Spectral characteristics of the seeded FEL using higher harmonic generation in gas at the SCSS test accelerator

Seeding a FEL with high harmonic generation from a Xenon gas cell at the SCSS test accelerator was successfully demonstrated at a wavelength of 160 nm (the 5th harmonic of a Ti: sapphire laser) in December 2006 [1]. After the improvement of the accelerator performance in 2007, we performed detailed studies of the spectral characteristics of the seeded FEL [2]. Compared to the spiky spectrum of SASE, the seeded FEL can produce a quasi-Gaussian spectrum with a single peak. In addition, the lasing wavelength is fixed at the wavelength of the seed, even if the deflection parameter K-parameter of the undulator is varied in a certain range. Therefore, it is expected that the spectral stability becomes much better than SASE against the fluctuation of undulator fields or beam energy.