Study of the Electron Beam Dynamics in the Fermi @ Elettra Linac

A study of the electron beam dynamics in the linac is conducted for the FERMI free electron laser (FEL) founded for construction at the Sincrotrone Trieste [1]. LAYOUT AND DESIGN PARAMETERS The FERMI accelerator layout is schematically shown in Figure 1. It consists of an injector, four linacs, two bunch compressors, a laser heater, and a spreader used to direct the electron beam into one of two undulator lines. Figure 1 A schematic of the FERMI accelerator. The entire length of the machine ~ 150 m. The electron beam energy after injector at the entrance of the laser heater is ~100 MeV and the peak current is ~70A. At the exit of Linac 4 the electron beam energy is approximately 1.2 GeV and the electron peak current is 500 A or 800 A, depending on the bunch length needed by the FEL processes. Three options have been developed: the short bunch (SB) option [2] with a bunch length ~ 200 fs, the medium bunch (MB) option with a bunch length ~ 700 fs and the long bunch (LB) option with a bunch length ~ 1.4 ps. The accelerator was designed with sufficient flexibility to accommodate such variations in bunch parameters. Only the medium and long bunch options are considered here. Other important electron beam parameters include the normalized slice electron beam emittance and the slice energy spread, which are 1.5 micron and 150 keV respectively for all three bunch lengths. Table 1 shows a list of major design parameters. A new parameter, the “flatness”, defines the value of the quadratic component of energy variation along the bunch for which the increase in bandwidth of the x-ray signal due to this variation becomes equal to the Fourier transform limited bandwidth defined by the bunch length. Table 1: Main electron beam parameters