Insertion Devices

Elliptical Undulators

Motivated by the increasing demand for circularly polarized radiation, a series of six variable polarization undulators have been designed and constructed at ELETTRA, whose main parameters are shown the table below. They will produce photons in the UV and soft X-ray range for the new APE, BACH and FEL/NANOSPECTROSCOPY beamlines.

Field strength (T) and corresponding fundamental photon energy (eV) for the various undulators

period Np Horizontal Polarization Circular Polarization Vertical Polarization
l0 (cm) # B0 e1 (eV) B0 e1 (eV) B0 e1 (eV)
4.8 44 0.58 178 0.29 287 0.34 366
6.0 36 0.78 59 0.42 94 0.51 123
7.7 28 0.92 21 0.53 32 0.64 43
10.0 2x20 1.02 8 0.63 11 0.77 14
12.5 17 0.77 8 0.48 10 0.59 13

These undulators are based on the so-called APPLE-II structure [1], which can generate elliptically polarized radiation including, as special cases, horizontal, circular and vertical polarization.

The development of these magnetic structures [2,3,4], represented a major challenge and required significant improvements in several technological areas such as control of magnetic material properties, precision mechanical design and more efficient field error correction methods. This project has reached its conclusive milestone in September 2000, with the installation of the last undulator magnet (EU4.8).

The arrangement of the six modules in the storage ring has been dictated by the different design choices and operating modes of the various beamlines.

BACH will use one of the two available sources (each covering a different photon energy range) whose flux is channeled through one single beamline. The corresponding undulators (EU4.8 and EU7.7) are therefore simply placed collinearly along the straight section.

APE will make simultaneous use of the two sources (EU6.0 and EU12.5). This is made possible by a chicane-like steering of the e-beam. For this reason, a supplementary dipole electromagnet has been placed between the two undulators, producing a 2 mrad horizontal deflection angle of the electron beam, and a corresponding angular separation of the radiation emerging from the two undulators.

Figure 4: Layout of the APE undulators in straight section #9

Figure 5: APE undulators with the deflection magnet in between

Finally, the two identical EU10.0 devices, together with a phase modulation electromagnet, are arranged in an optical klystron configuration. This will enable the two undulators to be properly phased, thus effectively doubling the undulator length and the useful flux. Furthermore, during FEL operation, this configuration offers the benefit of an increased laser gain.

Figure 6: FEL/NANOSPECTROSCOPY undulators and modulator

Despite the greater complexity of the new structures, the overall magnetic field quality achieved for these devices is similar or better than for the previously constructed conventional undulators, providing close to ideal intensity also on the high order harmonics. Multipole field errors, potentially harmful for the dynamics of the stored electron beam, were minimized by shimming tecniques to a level where they should have a negligible impact on the ring operation [4]. The residual perturbation to the closed orbit will be compensated, as for the other devices, by properly calibrated correction coils placed at the ends of each module.

Undulator EU12.5 deserves an additional comment. This is a special magnet, in which a quasi-periodic modulation of the magnetic field distribution has been implemented [4,5], in order to reduce contamination from higher harmonics which could not be effectively suppressed by the beamline optical system.

Last Updated on Monday, 20 April 2015 11:04