An FEL beam injection system with adjustable isochronism

A design study is presented for a beam injection system suitable for a free-electron laser (FEL) and other purposes, which in its normal mode is isochronous to second order in the particle co-ordinates, yet which may be progressively shifted from the isochronous condition by alteration of the strengths (and for larger deviations the locations) of some of its magnetic elements. The system uses a minimum number of magnetic components (3–4 dipoles and 2 quadrupole triplets), and can produce both positive and negative deviations from isochronism within the range ±30 ps/% momentum deviation. The major second-order chromatic and geometric aberrations are minimised by the use of sextupoles. Use of the system for electron bunch compression in an FEL is discussed.