Experimental study of a FEM based on a 2D distributed feedback cavity

We report operation of a co-axial free-electron maser (FEM) based on two-dimensional distributed feedback. The 37.3GHz FEM was driven by an annular electron beam (475kV, 500A, 250ns) and generated 15MW /spl plusmn/1.5MW of radiation with a 6% conversion efficiency. To investigate an increase in FEM output power and efficiency a new cavity configuration based on a copper 2D Bragg reflector is studied.