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
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.
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.