DEEI, Trieste
ELETTRA, Basovizza
SLAC, Menlo Park, California
In a harmonic generation free electron laser (HG FEL), the electron beam entering the undulator can have an initial energy curvature besides an initial energy chirp. Solving the Vlasov-Maxwell equations within the 1D model, we derive an expression for the Green function for the seeded HG FEL process for the case of the electron beam having both an energy chirp and an energy curvature. We give an asymptotic closed form which is a good approximation in the exponential growth regime, and a series expression that allows the evaluation of the field envelope along the undulator in both lethargy and exponential growth regime. The latter is useful to study the HG FEL behavior in the short modulator, like that of the FERMI@Elettra project. The FEL radiation properties such as central frequency shift and frequency chirp are studied considering Gaussian laser seeds of different temporal duration in respect to the Green functions temporal duration. The energy chirp and curvature of the electron bunch result in a time dependent bunching factor for the FEL start-up process in the radiator of the HG FEL like the FERMI@Elettra. The coherence properties of the FEL are examined.
UMD, College Park, Maryland
ELETTRA, Basovizza
SLAC, Menlo Park, California
Recent observations of coherent optical transition radiation (COTR) at LCLS and other laboratories have been recognized as a signature of theμbunching instability, which affects the longitudinal phase space of the electron beam and ultimately the performance of the Free Electron Laser. In addition, the COTR emission limits the utility of OTR screens as beam profiling diagnostics. In an effort to understand and predict the extent of COTR emission and to help specify required instrumentation for new FELs, we have developed codes at UMD and SLAC-LCLS that use the output from the ELEGANT particle tracking code to predict the emission of COTR at specific wavelengths or within a band width. The COTR codes provides plots of the intensity patterns in the transverse plane, simulating a virtual OTR screen. Both incoherent and coherent intensities are produced thus providing an estimate of theμbunching gain at the observed wavelengths. Since the ELEGANT simulation of microbunching strongly depends on the number of particles, efforts have been carried out to speed up the COTR code analysis. The results of these codes applied to the LCLS and FERMI@elettra linac FELs are presented.
UW-Madison/SRC, Madison, Wisconsin
SLAC, Menlo Park, California
In high-current magnetic bunch compression, shot-noise-induced energy and current fluctuations at the chicane entrance may cause microbunching. For the case where the energy fluctuations are the primary cause of microbunching, we perform approximate simulations with fewer particles than the bunch population by using a reduced value of the space-charge impedance upstream of the chicane. This method is applied to bunch-compressor designs for the Wisconsin Free Electron Laser (WiFEL).
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.
ASCo, Clayton, Victoria
Argonne National Laboratory, Office of Naval Research Project, Argonne
Monash University, Faculty of Science, Monash University
SLAC, Menlo Park, California
This paper describes the implementation of a neural network hybrid controller for energy and bunch length stabilization. The structure of the controller consists of a neural network (NNET) feed forward control, augmented by a conventional Proportional-Integral (PI) feedback controller to ensure stability of the system. The system is provided with past states of the machine in order to predict its future state, and therefore apply appropriate feed forward control. Experiments performed at the Australian Synchrotron showed the ability of the NNET to cancel multiple frequency energy jitter and the successful augmentation of the system by a PI algorithm. The LCLS experiments showed that the system can be expended to predict and correct coupled energy-bunch length deviations, and showed the improved jitter attenuation by the NNET system in comparison to the PI algorithm alone. Focus is also made on the machine response that needs to be accurately known to best operate the correction. When machine settings are modified, the response is re-calculated with the help of a model, and slight adjustments are made to optimize the energy jitter reduction as the control is operating.
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.