X-Ray Free Electron Lasers

X-ray Free-Electron Lasers (XFELs) are large-scale accelerator facilities generating radiation beams with intensity far exceeding that of any other available source of high-energy photons. The beams from an XFEL have important applications across a broad range of science: for example, they can be used to determine the detailed structure of complex biological structures such as
proteins and viruses, and to observe with unprecedented detail the processes taking place (on timescales of order 10-15 seconds) in chemical reactions. XFELs are essential tools for research in many different fields, including the life sciences, materials science and physical sciences.

However, effective operation of an XFEL requires extremely precise control of the high-energy electrons used to generate the photon beams. A wide range of effects in the systems used to accelerate and manipulate the electron beams can easily degrade the quality of the electron and, consequently, the photon beams. The magnets used to steer and focus the charged particle beams and (ultimately) to generate the photon beams play a critical role in achieving and maintaining the necessary electron and photon beam quality.

Conventional accelerators have generally relied on a few standard types of magnet for control of charged-particle beams. Typically, accelerator magnets are designed so that the field varies in a simple way in directions perpendicular to the beam motion, and is independent of distance along the magnet in a direction parallel to the beam motion. However, as machines and facilities become more ambitious in terms of beam parameters, more exotic magnet designs are being explored.

A further issue in any high-energy particle accelerator is that relativistic effects mean that any manipulation of the particle beams usually requires large magnets with strong fields, arranged over large distances (tens or hundreds of metres in the case of a high-power XFEL). The greater design flexibility in novel...