SLAC, Menlo Park, California
If the energy spread of a beam is larger then the Pierce parameter, the FEL gain length increases dramatically and the FEL output gets suppressed. We show that if the energy distribution of such a beam is modulated on a small scale, the gain length can be noticeably decreased. Such an energy modulation is generated by first modulating the beam energy with a laser via the mechanism of inverse FEL, and then sending it through a strong chicane. We show that this approach also works for the optical klystron enhancement scheme. Our analytical results are corroborated by numerical simulations.
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
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
LCLS capabilities can be significantly extended with a second undulator aiming at the soft x-ray spectrum (0.5- 5 nm). To allow for simultaneous hard and soft x-ray operations, 14 GeV beams at the end of the LCLS accelerator can be intermittently switched into the SLAC A-line (the beam transport line to End Station A) where the second undulator may be located. Recently, a new optics has been designed to transport the LCLS beam through the A-Line while preserving the beam brightness. In this paper, we discuss the A-Line Soft X-ray FEL design ─ parameter selections and performance expectations with an energy-chirped LCLS beam as required by the A-Line optics. Start-to-end simulations using realistic LCLS beams show that it is possible to generate ~100 GW FEL power with the pulse duration as short as 1-fs.
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.
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.