SLAC, Menlo Park, California
The very bright electron beam required for an x-ray free-electron laser (FEL), such as the LCLS, is susceptible to a microbunching instability in the magnetic bunch compressors, prior to the FEL undulator. Using a 'laser heater', the uncorrelated electron energy spread in the LCLS can be increased by an order of magnitude to provide strong Landau damping against the instability without degrading the FEL performance. In this paper, we report the commissioning experience with the LCLS laser heater. We present detailed measurements of laser heater-induced energy spread, including the unexpected self-heating phenomenon when the laser energy is very low. We discuss the suppression of microbunching instability with the laser heater and its impact on the LCLS x-ray FEL performance. The experimental results are compared with theory and simulations where possible.
DESY, Hamburg
The free-electron laser user facility FLASH at DESY, Germany is world-wide the only SASE-FEL operating in the VUV and the soft X-ray wavelengths range. Since summer 2005, FLASH operates as a user facility providing almost fully coherent, 10 femtosecond long laser radiation in the wavelength range from 47 nm to 6.5 nm with an unprecedented brilliance - many orders of magnitude higher than conventional facilities. The SASE radiation contains also higher harmonics. Several experiments have successfully used the third and fifth harmonics, in the latter case down to a wavelength of 1.59 nm. Starting autumn 2009, FLASH will be upgraded with an additional superconducting TESLA type accelerating module boosting its beam energy to 1.2 GeV. This will allow lasing below 5 nm. In addition, a 3rd harmonic accelerating module will be installed, which improves the longitudinal phase space and the overall performance of the facility.
SLAC, Menlo Park, California
The effect of the drive laser transverse shape upon the electron beam emittance and FEL performance at 1.5 angstroms was studied at 250 pC for the Linac Coherent Light Source X-Ray FEL. Rectangular grids and cylindrically symmetric shapes were imaged onto the cathode and the emittance and FEL output were measured. Each pattern was truncated by a 1.2 mm diameter iris. The projected and time-sliced emittances as well as the electron bunch shape were measured at 135 MeV using a one micron thick optical transition radiation foil and a transverse RF deflecting cavity. The beam was then compressed and accelerated to 13.7 GeV and transported through the undulator. In our initial measurements, the 1.5 angstrom FEL pulse energy was determined from the energy loss of the electron beam. Future experiments will use an x-ray calorimeter. The gain length was obtained by measuring the FEL output along the undulator by deflecting the electron beam off the optical axis. These emittances and the FEL performance are compared with the nominal uniform transverse shape.
Vanderbilt University, Nashville, TN
High-quantum-efficiency photocathodes used for free-electron lasers tend to be fragile and demand complex drive lasers. Field-emitter arrays eliminate both these problems, but introduce other problems along with interesting new physics. Diamond field-emitter arrays are rugged and forgiving of poor vacuum. They are easily conditioned to give uniform emission, current density on the order of 100 A/cm2 before phase compression, and emittance smaller than 10 μm-radians. In gated versions the emission can be phased to the rf drive and the emittance can be reduced by the focusing effect of the gate. Experimental evidence from diamond pyramids and carbon nanotubes suggests that field emission is enhanced by resonant tunneling through molecules adsorbed on the surface. The emission from individual molecules appears to reach the fundamental limits imposed by the Heisenberg uncertainty principle and by the Pauli exclusion principle.
LBNL, Berkeley, California
At the Lawrence Berkeley National Laboratory, a high-brightness high-repetition rate photo-injector is under fabrication. The scheme is based on a normal conducting 187 MHz RF cavity operating in CW mode and capable of generating an electric field at the cathode plane of ~ 20 MV/m. The electron bunches will be accelerated to ~ 750 keV with peak current, energy spread and transverse emittance suitable for FEL and ERL applications. At the same time, the presence of a vacuum load-lock mechanism jointly with a vacuum system designed to operate in the 10 picoTorr range, will make of this injector a flexible cathode test facility. In particular, it will allow to use "delicate" high quantum efficiency cathodes to generate nC bunches at MHz repetition rate with present laser technology. Construction status and future plans are presented.