Parameter Optimizations and Performances for the Low-Charge Beams in PAL Free-Electron Laser

We investigated optimum choices of beam and accelerator parameters for low-charge beams of self-amplified spontaneous emission (SASE) free-electron laser (FEL) at PAL. The FEL system is designed to provide a short pulse beam to generate the high power of the wavelength of 1-Å in the undulator. The designed linac provides the pulses of 10 fs rms and 3.85 fs rms for the charges of 0.2 nC and 0.02 nC, respectively. Parameters of the two-stage bunch compressors are optimized to reduce the effects of coherent synchrotron radiation. The bunch compressors are also designed such that the effects of nonlinearities due to wakefields, rf curvature, and second-order momentum compaction become as small as possible in the linear accelerator. Through these parameter optimizations, we have minimized the bunch charges that could decrease effects of emittance dilution and growth of energy spread, which may deteriorate the SASE performance. Start-to-end tracking simulations include the longitudinal and transverse wakefields, and the effects of errors such as rf gun timing, rf phase, and rf voltage are investigated. We investigated the effectiveness of a laser heater that may give Landau damping to suppress the microbunching beam instability. It is shown that a laser heater may suppress the microbunching beam instability in the FEL system. We also investigated the effects of the ac and dc resistive wall wakefields in a long undulator. It is shown that energy variations induced within the bunch over the length of the undulator are acceptable. As a result, through these optimizations studies, we could get beam parameters and accelerator parameters that can provide the FEL power of the 60 GW with FEL pulse of 10 fs and 3 fs in the wavelength of 1-Å in around a 50-m-long undulator for the charges of 0.2 nC and 0.02 nC, respectively. It is shown by our optimizations that our designed FEL system shows required performances for the low-charge beams in the 10 GeV.