High Brightness SASE Operation of X-ray FELs

A method to achieve High-Brightness Self-Amplified Spontaneous Emission (HB-SASE) in the Free Electron Laser (FEL) is described. The method is a development of earlier work in which a series of unequal delays to the electron beam was shown to extend the SASE temporal coherence. It is now shown that the enhanced electron-radiation slippage delocalises the collective FEL interaction and breaks the dependence of the radiation coherence length on the FEL cooperation length. The method requires no external seeding or photon optics and is therefore applicable at any wavelength or repetition rate. Simulations show that the radiation coherence length can be increased by approximately two orders of magnitude over SASE with a corresponding increase in spectral brightness. Examples are shown where HB-SASE may generate near transform-limited X-ray FEL pulses. The dependence of the output on the undulator module length and the choice of delay sequence are discussed.