Comparison of planar helical undulator designs for spear beamline five

摘要

There is an increasing demand for circularly polarized soft x-rays in the study of magnetic materials, biological molecules, and other systems that exhibit circular dichroism. At present, most experiments have been done with bending magnet radiation that is circularly polarized above and below the horizontal midplane of the storage ring. A number of insertion devices generate elliptically polarized x-rays, such as bifilar solenoids, elliptical and asymmetric wigglers, crossed undulators, and planar helical undulators. Elliptically polarized light is generated when electrons enter a helical magnetic field; the helicity of the field determines the helicity of the x-ray emission. Among the various technologies, the pure permanent magnet planar helical undulator is probably the best choice for installation on BLV in summer, 1993. This approach was pioneered by Pascal Elleaume at ESRF; he has installed a device of this type called 'Helios'. Richard Walker and Shigemi Sasaki have developed alternative planar helical undulators, which improve on the basic design of Elleaume. This proposal is a discussion of planar helical undulator strategies from which implementation choices can be made. We will consider only pure RCP and LCP sources, since we are trying only to span the range from 500--1000 eV. If we needed a broader range, we might consider creating elliptically polarized light, and hence, harmonics which RCP and LCP sources do not have.