The free electron laser with inverse taper

The undulator design determines the physics of the free electron laser interaction. In the tapered undulator, the resonant electron energy decreases along the undulator length in order to enhance gain and efficiency in strong optical fields. With inverse taper, the resonant electron energy increases along the undulator length. Surprisingly, gain and efficiency are also increased in strong optical fields. In addition, the resultant electron beam energy spread is decreased compared to both the tapered and untapered undulator. A reduction of the final electron beam energy spread is useful when recirculating the beam current as in the Jefferson Lab FEL.