Free Electron Lasers Based on Multi-Stage Tapered RF Wigglers

We propose a high-efficiency regime of a “multi-stage” trapping in a Free Electron Laser (FEL). This FEL scheme uses strongly tapered flying radio frequency (RF) undulator sections to be fed by short (nanosecond) gigawatt-power microwave pulses produced by existing Backward Wave Oscillators (BWOs). In this regime, phase locking of the RF sources is not necessary. The proposed method promises an efficiency at level of several percents in FELs driven by modern laser-plasma accelerators producing bunches with ~1 % energy spread. Low-power tests with 30 GHz undulator prototypes have shown that up to 50% tapering of the periodicity is obtainable. Multi-stage tapering is also appealing for X-ray FEL oscillators (XFELO). An XFELO, consisted of not tapered and tapered undulators, can have self-modulated Q-factor of a cavity. This allows XFELO to start up faster and at the same time to provide a high efficiency in steady state regime. Multi-staging might also improve an XFELO concept where the first uniform undulator is inserted in an optical cavity, but the second external undulator works in Self-Amplified Spontaneous Emission (SASE) regime consuming the microbunched electron beam generated by the first undulator. The idea is based on the use of smart reflecting mirrors in a cavity with complicated shapes. These reflectors due to a proper design produce an eigenmode with zero-field in the center of one of mirrors so that electrons can be out-coupled flying straight through a hole in this mirror.