Most of the undulators at APS are planar undulators. They produce a vertical magnetic field on the undulator axis that results in a horizontal wiggle in the electron beam and horizontally polarized photons. An important characteristic of a planar undulator is its period length – the distance in the beam direction over which the undulator field goes through a full cycle. The different period lengths result in different tuning curves of photon brilliance as a function of photon energy, as shown in Fig. 1. Shorter-period undulators give a higher brilliance that is also shifted out to higher photon energy. When the period is short enough, however, the maximum field strength reachable with the undulator is limited and a gap will develop between the first and third harmonics, as can be seen for the 2.7- and 2.3-cm undulators. Therefore, when users must choose a single undulator period length, there can be a tradeoff between maximizing the photons in the energy range of interest and making sure the tuning range is wide enough.
A revolver undulator can help with this tradeoff. The concept is shown in Fig. 2. The user can choose two different period lengths for the two magnetic structures when the revolver is built, and then select the period length that is preferred for the measurement of the moment. Having a revolver installed means that for the physical length of one undulator, the user has the choice between either of two undulators. For some beamlines, the availability of a revolver does away with the need for a much more expensive long straight section.
The choice of the period lengths for the two revolver magnetic structures is based on the needs of the beamline. For some, maintaining a wide continuous tuning range is important, but there are particular applications that need as many photons as possible at a particular energy. In such cases, an undulator with a wide, continuous tuning curve is paired with an undulator that optimized to produce photons of the desired energy. An example of this is shown in Fig. 3.
When the continuous tuning range and access to lower photon energies is not as important as increased brilliance, a pair of short-period undulators can be chosen. This is shown in Fig. 4, for a combination of a 2.5 and a 2.3 cm undulator.
A revolver prototype is presently under development at APS.