LBNL, Berkeley, California, USA
In linac driven free electron lasers, the final electron beam quality is constrained by the low energy (<100 MeV) beam dynamics at the injector. In this paper, we present studies and the optimized design for a high-repetition (>1 MHz) injector in order to provide a high brightness electron beam. The design effort is also extended to multiple modes of operation, in particular different bunch charges. The effects of space charge and low energy compression on the electron beam brightness are also discussed for the different modes.
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.
LBNL, Berkeley, California, USA
In the self-seeding scheme, the longitudinal coherence and spectral density of an unseeded FEL can be improved [*] by placing a monochromator at a location before the radiation reaches saturation levels, followed by a second stage of amplification. The final output pulse properties are determined by a complex combination of the monochromator properties, undulator settings, variations in the electron beam, and wakefields. We perform simulations for the output of SASE and self-seeded configurations for a soft x-ray FEL using both idealized beams and realistic beams from start-to-end simulations. These studies include cross-planar undulators dedicated to polarization control [**].
[*] J. Feldhaus, E.L. Saldin, J.R. Schneider, E.A. Schneidmiller, and
M.V. Yurkov, Optics Commun. 140 (1997) 341-352.
[**] K.-J. Kim, Nucl. Instrum. Methods A 445 (2000) 329-332.
LBNL, Berkeley, California, USA
DESY, Hamburg, Germany
SLAC, Menlo Park, California, USA
The Linac Coherent Light Source (LCLS) is an x-ray Free-Electron Laser (FEL) facility now in operation at SLAC. One of the limiting effects on electron beam brightness is the coherent synchrotron radiation (CSR) generated in the bunch compressor chicanes, which can significantly dilute the bend-plane (horizontal) emittance. Since simple emittance measurements* do not tell the full story, we would like to see the time-dependent CSR-kicks along the length of the bunch. We present measured images and simulations of the effects of CSR seen on an intercepting beam screen just downstream of the LCLS BC1 chicane while powering a skew quadrupole magnet near the center of the chicane [ ]. The skew quadrupole maps the time coordinate of the pre-BC1 bunch onto the vertical axis of the screen, allowing the time-dependent CSR-induced horizontal effects to become clearly visible.
* K. Bane et al., Phys. Rev. ST Accel. Beams 12, 030704 (2009).
** K. Bertsche, P. Emma, O. Shevchenko, "A Simple, Low Cost Longitudinal Phase Space Diagnostic", PAC'09, Vancouver, BC, Canada, 2009.
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.
LBNL, Berkeley, California, USA
DESY, Hamburg, Germany
SLAC, Menlo Park, California, USA
There is great demand for a fast, accurate method to measure the absolute bunch length of an electron beam in a linac. Many ideas are available, with one of the most attractive based on the transverse RF deflector*. Since this specialized technology can be costly and unavailable, we introduce an alternate method using accelerating RF with the same robust characteristics (fast, accurate, and absolute). This method is based on the 'RF zero-phasing' scheme**, but includes several significant improvements based on experience with the RF deflector method.
* R. Akre et al., Proc. of PAC-01, p. 2353.
** D. X. Wang et al., Proc. of PAC-97, p. 2020.
SLAC, Menlo Park, California, USA
Laser-based techniques have been widely used for cleaning metal photocathodes to increase quantum efficiency (QE). However, the impact of laser cleaning on the cathode uniformity and final quality of the electron beam is not understood. We are evaluating whether the technique can be applied to revive photocathodes used for electron beam sources of advanced x-ray free electron laser (FEL) facilities, such as the Linac Coherent Light Source (LCLS) at the SLAC. The laser-based cleaning was applied to two separate areas of the LCLS photocathode on July 4 and July 26, 2011, respectively. Since the cleaning performed, routine operation has shown a slow evolution of both the QE and the transverse emittance, with a significant improvement of both over 2-3 weeks. Currently, the LCLS photocathode QE is constant at about 1.2·10-4 with a normalized injector emittance of about 0.3 μm for a 150-pC bunch charge. The laser cleaning technique becomes a viable tool to revive the LCLS photocathode. We present these observations of the QE and emittance evolution after laser-based cleaning of the LCLS photocathode,and the thermal emittance for different QE.