Design of a novel coaxial wiggler

Summary form only given, as follows. The Naval Research Laboratory is investigating innovative wiggler designs in order to reduce the beam energy requirements for short wavelength FEL operation and to enhance the gain, efficiency, and total output power. Recent work has focused on coaxial wiggler designs. The advantages of this are twofold. Firstly, it is advantageous to employ annular configurations in propagating high current electron beams. The hollow beam geometry permits the use of a central structure which can be used to enhance the wiggler field. This latter feature represents the second advantage; specifically, the enhanced wiggler field allows us to use shorter wiggler periods without sacrificing field strength. We will discuss a wiggler design that uses both outer and inner bifilar helical current windings. This wiggler design results in substantial enhancements in the periodic wiggler field experienced by the electron beam as compared with the fields in the absence of the central structure. Steady-state orbits are derived analytically which indicate that substantial enhancements in the transverse beam velocity can be obtained. Integration of single-particle orbits yields results which are consistent with the steady-state orbits, but only axis-encircling orbits appear to be stable. Nonlinear device simulations will also be discussed.