Periodic ion channel laser

Radially polarized radiation is amplified by a free electron laser (FEL) in which the undulator is an ion channel with periodic density. To ensure stable beam propagation, the undulator period is much shorter than the betatron wavelength. The gain at a given distance from the axis equals that of a planar magnetostatic undulator with the same quiver velocity. When an ultrarelativistic electron beam propagates in a periodic ion density of 10/sup 11/-10/sup 17/ cm/sup -3/, a short-wavelength FEL may be obtained.