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2.7 Insertion devices 2.7 Insertion devices 2.7.1 Introduction The Cornell ERL is designed for up to 14 x-ray beamlines with undulator magnets used as radiation sources. The type of the undulator magnet for each beamline is based on the beam line purpose. Tab. 2.7.1 lists x-ray beamlines according to function with the required photon energy range and space available for the various IDs. The unique properties of the ERL x-ray beams are determined by a high-quality electron beam passing through its insertion devices comprising periodic arrays of magnets or wire coils that `wiggle' the beam, and in the process, produce the optimal hard and soft coherent x-ray beams for experimentation at the ERL facility. Third-generation synchrotron radiation facilities have brought ID technology to a high state of optimization for storage-ring use. These developments form a solid base of principles and practices that can be advanced even further for ERL applications. The advancements involve optimizations to cover various photon energy ranges, taking advantage of the very small ERL source size, divergence, polarization control of the emitted x-rays, and ease of tunability. The ERL machine gives ID designers new freedom in its magnetics due to the interesting new machine properties. Since the ERL is not a storage ring, no extra horizontal aperture is required for injection and thus the ID magnets may be close to the electron beam. Small bore devices with an inner diameter of 5 mm or smaller are now practical. The electron beam energy is high enough at 5 GeV and the emittance and energy spread are small enough to make small-period IDs perform exceptionally well. Undulators with small gaps (and therefore small periods down to about twice the gap dimension) become feasible. More periods per ID unit length mean more flux and spectral brightness per unit length. In addition, the lattice and layout of the ERL can accommodate long ID lengths compared to standard storage-ring ID straights. The ERL's very low energy spread of ∆E=E of 2×10−4 means that constructive interference can be maintained even in 1000-period long devices. This is important not only for maintaining a very high average x-ray flux on the x-ray beamline, but may permit monochromator-less beamlines, thus removing all the optical issues of associated coherence preservation through the optics. A further feature is the practical use of the higher harmonics of the x-ray spectrum for high-photon energy experiments (up to 80{100 keV), a scheme that is well-developed by the current third generation storage-ring sources. There are, of course, concerns and optimizations for any technology that breaks new terri- tory. Some of the design concerns for ERL insertion devices (as well as for many existing IDs) are: • Cost per meter of the segment IDs • Predicted reliability over a 10 year period • Overall complexity of mechanics and magnetics • Radiation damage resistance of undulator technology • rms phase errors (in degrees) from period to period down the length of the device 303 Cornell Energy Recovery Linac PDDR Table 2.7.1: The 14 ID beamlines at Cornell's ERL. The beamlines are optimized for x-ray use in the indicated energy ranges. The purposes of the beamlines are indicated and range from coherent diffraction to several kinds of nanoprobes. beamline application mode ID length E (keV) BL length Coherent diffraction Microscopy and dynamics Hi-Coh 25 m 5-25 80 m and XPCS at nm Nanoscope, TXM/STXM with high Hi-Coh 5 m 5-25 75 m Nanoprobe NA Soft x-ray Microscope for biomateri- Hi-Coh 5 m 0.1 - 5 75 m als, XMCD nanomagnetic imaging, ARPES Nanoprobe General, 1 nm beam Hi-Coh 5 m 1-10 80 m Protein Crystallogra- High throughput, microfo- Hi-flux 5 m 5-25 40 m phy I cus Protein Crystallogra- Wide range tunability Hi-flux 5 m 5-25 40 m phy II Inelastic x-ray Scatter- 1 meV resolution Hi-flux 25 m 5-25 80 m ing Femtosecond timing Charge density waves Ultra-fast 25 m 2-25 80 m Materials Science I High pressure science Hi-flux 5 m 5-100 75 m Materials Science II General material science Hi-flux 5 m 5-25 45 m Resonant Scattering x-ray science Hi-flux 5 m 5-25 80 m SAX I, XPCS, grazing Mesoscopic science Hi-flux 5 m 5-25 80 m incidence SAX II Mesoscope science Hi-flux 5 m 5-25 80 m Diagnostics Beamsize and position All modes 5 m 5-25 40 m measurements • Ability to correct phase errors when observed • Metrology to determine field qualities and errors from Hall probe, scanning wires, etc. • Metrology to verify what has been produced and if the magnetic field qualities change over years of use • Amount of development needed for prototypes and production versions • What is the practical limit to useful harmonic number? • How quickly (in hours or days) does it take to remove the device after a damage incident and replace it with a spare? 