TUOAI02
ANL, Argonne, USA
SLAC, Menlo Park, California, USA
TISNCM, Troitsk, Russia
LBNL, Berkeley, California, USA
Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
The Linac Coherent Light Source (LCLS) has produced extremely bright hard x-ray pulses using self-amplified spontaneous emission (SASE) since 2009. In SASE, the electron beam shot noise initiates the FEL gain, resulting in output radiation characterized by poor temporal coherence and a fluctuating spectrum whose normalized width is given by the FEL bandwidth. Recently, colleagues at DESY suggested a self-seeding scheme for the LCLS to reduce the bandwidth*. Here, the SASE produced in the first half of the undulator line is put through a simple diamond-based monochromator; the resulting monochromatic light trailing the main SASE pulse is used to seed the FEL interaction in the downstream undulators. We report on the experimental results implementing such a scheme at the LCLS, in which we have measured a reduction in bandwidth by a factor of 40-50 from that of SASE at 8-9 keV. The self-seeded FEL operates close to saturation, with the maximum output energy approximately equal to that with no seeding for low charge. The observed level of power fluctuations in the seeded output is presently rather large, and future plans focus on discovering their origins and reducing their magnitude.
* Geloni, V. Kocharyan ,and E.L. Saldin, DESY 10-133, arXiv:1008.3036 (2010)
ANL, Argonne, USA
Fluctuations in relativistic electron beams for free-electron lasers (FELs) exhibit both collective and individual particle aspects, similar to that seen in non-relativistic plasmas. We show that the density fluctuations are described by a linear combination of the collective plasma oscillation and the random individual motion of Debye-screened dressed particles. The relative importance of the individual to the collective motion is determined by comparing the fluctuation length scale divided by two pi with the relativistic beam Debye length. Taking into account the fact that the velocity spread is caused by both the energy spread and the angular divergence, we derive a simple formula for the minimum value of the Debye length using a solvable 1-D model. For electron beams used for x-ray self-amplified spontaneous emission (SASE) we find that the Debye length is comparable to the radiation wavelength, and that therefore the collective motion is not relevant.