| Here we study a new approach for producing coherent X-rays by cascading several stages of a High-Gain Harmonic Generation (HGHG) Free-Electron Laser (FEL). Our calculation shows that such a scheme is feasible within the present technology. Compared with the Self-Amplified Spontaneous Emission (SASE) FEL scheme, the HGHG FEL scheme could provide radiation with the following advantages: better longitudinal coherence, narrower bandwidth, a more stable central wavelength, controllable pulse length, and much better stability of the performance. In addition, the HGHG FEL Scheme weakens the requirements for the electron beam emittance. This is called the Natural Emittance Effect Reduction (NEER) mechanism. In contrast to this, in a SASE FEL, the requirements are stringent for both beam emittance and local energy spread. This NEER mechanism makes possible a promising new operation mode using a higher-current; even though unavoidably; higher-emittance electron beam in the Harmonic Generation stages. The coherent X-ray so produced could have a high enough brightness. It could also be used as the seed for exponential growth in a following amplifier where a low-emittance electron beam is used for the FEL interaction. For the purpose of comparison with the SASE FEL scheme, the electron bunch parameters used in our current study are restricted to those used in the SASE FEL of the DESY TTF project and the SLAC LCLS project. To achieve similar out-put power until saturation, the HGHG scheme has a shorter total undulator length than that of the SASE FEL scheme, besides the above mentioned better qualities. |
