Conceptual Design of a Hybrid-Type Elliptically Polarizing Undulator*

There are several means of generating circularly polarized light. The use of an electromagnetic helical device is very popular for the experiments using lower photon energy radiation. An advantage of the electromagnetic device is the capability of fast switching of polarization. However, due to its complicated structure and a limited maximum current, construction of a shorter period (< 100 mm) device with normal-conductor coils is not practically possible. Similar to a conventional planar device, the permanent magnet APPLE device with a shorter period can be constructed without any restrictions. However, there are some disadvantages of the APPLE device. One is the difficulty of fast polarization switching. But this problem can be solved by installing two identical devices in series in the same straight section. Another disadvantage is related to the magnetic tuning. Usually, a shimming technique is used to adjust the magnetic field. For pure permanent magnet undulators, thin iron shims or magnet shims are applied to the surface of the undulator magnets at the cost of reducing the undulator gap. On the other hand, for a hybrid type undulator, the shimming of magnet blocks can be applied without reducing the gap because the gap distance is determined by a pole-to-pole distance.