C. Bruni

Longitudinal detuning for an SRFEL

The interaction between laser pulse and electron beam of a storage ring free electron laser (SRFEL) can be described in the longitudinal phase space as a function of the longitudinal detuning, which we define as the delay per pass between the laser pulse and the electron bunch. SRFEL dynamics versus detuning is studied by comparison of measurements performed at Super ACO with a numerical code. Experimental phenomena regarding detuning have been properly predicted by simulations. Quantitative agreement for pulse duration, pulse repetition frequency, laser power, etc. has been found.

Local heating induced by Coherent Harmonic Generation on electron beam dynamics in storage ring

We investigate the dynamical response of an electron bunch in a storage ring to a brief and intense excitation of an external laser. We present a model that simulates the evolution of the electronic distribution in the Coherent Harmonic Generation (CHG) configuration: the energy exchange between photons and electrons occurs within an undulator magnetic field, and further leads to a coherent radiation process. Using those simulations and experimental results on the UVSOR-II CHG free electron laser, we demonstrate that the electronic distribution reaches an equilibrium regime thanks to energy spread relaxation in between two laser injections. We also show that saturation is driven by laser-induced local heating, leading to global effect over the whole bunch.

Super-ACO FEL oscillation with longitudinal to transverse coupled beam dynamics

The Super-ACO storage ring Free Electron Laser (FEL) has been operated using an alternative operating point characterized by a non-zero dispersion function in the undulator straight section, linking the transverse motion of the electrons to the energy spread, and by a low momentum compaction factor. The longitudinal to transverse coupled electron beam motion induces a modified dynamics of the Super-ACO FEL. The FEL characteristics, such as the FEL power, pulse duration and spectral width are presented and analyzed in terms of storage ring FEL dynamics.

Simulations of the Acceleration of Externally Injected Electrons in a Plasma Excited in the Linear Regime

We have investigated numerically the coupling between a 10 \si{MeV} electron bunch of high charge (§I{> 100}{pc}) with a laser generated accelerating plasma wave. Our results show that a high efficiency coupling can be achieved using a §I{50}{TW}, §I{100}{μ \meter} wide laser beam, yielding accelerating field above §I{1}{ GV/m}. We propose an experiment where these predictions could be tested.

Chaos in free electron laser oscillators

AbstractThe chaotic nature of a storage-ring free electron laser (FEL) is investigated. The derivation of a low embedding dimension for the dynamics allows the low-dimensionality of this complex system to be observed, whereas its unpredictability is demonstrated, in some ranges of parameters, by a positive Lyapounov exponent. The route to chaos is then explored by tuning a single control parameter, and a period-doubling cascade is evidenced, as well as intermittence.

Linewidth Narrowing and Etalon Fabry-Perot experiment at ELETTRA

Publisher Summary The free-electron laser (FEL) oscillators are pulsed high coherent and tunable sources, showing quite large linewidths. The number of lasing longitudinal modes is quite high because of the long optical cavity of storage ring FELs (SRFELs). The spectral linewidth can be reduced inserting a Fabry–Perot etalon in the cavity allowing the selection of a limited number of longitudinal modes. The spectral features of a FEL oscillator can be easily estimated from the gain spectral distribution and the gain over losses ratio. In SRFELs, the use of an optical klystron and the relatively low gain of the system limits the linewidth narrowing. This chapter discusses the validation of SRFEL theories and codes. It further discusses the characterization of the SRFEL dynamics in a complete and totally controlled Fourier-transformed laser pulse. The chapter explains the possible exploitation of the new spectral features for spectroscopy in the UV range, providing a high spectral resolution for the FEL users.

LASER HEATING AND MICROWAVE INSTABILITY IN THE SUPER-ACO FEL

Abstract In a storage ring FEL, both laser heating and microwave instability concour to define the equilibrium state of the longitudinal beam dynamics. Their competitive interplay can be studied by analysing the equilibrium laser power. Preliminary results of the laser power analysis and longitudinal dynamics characterization - carried out at the Super-ACO storage ring FEL - are hereby presented. PACS codes: 41.60. Cr; 29.20. Dh