The ESRF Insertion Devices.

The European Synchrotron Radiation facility is presently operating 47 segments of insertion devices (IDs). A record brilliance of 1 x 10(20) photons s(-1) (0.1% bandwidth)(-1) mm(-2) mrad(-2) has been reached. Almost all devices are built with permanent magnets with or without iron pole pieces. They have been mechanically and magnetically designed and field-measured in house. Multipole shimming has been applied to all devices to remove the integrated dipole and higher-order multipole fields, thereby reducing the interaction between the IDs and the stored beam. For all undulators, the field errors have been corrected further using spectrum shimming in order to achieve ideal spectral brilliance on all harmonic numbers from 1 to 15. A significant effort has been made to optimize the magnet terminations for both field-integral correction and phasing. A phasing scheme of the undulator segments has been developed which allows the independent manufacture and operation of individual segments. Several designs for undulator phasing are presented, together with a comparison between hybrid and pure-permanent-magnet technology. A new type of variable-polarization helical undulator is presented.

SPring-8 in-vacuum undulator beam test at the ESRF.

Before the commissioning of SPring-8, the in-vacuum hybrid undulator developed at SPring-8 had been brought to the ESRF for the first beam test in the summer of 1996. The purpose of this test was to investigate the influence of the in-vacuum undulator on the beam and check its vacuum system. However, heating by the resistive wall impedance turned out to be a critical issue for the in-vacuum undulators.

A three-dimensional magnetostatics computer code for insertion devices.

RADIA is a three-dimensional magnetostatics computer code optimized for the design of undulators and wigglers. It solves boundary magnetostatics problems with magnetized and current-carrying volumes using the boundary integral approach. The magnetized volumes can be arbitrary polyhedrons with non-linear (iron) or linear anisotropic (permanent magnet) characteristics. The current-carrying elements can be straight or curved blocks with rectangular cross sections. Boundary conditions are simulated by the technique of mirroring. Analytical formulae used for the computation of the field produced by a magnetized volume of a polyhedron shape are detailed. The RADIA code is written in object-oriented C++ and interfaced to Mathematica [Mathematica is a registered trademark of Wolfram Research, Inc.]. The code outperforms currently available finite-element packages with respect to the CPU time of the solver and accuracy of the field integral estimations. An application of the code to the case of a wedge-pole undulator is presented.

XAFS and X-MCD spectroscopies with undulator gap scan.

The first experimental applications of the undulator gap-scan technique in X-ray absorption spectroscopy are reported. The key advantage of this method is that during EXAFS scans the undulator is permanently tuned to the maximum of its emission peak in order to maximize the photon statistics. In X-MCD or spin-polarized EXAFS studies with a helical undulator of the Helios type, the polarization rate can also be kept almost constant over a wide energy range.

Two-plane focusing of 30 keV undulator radiation.

An imaging experiment using a two-plane focusing refractive lens made of aluminium and operated in the hard X-ray range is described. The lens is made of a series of 1 and 0.8 mm-diameter lenses drilled through a 2 mm aluminium plate. It is exposed to the white beam from an undulator with total power as high as 1.7 kW and normal-incidence power density of 100 W mm(-2). The measured r.m.s, size of the image is 0.12 x 0.06 mm at a photon energy of 30 keV. Theoretical estimates for the transmission, aperture and tolerance of alignment of such lenses are made. It is found that the aperture of the lens is dominated by photoelectric absorption (Compton scattering) in the low (high) energy range of the spectrum. Beryllium is the most promising material.

Phasing multi-segment undulators.

An important issue in the manufacture of multi-segment undulators as a source of synchrotron radiation or as a free-electron laser (FEL) is the phasing between successive segments. The state of the art is briefly reviewed, after which a novel pure permanent magnet phasing section that is passive and does not require any current is presented. The phasing section allows the introduction of a 6 mm longitudinal gap between each segment, resulting in complete mechanical independence and reduced magnetic interaction between segments. The tolerance of the longitudinal positioning of one segment with respect to the next is found to be 2.8 times lower than that of conventional phasing. The spectrum at all gaps and useful harmonics is almost unchanged when compared with a single-segment undulator of the same total length.

Measuring Beam Sizes and Ultra-Small Electron Emittances Using an X-ray Pinhole Camera.

A very simple pinhole camera set-up has been built to diagnose the electron beam emittance of the ESRF. The pinhole is placed in the air next to an Al window. An image is obtained with a CCD camera imaging a fluorescent screen. The emittance is deduced from the size of the image. The relationship between the measured beam size and the electron beam emittance depends upon the lattice functions alpha, beta and eta, the screen resolution, pinhole size and photon beam divergence. The set-up is capable of measuring emittances as low as 5 pm rad and is presently routinely used as both an electron beam imaging device and an emittance diagnostic.

Helios: a new type of linear/helical undulator.

A new class of undulator capable of producing linear and/or helical polarization is described. The magnetic field, power, spectral flux, brilliance and interactions with the electron beam of such undulators are discussed. The case of Helios, an undulator presently installed on the ESRF, is discussed in detail.

Diagnostic techniques and UV-induced degradation of the mirrors used in the Orsay storage ring free-electron laser.

Due to its low gain, the Orsay storage ring free-electron laser necessitates the use of high reflectivity mirrors. Three techniques for measuring the mirror losses are presented, based on cavity decay time measurements using either an external laser, the synchrotron radiation stored in the cavity, or the free-electron laser itself. The high signal-to-noise ratio allowed the detection of loss variations as low as 10(-7)/sec(1/2). From these diagnostics three distinct processes of UV-induced degradation of TiO2/SiO2 dielectric mirrors were identified. One was a surface absorption of the upper SiO2-air interface; it was not affected by annealing. The other two corresponded to a volume absorption of the layers which completely recovered after annealing.