The study of magnetic and biological materials which exhibit magnetic circular dichroism has created a demand for circularly polarized x-rays whose helicity may be switched between the right and left hand senses. At present, circularly polarized x-rays are obtained from storage ring bending magnets by accepting radiation from above and below the axis, but insertion device sources are desired for greater intensity. Planar helical undulators are magnetic insertion devices that generate helical

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... netic fields. The charged particle beam executes a helical trajectory in the device, and produces elliptically polarized x-rays. The .-special case of circularly polarized x-rays is most in demand.