Free electron laser spectrum control with a quasiperiodic wiggler configuration

Recently, we have studied an unusual wiggler magnet configuration comprisin g two conventional periodic wigglers. It was shown that if the periods of t he two constituent wiggler components are not integrally but irrationally r elated then considerable radiation is generated at frequencies that are irr ational multiples of the fundamental radiation frequency. The generation of these irrationally related harmonics modifies considerably the basic free electron laser (FEL) mechanism and as such they may be used to manipulate t he FEL spectrum. In this contribution, we consider the spontaneous emission of electrons in such a quasiperiodic wiggler structure with and without th e electron energy spread effects. Numerical examples are used to demonstrat e that significant radiation can be generated simultaneously at both the us ual FEL harmonics and irrationally related frequencies. When the wiggler pa rameters are appropriately adjusted, the usual integrally related FEL harmo nics are suppressed considerably. This feature may be used to relieve the m irror degradation problem in some ultraviolet and X-ray FELs. (C) 2000 Else vier Science B.V. All rights reserved.