The first experimental study of a coaxial free-electron maser (FEM) based on two-dimensional (2D) distributed feedback is presented. A new type of cavity formed with coaxial 2D surface photonic band gap structures was used. The FEM was driven by a large diameter (7 cm), high-current (500 A), annular electron beam of energy 475 keV. By tuning the amplitude of the undulator or guide magnetic field, modes associated with the different band gaps of the 2D structures were excited. The $Ka$-band coaxial FEM generated 15 MW of radiation with a 6% conversion efficiency, in excellent agreement with theory.

• Received 27 September 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.035002