SLAC, Menlo Park, California, USA
LCLS is presently operating with a third copper photocathode in the original rf gun, with a quantum efficiency (QE) of ~1x10-4 and projected emittance eNx,y=0.45 μm at 250 pC bunch charge. The spare LCLS gun is installed in the SLAC Accelerator Structure Test Area (ASTA), processed to the design rf gradient of >120 MV/m. As part of a wider photocathode R&D program, a UV laser system and additional gun diagnostics are being installed at ASTA to measure QE, QE lifetime, and electron beam emittance under a variety of operating conditions. The near-term goals are to test and verify the spare photocathode production/installation sequence, including transfer from the final holding chamber to the rf gun. Mid- and longer-term goals include development of a rigorous understanding of plasma and laser-assisted surface conditioning and investigation of new, high-QE photocathode materials. In parallel, an x-ray photoemission spectroscopy station is nearing completion, to analyze Cu photocathode surface chemistry. In this paper we review the status and anticipated operating parameters of ASTA and the spectroscopy test chamber.
SLAC, Menlo Park, California, USA
The Facility for Accelerator science and Experimental Tests (FACET) at SLAC provides a high peak current, sub-ps bunched beam that is ideal for THz photon generation via coherent transition radiation. This paper presents preliminary characterization of the THz pulses generated by FACET electron beam. A one-micron thick Ti foil has been inserted into the beam path and the radiated photons collected. Michelson spectroscopy yields frequency content spanning from 0.25 THz to 2.3 THz and peaked at around 0.5 THz. Multiple scans at different bunch compression show a monotonic increase of the peak radiation frequency as the electron bunch gets shorter. Using the Kramers-Kronig relation, the temporal profile of the THz pulse is reconstructed from the power spectrum indicating a ~4 picosecond main pulse followed by a long oscillating tail due to the water absorption lines and detector response. Knife-edge scans measure a 4.4 mm x 4.8 mm transverse spot size at the focal point of the THz optical path. The total collected energy per pulse is 0.69 mJ measured by a Joulemeter. Fitting this total energy to the spatiotemporal profile of the THz pulse yields peak e-field amplitude of 1.5 MV/cm.
WEYB02
SLAC, Menlo Park, California, USA
ANL, Argonne, USA
TISNCM, Troitsk, Russia
We report on experimental results of FEL self-seeding with Angstrom wavelengths at the Linac Coherent Light Source (LCLS) at SLAC. The scheme, suggested at DESY*, replaces the 16th 4-m long undulator segment (out of 33 total) with a weak magnetic chicane and a diamond-based monochromator in Bragg transmission geometry. The monochromatized SASE FEL pulse from the first half of the undulator line then seeds the second half. This demonstration of hard x-ray self-seeding is shown to narrow the FEL bandwidth by a factor 40-50, allows longitudinally coherent x-ray pulses near the Fourier-transform limit, and may eventually allow an increases in peak brightness by 1-2 orders of magnitude after applying an aggressive undulator field taper.
* G. Geloni, V. Kocharyan, E. Saldin, DESY 10-133, Aug. 2010.
