Free Electron Laser Based on a Multi-Stage System of RF Wigglers

We propose a high-efficiency regime of a “multi-stage” trapping in FELs. This FEL scheme use strongly tapered flying RF undulator sections to be fed by short (nanosecond) high-power RF pulses produced by already existing BWOs. In this regime, phase locking of the RF sources is not necessary. The proposed method allows efficiency at level of several per cents in FELs driven by modern laser-plasma accelerators producing bunches of high energy spread.