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.
SLAC, Menlo Park, California, USA
Self-seeding of a hard x-ray FEL was demonstrated at LCLS in January 2012 and produced a factor of 40-50 bandwidth reduction using a electron bunch charge of 20-40 pC*. For many hard x-ray users, the photon intensity after a monochromator is an important performance parameter. In this paper, we report results from a subsequent study of self-seeding performance using the Si (111) K-monochromator with a full bandwidth of 1.2 eV at 8.2 keV. These include a direct comparison of the average intensity of the monochromatized seeded beam with that of a monochromatized fully tuned-up SASE beam, in both cases using 150 pC bunch charge. The intensity distribution, fluctuations, and spatial profiles of the monochromatized radiation are studied and compared.
* J. Amann, et. al, Nature Photonics, to be published
