Optical-klystron Configuration for a High-gain X-ray Free Electron Laser

The amplified spontaneous-emission (ASE) regime of a free electron laser (FEL) can be used to produce coherent soft X-ray radiation in a long undulator (5 5 L -< 10 m) [1-6] . In such a regime, the initial incoherent radiation, with an optical power proportional to the number of electrons, is amplified by the electron beam itself as it traverses the interaction region subject to the combined periodic magnetic and radiation fields . This collective phenomenon leads to exponential growth until the system reaches saturation [5,6]. To achieve this situation, the undulator has to be made sufficiently long and the accelerator has to provide a high-brightness electron beam, well above the values required for a multipass oscillator experiment. However, 1 .3E has a number of advantages in comparison with an oscillator : first, good-reflectivity soft X-ray mirrors are not needed ; second, an ASE-FEL only needs a single pulse traversing the undulator reducing the beam-loading problem [7] in the electron-beam accelerator, and thus makes it easier to produce a high-brightness, nearly monochromatic beam . In this work we discuss one possible alternative undulator configuration that will reduce its length, keeping the total power output of the laser approximately constant. An optical klystron [8,9] has been used to improve the gain of FELs in a weak optical field and small gain . We show that the same concept is useful in a short-wavelength ASE-FEL thus allowing for a moderately long undulator. An optical klystron consists of two undulators separated by either a long drift space or a shorter dispersive magnet. This magnet configuration translates the energy spread created in the first Lndulator section (modulator) into significant changes in the electron phase ~ = (k + ko )z wt when the electron pulse arrives at the second undulator section (radiator) . If these changes are arranged in a suitable manner, we can achieve significant bunching and, consequently, a larger optical gain .