanced from each other in a direction parallel to the longitudinal axis (11).