A device is disclosed for amplifying electromagnetic waves propagating al a meander line slow wave structure by means of magnetic cylindrical domains or bubbles which are propagating at substantially the same velocity as the RF wave in an adjacent platelet of orthoferrite having associated with it a propagation arrangement of the conventional type which is adapted to move the domains from one end of the platelet to the other. Since the magnetic bubbles are magnetic dipoles that produce magnetic surface charges, a portion of their energy is transferred to the propagating RF wave, resulting in amplification of the RF wave.