Beam Dynamics Around Transition in a High Brightness Linac for Short Wavelength SASE-FEL Experiments
M. Ferrario, M. Boscolo, V. Fusco, M. Migliorati, L. Palumbo, B. Spataro, C. Vaccarezza, INFN-LNF
L. Giannessi, M. Quattromini, C. Ronsivalle, ENEA
L.Serafini, INFN-MI,
J. B. Rosenzweig, UCLA
INFN-LNF

Short wavelength (nm-range) SASE-FEL experiments require the generation and transport up to the undulator entrance of low emittance (~ 1-2 mm-mrad) high peak current (~1-3 kA) electron beams with energy around 1-3 GeV. The evolution of a beam with such parameters takes place in a transition regime from space charge to emittance dominated dynamics, as the beam energy increases. In addition, downstream magnetic compressor devices where the peak current increases up to kA range, the transition may occur again. Under this conditions the electron beam has to be properly matched to the Linac accelerating sections in order to keep under control emittance oscillations driven by residual space charge effects. Generalized invariant envelope matching conditions are discussed in this paper, showing that an equilibrium between RF focusing forces and space charge/emittance defocusing forces can be attained without any additional external focusing along the Linac (no quadrupoles), thus reducing alignment problems. As a specific example the preliminary design of the SPARXINO Linac will be discussed.