Beam-energy replacement in a compact fel

To achieve high energy extraction from a compact FEL requires a method to negate the energy loss from the electron beam. A number of proposals have been made to incorporate an accelerating system within the FEL to overcome this restriction. The present study has examined two RF structures that can be conveniently be interleaved with undulator magnets. A ‘muffin tin’ type of structure seems feasible for output wavelengths of the order of 100 μm, but a new type of structure using rectangular waveguide cavities, powered from a parallel coaxial line, looks promising for wavelengths in the range of 10–20 μm. Solutions for practical FELs are proposed and appropriate magnet designs considered.