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Conceptual Design Report

CDR chapter 4 - FEL Physics

The FERMI@ELETTRA project is based on the harmonic up-shifting of an initial “seed” signal in a single-pass FEL amplifier employing multiple undulators. The basic principles which underlie this approach are: the energy modulation of the electron beam via the resonant interaction with an external laser seed in a first undulator (modu-lator); the use of a chromatic dispersive section to then develop a strong density modulation with large harmonic overtones; the production of coherent radiation by the microbunched beam in a downstream undulator (radiator).  The first stage of the project, FEL-1, aims at generating coherent output radiation in the 40-100 nm spectral range. For these wavelengths, users require short (<100 fs) pulses with adjustable polarization and high temporal and spatial reproducibility. The goal of project’s second stage, FEL-2, is to extend the spectral range down to 10 nm. Present users’ requirements point to long (narrow-bandwidth) pulses with high peak brilliance and adjustable polarization. FEL-1 relies upon a single-stage scheme (i.e., modulator-dispersive section-radiator), like the one already operational at Brookheven [refs]. As for FEL-2, a two-stage harmonic cascade is necessary for reaching short wavelengths. The selected configuration is based on the so-called “fresh bunch” approach [ref], in which the radiation from the first radiator is used to energy-modulate in a subsequent modulator a part of the electron beam that did not interact with the external seed. This chapter is organized as follows. After a general Introduction to the schemes adopted for the design of both FEL-1 and FEL-2 (section 4.1), in section 4.2 an overview is given of FEL basic output requirements and of main phenomena which may affect the FEL performance. Section 4.3 is devoted to the design of undulators and transport lattice. Design and expected performance of FEL-1 and FEL-2 are addressed in Sections 4.4 and 4.5, respectively. Calculations, which rely both upon time-steady input parameters and full start-to-end time-dependent simulations, have been performed using the 3D numerical codes Genesis [ref] and Ginger [ref]. As a benchmark for FEL-2 “fresh bunch” scheme, calculations are also presented for a possible alternative configuration, named “whole bunch”, in which the entire electron bunch is seeded and the second modulator is eliminated. Section 4.6 contains conclusions and perspectives.


Ultima modifica il Giovedì, 17 Ottobre 2013 09:25