The high-gain harmonic-generation free-electron laser experiment

The High Gain Harmonic Generation Experiment has been performed at the Accelerator Test Facility and the measured results are presented. The 41.3 MeV electron beam is seeded by 10.6 μm 0.7 MW power CO₂ laser beam. The FEL interaction takes place in a periodic magnetic array (wiggler) comprising three elements: Modulator, dispersive section and radiator. The energy of the electrons is modulated by a seed laser in the modulator, which is tuned to 10.6 μm wavelength. The energy modulation is converted into a density modulation in the dispersive magnet to produce microbunching. The radiator is tuned to one of the higher harmonies of the seed laser beam, which is 5.3 μm in this case. In the radiator section, the microbunched electron beam starts radiating coherently and the radiation is amplified exponentially until it reaches saturation.; When the seed laser is turned off, the radiator may still produce radiation, based on the Self Amplified Spontaneous Emission (SASE) process. The energy, spectrum and the gain of the SASE FEL were measured.; HGHG radiation is produced with the seed laser turned on. The HGHG output has been characterized. Properties such as the energy, the spectrum and the pulse length were measured and compared with the SASE case. The HGHG spectrum was measured to be of the order of magnitude narrower than the SASE case and longitudinal coherence was demonstrated. The results were found to be in agreement with simulations. Special purpose diagnostics were developed for carrying out some of these measurements.; The preliminary harmonic content of the HGHG was measured. The second (2.65 μm) and the third (1.77 μm) harmonics of the fundamental (5.3 μm) were observed. The rapid amplification of the higher harmonic reveals that a saturation (or near saturation) condition has been achieved. The measured ratio of the harmonics to the fundamental was found to be consistent with the simulations.; Finally the HGHG output was steered using the electron beam stewing. This measurement may have importance when we want to cascade the HGHG process to the X-Ray regime, which requires more than one HGHG sections.