Insertion Devices

The insertion devices can be divided in three categories:

1. Conventional (vertical field) undulators and wiggler. They represent the 'first generation' of magnets built for ELETTRA, and provide fixed polarisation.
2. The Electromagnetic Elliptical Wiggler (EEW). Designed to allow circular polarisation with fast helicity switching.
3. Elliptical undulators. The development of these 'second generation' devices was stimulated by strong users' demand for variable polarisation sources.
4. Figure-8 undulator

Also shown in red in the above table are two new magnets (presently under construction):

Additionally, a prototype short undulator has been recently completed. It will be installed in a dispersive section as a test of the feasibility of adding new radiation sources in the short straights of the storage ring.

Electromagnetic Elliptical Wiggler

In January 1998, an Electromagnetic Elliptical Wiggler (EEW) was installed in Elettra. This project commenced in Febrary 1996, with partial European Commision funding as a collaboration between Sincrotrone Trieste, Bessy and Max-Lab.

The device, built by DANFYSIK, is a source of circularly polarized light in VUV/Soft X ray region with variable helicity (in the range 5 eV - 1.3 KeV) and combines horizontal and vertical magnetic field in the same structure. The vertical field component is powered with d.c., while the horizontal field has 3 different operation mode:

• D.C.
• Trapezoidal from 0.1-1 Hz
• Sinusoidal from 10-100 Hz.

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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

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.

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.

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.

Superconducting wiggler

The multipole superconducting wiggler is designed to produce a high flux and brightness source in the 10-25 keV range for the second Diffraction beamline (XRD-II). The main parameters of the device are given in the table below:

As shown in fig. 1, a factor of 3 (14) higher flux will be produced compared to the permanent magnet wiggler of the existing Diffraction beamline (XRD) at 12.5 keV (25 keV). A further significant improvement is gained from the smaller source size resulting from the shorter wiggler length (1.4 m instead of 4.5 m). The detailed design and manufacture of the wiggler is now in progress at the Budker Institute of Nuclear Physics, Novosibirsk. Installation is scheduled for June 2002. The wiggler has a cold-bore design with internal 20 K copper liner to efficiently absorb the thermal load from scattered photons and r.f. heating.

Short insertion device

In order to increase the number of insertion devices installed in the storage ring and since all eleven ID straight sections are already defined, we are studying the possibility to install short insertion devices in the dispersive regions.

To test this new idea, the construction and the installation of a prototype short ID with a period of 56 mm and a min. gap of 23 mm (max. magnetic field » 0.5 T), was approved at the beginning of 2000.

A new type of insertion device support structure has been developed and constructed (by ICAP, Italy): each beam is moved by only one motor (instead of 2) with a maximum magnetic load (per beam) of 4 tons.

On this new type of carriage it is possible to install magnet arrays with a length from 0.7 m to 1 m. The absolute gap setting accuracy, independent of load, is less than 20 µm and measured by 2 Heidenhain encoders with a resolution of 1 µm.

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Figure-8 Undulator

The radiation source for the IUVS beamline at ELETTRA is required to provide a photon flux of at least 10¹⁵ photons/s in the range 5 to 10 eV. This calls for an undulator of the maximum length compatible with available length in the straight sections of the storage ring (4.5 m), and having the maximum possible number of periods. This requirement naturally implies a very high emitted power and power density, which can be harmful to the optical elements of the beamline. For this reason an exotic insertion device, the Figure-8 undulator, has been considered as an alternative to the stardard vertical field device.

The main advantage of this solution is a much reduced on-axis power density which is obtained with no penalty on the useful photon flux.

The figure-8 undulator, as originally proposed by Tanaka and Kitamura [1,2], is made of six periodic magnetic arrays: the central rows generate a vertical field with spatial period lo, while the side blocks create a horizontal field with twice that periodicity (2lo).

Figure 2: Predicted radiation spectrum at minimum gap through a 0.6 x 0.6 mrad² pinhole.

The angular distribution of the emitted power (see picture below) is peaked off-axis so that a suitable pinhole can filter-off most of the unwanted power, thus reducing by a large factor the heat load on optics.

Figure 3: Power density angular distribution at minimum gap

Due to the large emission angle of the fundamental compared to that of the higher-order harmonics (which carry most of the energy) this is achieved with no significant reduction on the useful photon flux, as can be seen in the next graph where the spectral intensity produced by an equivalent conventional undulator is shown for comparison [3].

Figure 4: Predicted radiation spectrum of a conventional undulator through a 0.6 x 0.6 mrad² pinhole.

Main Figure-8 undulator parameters

Parameters

Abbreviations and symbols:

Daniele MILLO


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Roberto BRACCO


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Bruno DIVIACCO


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