SLAC, Menlo Park, California
LBNL, Berkeley, California
We present experimental studies of the gain length and saturation levels from 1.5 nm to 1.5 Å for a variety of conditions at the Linac Coherent Light Source (LCLS). By disrupting the FEL process with an orbit kick, we are able to measure the X-ray intensity as a function of the undulator length. This kick method is cross-checked with the method of removing undulator sections. We measure the FEL gain length as a function of X-ray wavelength, laser-heater induced energy spread, beta function and peak electron current. We also study the X-ray intensity level and FEL-induced electron energy loss after saturation as a function of undulator K value to determine the optimal taper. The experimental results are compared to analytical formulae and simulations.
SLAC, Menlo Park, California
Undulator magnets for the LCLS need to be maintained at a very stable and accurate temperature in order to stay within the tolerance required for the FEL. At the LCLS the temperature of the undulator magnets is mainly determined by the temperature of the surrounding air. Furthermore, the climate control system which controls the temperature of the air must never accidentally go out of a safe operating range of ± 2.5 C or the magnets may lose calibration and have to be removed and remeasured. This was one motivation for the sighting the Unduator Hall underground in a tunnel where the thermal inertia of the surrounding earth provides stability. In this paper we describe the technical solution adapted by the LCLS for controlling the air temperature in the Undulator Hall and its initial performance. We also discuss thermal issues of heat balance and steady state and transient temperature behavior of the undulators system and the surrounding earth.
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.
SLAC, Menlo Park, California
LLNL, Livermore, California
Stanford University, Stanford, California
European X-ray Free Electron Laser Project Team, c/o DESY, Hamburg
Precision in-situ measurements of relative undulator segment K parameters were made at the LCLS and are reported here. We describe the methods used, systematics errors, and signal levels. A method for determining the central ray from each undulator segment was developed to control the effect of angle-energy correlation of the spontaneous radiation on the photon energy spectrum. A variety of photon-energy sensitive detectors were employed, including: Ni foil, the yttrium component in a YAG screen, and a narrow band monochromator followed by either a photodiode or a YAG screen. Different harmonics of the spontaneous radiation were also used.
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.