A high gain harmonic generation (HGHG) FEL can provide high-power radiation with a full transverse and longitudinal coherence and a shorter undulator than a conventional SASE FEL wherein the radiation starts from noise. This method is recognized as an advanced source of coherent radiation as in the FERMI FEL at Trieste. More instances would be the flagship FLASH II facility under construction at DESY or the proposed NGLS at Berkley. But the HGHG effect was demonstrated for the first time at the ATF in the experiment that is schematically shown below.
Configuration for the HGHG FEL experiment.
In this experiment, the FEL was seeded with a CO2 laser at a wavelength of 10.6 Ám and produced, a longitudinally coherent, Fourier transform limited output at a second harmonic of the seed laser (5.3 Ám). The experiment verified the theoretical foundation of this concept and prepared the way for applying this technique in other spectral regions
Doyuran, A., et al., Characterization of a High-Gain Harmonic-Generation Free- Electron Laser at Saturation. Phys Rev Lett, 2001. 86(26): p. 5902-5905.70.