Tables for
Volume F
Crystallography of biological molecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F, ch. 8.1, pp. 155-156   | 1 | 2 |

Section 8.1.3. Insertion devices (IDs)

J. R. Helliwella*

aDepartment of Chemistry, University of Manchester, M13 9PL, England
Correspondence e-mail:

8.1.3. Insertion devices (IDs)

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These are multipole magnet devices placed (inserted) in straight sections of the synchrotron or storage ring. They can be designed to enhance specific characteristics of the SR, namely

  • (1) to extend the spectral range to shorter wavelengths (wavelength shifter);

  • (2) to increase the available intensity (multipole wiggler);

  • (3) to increase the brilliance via interference and also yield a quasi-monochromatic beam (undulator) (Fig.[link] shows the distinctly different emission from an undulator);

  • (4) to provide a different polarization (e.g. to rotate the plane of polarization, to produce circularly polarized light etc.).

The classification of a periodic magnet ID as a wiggler or undulator is based on whether the angular deflection, δ, of the electron beam is small enough to allow radiation emitted from one pole to interfere directly with that from the next pole. In a wiggler, [\delta \gg \gamma^{-1}], so the interference is negligible and the spectral emission (Fig.[link]) is very similar in shape to, but scaled up from, the universal curve (i.e. bending magnet spectral shape). In an undulator [\delta \leq \gamma^{-1}] and the interference effects are highly significant (Fig.[link]). If the period of the ID is [\lambda_{u}] (cm), then the wavelengths [\lambda_{i}] (i integer) emitted are given by [\lambda_{i} = {\lambda_{u} \over i2\gamma^{2}}\;\left(1 + {K^{2} \over 2} + \gamma^{2} \theta^{2}\right), \eqno(] where [K = \gamma \delta].

The spectral width of each peak is [\Delta_{i} \simeq 1/iN, \eqno(] where N is the number of poles.

The angular deflection, δ, is changed by opening or closing the gap between the pole pieces. Opening the gap weakens the field and shifts the emitted lines to shorter wavelengths, but decreases the flux. Conversely, to achieve a high flux means closing the gap, and in order to avoid the fundamental emission line moving to long wavelength, the machine energy has to be high. Short-wavelength undulator emission is the province of the third-generation machines, such as the ESRF in Grenoble, France (6 GeV), the APS at Argonne National Laboratory, Chicago, USA (7 GeV), and SPring-8 at Harima Science Garden City, Japan (8 GeV). Another important consideration is to cover the entire spectral range of interest to the user via the tuning range of the fundamental line and harmonics. This is easier the higher the machine energy. However, important developments involving so-called narrow-gap undulators (e.g. from 20 mm down to ~7 mm) erode the advantage of higher machine energies ≥6 GeV for the production of X-rays within the photon energy range of primary interest to macromolecular crystallographers, namely ~30 keV down to ~6 keV.

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