PSI XFEL SIMULATIONS WITH SIMPLEX AND GENESIS

The numerical simulation results of the SASE FEL process for PSI XFEL project are presented. The main purpose of the investigations using FEL simulation codes SIMPLEX and GENESIS is the reliable definition of the undulators design parameters (K value, period, segment length, number of segments) that provide desirable radiation characteristics such as wavelength, bandwidth, saturation length, peak power and the brightness. INTRODUCTION One of the approaches to significantly reduce the costs for the construction of an X-Ray FEL would be the use of new electron sources with reduced transverse beam emittance combined with sufficient peak current. In PSI XFEL project [1] this is supposed to achieve by developing a Low Emittance Electron Gun based on field emitter technology [2]. After a 250 MeV injector and first bunch compressor, the electron beam with normalized emittance of 0.2 mm-mrad is accelerated to the maximum energy of 6 GeV in two S-Band linear accelerators separated by the second bunch compressor. The compressed electron beam with 1.5 kA peak current is delivered at different energies to three undulator sections to produce SASE FEL (Fig.1). It is assumed that output radiation energy should be tunable between the limits ~0.124 and ~12.4 keV, which corresponds to the wavelength region 0.1 – 10 nm. The photon beam wavelength is tuned by electron energy and undulator gap variations. Branch 1 undulator will produce linearly polarized FEL radiation with wavelengths starting of 0.1 nm to 0.3nm utilizing electron beam with the energy that can be varied up to 5.8 GeV. For that purpose a separate linear accelerator segment is foreseen. Branch 2 and branch 3 undulators’ radiation fundamental wavelengths are also tuneable within the range of 0.3 – 1.0nm and 1.0 – 10nm correspondingly. They can produce radiation with both linear and circular polarization. Each of the undulator systems should be compact enough (~70 m) to fit the area anticipated for the construction of the PSI XFEL facility. In those three FELs three different methods of the fundamental mode wavelength tuning will be applied. In the branch 2 and branch 3 undulators the gap is variable and additional linac segment enables one to change also the energy of the beam delivered to branch2 FEL. At the design stage one can use the theoretical considerations [3] and the variational solution of the FEL dispersion relation equation [4] to obtain crude estimates for the most important FEL performance parameters. We carried out numerical simulation study of the FEL process applying two different three-dimensional simulation codes SIMPLEX [5] and GENESIS [6] to find out FEL design parameters. The main design parameters of the PSI XFEL project are presented in Table 1. The electron bunch charge is 0.2 nC, the rms length after BC1 and BC2 is 40 μm and the required energy spread is 0.5 MeV. Figure 1: Schematic layout of the PSI XFEL. Table 1: PSI XFEL main design parameters FEL Branch 1 Branch 2 Branch 3 Beam energy [GeV] 5.8-3.4 5.3– 4.5 3.7 Wavelength [nm] 0.1– 0.3 0.3 – 1.0 1.0 – 10 Wavelength tuning energy energy and gap gap Type Planar APPLE APPLE Period [mm] 15.0 36.6 52.0 Section length [m] 4.50 4.39 4.16 FODO period [m] 10.5 10.3 9.8 Beta function [m] 15 15 15 Saturation length [m] 31.4 40.5 35.4 Peak power [GW] 2 – 6 10 – 20 10 – 20 BEAM FOCUSING Since the total length of each undulator system is important the numerical simulations have been performed to get the optimal value for the average beta function that minimizes the saturation length. The focusing lattice of each branch of the undulators is FODO type and comprised of a set of single focusing or defocusing quadrupole magnets inserted into every 75 cm drift section between the undulator segments. Both analytical and numerical simulation studies indicate that saturation length decreases with the reduction of the average beta function. The SIMPEX simulation results for branch1 is presented in Table 2, where P is the peak power, L is the saturation length. The same pattern is observed for all three undulators. The minimal possible values for the averate beta function ___________________________________________ khachatryan@asls.candle.am Proceedings of EPAC08, Genoa, Italy MOPC012 02 Synchrotron Light Sources and FELs A06 Free Electron Lasers