Using VUV high-order harmonics generated in gas as a seed for single pass FEL

In this paper we illustrate the main features of a new scheme which aims at obtaining coherent radiation at very short wavelengths (down to 1.5 nm) by using the Harmonics of an intense laser, generated in a gas, as a seed to inject in a long undulator, either in the pure SASE or in the HGHG configuration. The unique spatial and temporal coherence properties of the High-Order Harmonics are then conserved, and the High Gain FEL amplifier system saturates up to the GW level. An experiment is here proposed, to be performed on the new compact LINAC-based source SCSS, currently under development in SPring-8.