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 mod e is isochronous to second order in the particle co-ordinates, yet whi ch may be progressively shifted from the isochronous condition by alte ration of the strengths (and for larger deviations the locations) of s ome of its magnetic elements. The system uses a minimum number of magn etic components (3-4 dipoles and 2 quadrupole triplets), and can produ ce both positive and negative deviations from isochronism within the r ange +/-30 ps/% momentum deviation. The major second-order chromatic a nd geometric aberrations are minimised by the use of sextupoles. Use o f the system for electron bunch compression in an FEL is discussed.