Developments in Polarization and Energy Control of APPLE-II Undulators at Diamond Light Source

Abstract

Abstract.A pair of 2m long APPLE-II type undulators have been built for the I10 BLADE beamline at Diamond Light Source. These 48mm period devices have gap as well as four moveable phase axes which provide the possibility to produce the full range of elliptical polarizations as well as linear polarization tilted through a full 180deg. The mechanical layout chosen has a 'master and slave' arrangement of the phase axes on the top and bottom. This arrangement allows the use of symmetries to provide operational ease for both changing energy using only the master phase while keeping fixed linear horizontal or circular polarization, as well as changing linear polarization angle while keeping fixed energy [1]. The design allows very fast motion of the master phase arrays, without sacrifice of accuracy, allowing the possibility of mechanical polarization switching at 1Hz for dichroism experiments. We present the mechanical design features of these devices, as well as the results of magnetic measurements and shimming from before installation. Finally, we present the results of characterization of these devices by the beamline, including polarimetry, which has been done on the various modes of motion to control energy and polarization. These modes of operation have been available to users since 2011. IntroductionDiamond Light Source is a 3GeV Synchrotron with provision for up to 22 Insertion Device (ID) beamlines. Several beamlines require APPLE-II type IDs for the provision of variably polarized photons and to date, four such devices have been constructed and installed, with another three due for construction within the next 18 months. In the case of the I10 BLADE beamline, the available polarization modes include all elliptical polarizations, as well as arbitrary linear polarization variable between 0° and 180°. In order to provide switching between two different polarizations relatively quickly for experiments such as circular dichroism studies, a relatively common solution is for two IDs to be installed in a single straight where then an external method of switching or filtering between the two polarizations is implemented [1]. The I10 BLADE beamline has such a layout.The design of the beamline straight section includes provision of five fast switching chicane magnets which will provide switching between the beams at up to 10Hz, but as a fallback option the electromechanical design of the IDs provides the possibility to do polarization switching at up to 2Hz mechanically. This capability relies fundamentally on the implementation of a "master-slave" arrangement of the magnet arrays which will be shown to provide several further advantages over other APPLE-II designs including ease of energy and polarization control [3].

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