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

A quasiperiodic wiggler configuration made from two periodic wigglers of ir rational period ratio is considered for application to free-electron lasers (FEL's). With an analytical formulation of its spontaneous emission, spect ral behavior of a quasiperiodic wiggler is studied. it is found that the no nlinear 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 mul tiples of the fundamental FEL radiation frequency. By adjusting the relativ e field strengths of the two constituent wigglers and their period ratio, t he usual FEL harmonics can be suppressed considerably. This unique spectral characteristic of quasiperiodic wigglers may be used to relieve the proble m of minor damage in some free electron lasers operating in ultraviolet and x-ray wavelengths. [S1063-651X(99)14702-0].