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)
SLAC, Menlo Park, California, USA
LBNL, Berkeley, California, USA
The complete design for self-seeding the LCLS at soft X-ray energies from 400 to 1000 eV based on a grating monochromator is described. The X-ray optics system consists of a toroidal variable-line-space (VLS) grating with a resolving power greater than 5000 for creating a nearly transform-limited seed pulse from the upstream SASE undulator for pulse durations of the order of 25 fs, and focusing mirrors for imaging the seed pulse onto the downstream seeding undulator. Diagnostics for ensuring overlap with the reentrant electron beam are included in the design. The optical system is sufficiently compact to fit within a single 3.4 m LCLS undulator segment. The electron chicane system which serves to delay the electron beam to match the less than 1 ps delay from the optical system is similar to the chicane used in the hard X-ray self-seeding at LCLS. The seeded FEL pulse is expected to be nearly transform-limited with a bandwidth in the 10-4 range, potentially increasing the low-charge FEL X-ray peak brightness by 1-2 orders of magnitude.
SLAC, Menlo Park, California, USA
UCLA, Los Angeles, California, USA
We study an improved-SASE (iSASE) scheme to generate narrow bandwidth Free Electron Laser (FEL) by introducing phase shifter between the undulator segments to speed up the slippage. Due to the shift of the FEL pulse with respect to the electron bunch, spikes with phase relation develop; therefore the coherent length increases faster. Furthermore, due to the similarity of these spikes in the temporal domain with respect to the spikes generated in the previous sections, the spectrum of such an FEL containing a regular temporal spike train is intrinsically narrower than that of a conventional SASE FEL. Here, we report study results for a soft x-ray FEL at 6 nm and a hard x-ray at 0.15 nm. With a narrower bandwidth, the FEL responds to a tapered undulator more efficiently than a conventional SASE FEL does. This then make it possible to reach high power. Analysis is carried out with GENESIS numerical simulation as well as 1-D analytical calculation.
SLAC, Menlo Park, California, USA
Tolerance and stability are important issues for designing and operating accelerator and FEL. Jitter can come from various sources. We identify and study well known sources as well as some particular ones, important for a seeded tapered high power FEL. Seed laser phase error, electron bunch current profile, self-seeding residual density bunching and energy modulation after the chicane, undulator wakefield, and phase errors in the undulator breaks are just a few important examples. Schemes to improve the stability of a seeded FEL are also discussed.
USTC/NSRL, Hefei, Anhui, People's Republic of China
SLAC, Menlo Park, California, USA
THOC04
SLAC, Menlo Park, California, USA
LBNL, Berkeley, California, USA
SINAP, Shanghai, People's Republic of China
[*] P. Emma et al., PRL 92, 074801 (2004).
[**] G. Geloni et al., DESY 10-008.