Coherent hard X-ray production by cascading stages of High Gain Harmonic Generation

We study a new approach to produce coherent hard X-rays by cascading several stages of High Gain Harmonic Generation (HGHG). Our calculation shows that such a scheme is feasible within the present technology. Compared with the Self-Amplified Spontaneous Emission (SASE) scheme for the Linac Coherent Light Source (LCLS), the HGHG scheme can provide radiation with full longitudinal coherence, narrower bandwidth, more stable central wavelength, more controllable pulse length, and much higher performance stability. The electron bunch parameters used in our calculation are those of the DESY TTF and SLAC LCLS projects. To achieve similar output power, i.e. about 15 GW, the total length of the HGHG scheme will be 88 m, while that of SASE scheme will be around 100 m.