Free electron laser spectrum control with a quasiperiodic wiggler configuration

Recently, we have studied an unusual wiggler magnet configuration comprising two conventional periodic wigglers. It was shown that if the periods of the two constituent wiggler components are not integrally but irrationally related then considerable radiation is generated at frequencies that are irrational 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 the FEL spectrum. In this contribution, we consider the spontaneous emission of electrons in such a quasiperiodic wiggler structure with and without the electron energy spread effects. Numerical examples are used to demonstrate that significant radiation can be generated simultaneously at both the usual FEL harmonics and irrationally related frequencies. When the wiggler parameters are appropriately adjusted, the usual integrally related FEL harmonics are suppressed considerably. This feature may be used to relieve the mirror degradation problem in some ultraviolet and X-ray FELs.