Control of harmonic generation in a free-electron laser with a quasiperiodic wiggler configuration

A quasiperiodic wiggler configuration made from two periodic wigglers of irrational period ratio is considered for application to free-electron lasers (FEL’s). With an analytical formulation of its spontaneous emission, spectral behavior of a quasiperiodic wiggler is studied. It is found that the nonlinear coupling between the two constituent wigglers of the quasiperiodic wiggler configuration leads to significant spontaneous emission at both the usual FEL harmonic frequencies and new frequencies that are irrational multiples of the fundamental FEL radiation frequency. By adjusting the relative field strengths of the two constituent wigglers and their period ratio, the usual FEL harmonics can be suppressed considerably. This unique spectral characteristic of quasiperiodic wigglers may be used to relieve the problem of mirror damage in some free electron lasers operating in ultraviolet and x-ray wavelengths.