Fractional harmonic radiation from a prebunched free electron laser

Summary form only given. Free electron laser (FEL) devices driven by prebunched electron beams have been a subject of intensive research. One common feature of these prebunched FEL systems is that the electron beam receives a sinusoidal modulation. However if the electron beam is not modulated at one sinusoidal frequency, unconventional interaction mechanisms may be uncovered and this can lead to an improved system performance. In this contribution, we consider the usage of two sinusoidal modulation signals at different frequencies, instead of one sinusoidal modulation signal. The electron modulation is thus not periodic but quasi-periodic. This quasi-periodicity may be realized either from substantially nonlinear beam-wave interaction, or from an external modulation induced by a quasi-periodic device for instance a double-period wiggler magnet structure. This approach is motivated by recent studies of quasi-periodic systems in very different context, for instance quasicrystals in crystallography.