An Experiment on FEL Efficiency Enhancement with a Variable Wiggler

At TRW, an experiment is in progress to deomonstrate efficiency enhancement on a FEL. This is accomplished by trapping the electrons in the pondero-motive potential well generated by a linearly polarized CO2 laser beam and a tapered magnetic wiggler. An adiabatic reduction in the wiggler amplitude and/or wavelength results in an electron beam energy loss which is transformed into radiation. In the TRW experiment, the electron beam is generated by an rf linac with an energy of 25 MeV and 40 A in the microbunch. The CO2 laser driver is a 1 Joule injection mode locked TEA laser delivering 100 MW in the interaction region. The 3 kG wiggler field is generated by SmCo permanent magnets with a magnet gap of 1.45 cm and a wavelength of ¿w = 3.56 cm. The interaction region is 300 cm long. The system is operated at a high repetition rate to facilitate diagnostics. A trapped electron extraction of approximately 3% is expected based on theoretical predictions.