SLAC, Menlo Park, California
Recent experiments on the LCLS accelerator have demonstrated low emittances for 20-pC bunches, with evidence for few-femtosecond electron bunch lengths, although the existing beam diagnostics do not allow a direct measurement of the bunch length. Simulations confirm that the LCLS accelerator can be operated at low charge (20 pC) while maintaining the nominal 3 kA peak current and with transverse emittances below 0.4 microns. An x-ray pulse duration of 2 femtoseconds with 3× 1011 photons is predicted, and nearly a single longitudinal spike may be obtained for soft x-ray wavelengths. We report on the operation of the accelerator and undulator with short electron bunches and present supporting simulation results.
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
JAI, Oxford
USTRAT/SUPA, Glasgow
The New Light Source (NLS) project was launched in April 2008 by the UK Science and Technology Facilities Council (STFC) to consider the scientific case and develop a conceptual design for a possible next generation light source based on a combination of synchronised conventional laser and free-electron laser sources. The requirement identified for the FELs was continuous coverage of the photon energy range 50-1000eV with variable polarisation, 20fs pulse widths and good temporal coherence to as high a photon energy as possible. This paper presents a design study of three separate FELs which in combination satisfy these requirements. It is proposed to use an HHG seed source tunable from 50-100eV giving direct seeding at the fundamental FEL wavelength up to 100eV, then one or two stages of harmonic upconversion within the FEL to reach the higher photon energies. FEL simulations using realistic electron beam distributions tracked from the gun to the FEL will be presented, illustrating the predicted coherence properties of the FEL output at different photon energies.
SOLEIL, Gif-sur-Yvette
Imagine Optic, Orsay
BNL, Upton, Long Island, New York
LOA, Palaiseau
RIKEN/SPring-8, Hyogo
JASRI/SPring-8, Hyogo-ken
The VUV Self-Amplified Spontaneous Emission of the SPring-8 Compact SASE Source (SCSS) Test Accelerator is characterized at different stages of amplification up to saturation [1]. Experimental measurements are performed by use of a VUV Hartmann wavefront sensor. This kind of sensor gives access to both intensity and phase profiles of the incoming beam. We characterize the mode selection when approaching the saturation regime of the FEL. Optical quality of the saturated SASE radiation is measured to be better than Lambda/5 PV and Lambda/22 rms (Lambda = 61.5 nm) depending on the machine optimization. Moreover, pointing of the beam as well as spatial structure, size and position of the source are retrieved and their shot-to-shot fluctuations investigated. Analytical [2] and numerical calculations [3], using SRW and GENESIS codes, show good agreement with the experimental measurements. All these elements are of crucial importance for a better understanding and optimization of the FEL and of course for user applications requiring a stable focused beam on their samples. We are grateful to the SCSS Test Accelerator Operation Group at RIKEN for continuous support in the course of the studies
[1] R. Bachelard et al., "Wavefront and Transverse Structure of the FEL Self-Amplified Spontaneous Emission", to be submitted
[2] M. Xie, NIMA 445, 59(2000)
[3] O. Chubar et al., Proc. EPAC-98, 1177(1998)
BNL, Upton, Long Island, New York
SAIC, McLean
We report the first experimental characterization of efficiency and spectrum enhancement in a laser-seeded free-electron laser (FEL) using a tapered undulator. Output and spectra in the fundamental and 3rd harmonic were measured versus distance for uniform and tapered undulators. With a 4% field taper over 3 m, a 300% (50%) increase in the fundamental (3rd harmonic) output was observed. A significant improvement in the spectra with the elimination of side-bands was observed using a tapered undulator. The experiment is in good agreement with predictions using the MEDUSA simulation code.