304 2.7 Insertion devices • Vacuum qualities - Does it take a lot of conditioning to get it ready to work in the ERL? • How quickly (in minutes or seconds) will it take to tune from circular to linear polariza- tion? • How reproducible will the magnetic fields be after a change from linear to circular polar- ization and back to linear again? (or for any other mechanical or temperature change) • How can the trajectory be tuned so that the integral of B*dL over the device length is low enough not to disturb the ERL electron optics? • How will beam image currents and HOMs be dealt with? • Will separate quadrupoles and collimators be periodically needed along the 25 m ID magnetic length device? What are the magnetic field tolerances needed for a 1000-pole undulator to have a relative line width equal to 1 divided by the number of periods? What are the operating and maintenance costs predicted to be over the life of the ID? The ERL insertion device group is investigating insertion devices based on pure permanent technology as well superconducting technology. The state of progress in each of these areas is discussed in the next two sections. 2.7.2 Permanent magnet insertion devices General remarks ERL insertion devices based on permanent magnets (PM) are being designed to take advantage of unique ERL operational characteristics. In contrast to storage rings, an ERL does not require additional space around electron beams for newly injected particles. Thus, the beam aperture size (and the ID gap) can be significantly reduced and made round. This feature alone allows placement of magnetic material very close to the beam axis as well as permitting a symmetrical magnetic structure around the electron beam. The consequence of a smaller gap is a stronger magnetic field. The opportunity for a magnetic structure with 4-fold symmetry simplifies construction and magnetic field tuning. Both of these features are incorporated into the Delta undulator design. The Delta design may also be useful for Free Electron Laser (FEL) applications. Although ERL and FEL machines use the same type of electron beam and their operations are similar, the ID requirements are quite different for each application. Early FEL undulators were generally not adjustable in their magnetic field value and the x-ray energy was adjusted by varying the energy of electron beam. This is not practical with an ERL facility where it is important to provide x-rays with various energies for several experimental stations working simultaneously and to dynamically tune the photon energy as the experiment progresses. Another significant difference between FEL and ERL machines is that the average beam current in the Cornell ERL is planned to be two orders of magnitude larger than at an XFEL such as the LCLS and consequently the beam-generated heat load inside the ID is much higher. Two candidates for permanent magnet ID designs that can be used at an ERL are considered. The prime candidate is of an innovative `Delta' type. A second plan is to adapt a more conservative in-vacuum planar-type undulator approach. 305 Cornell Energy Recovery Linac PDDR State of the art At present, the brightest x-ray beams are generated by planar undulators based on permanent magnet (PM) technology. Advanced types of PM undulators developed recently are listed below. In-vacuum undulators (IVU) The first in-vacuum undulator was constructed and put into operation at KEK (Japan) in 1991 [1]. IVUs have magnet arrays enclosed in a vacuum vessel. The enclosure eliminates the need for the vacuum beam pipe inside the ID and allows reduction of the undulator gap down to a size of a few millimeters. Smaller gaps allow for a stronger magnetic field, which in turn can be translated into a device with more poles per unit length and, as a consequence, can produce more x-ray flux and higher x-ray brightness than larger gap devices. Presently the IVU technology is well established. In-vacuum undulators are considered standard devices at ESRF [2], [3] and other laboratories. The IVU operating at NSLS beamline X25 has the smallest gap of 3.3 mm [4]. Cryogenic permanent magnet undulators Cryogenic Permanent Magnet Undulators (CPMU) have evolved directly from the IVU type. In [5], the authors propose to utilize a ∼ 20% increase in the remnant field strength of NdFeB PM material as well as significant increases in the intrinsic coercivity at cryogenic temperatures of ∼ 150 K. The latter characteristic is critical for the magnet to resist radiation damage coming from the nearby electron beam.
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