a fast pulsed excitation , electromagnetic undulator or wiggler , employing geometrically alternating substacks of thin laminations of ferromagnetic material , together with a single turn current loop excitation of the composite assembly , of such shape and configuration that intense , spatially alternating , magnetic fields are generated ; for use as a pulsed mode undulator or wiggler radiator , for use in a free electron laser type radiation source or , for use in an inverse free electron laser charged particle accelerator .