SOLEIL, Gif-sur-Yvette
CNRS/CPT, Marseille
ELETTRA, Basovizza
Università di Firenze, Florence
Université Libre de Bruxelles, Center for Nonlinear Phenomena and Complex Systems - CP 231, Bruxelles
The regime of intensity (and bunching) oscillation following the FEL saturation is investigated using the Colson-Bonifacio model. This regime is understood as an out-of-equilibrium metastable state, which slowly relaxes toward thermodynamical (Boltzmann) equilibrium. This dynamics is also characterized by a strong regularity, unexpected for an interaction between waves and large number of particles, as well as by low-dimensional phase-space structures in the electron-beam phase space. In this context, the switch from regimes associated to high gain (for small electron-beam energy spread) or very low gain (for large energy spread) can be interpreted as out-of-equilibrium phase transitions, a phenomenon which was recently explained by a mechanism of entropy maximization.
SOLEIL, Gif-sur-Yvette
LOA, Palaiseau
ELETTRA, Basovizza
ENEA C.R. Frascati, Frascati (Roma)
In the quest for compact FEL ultra-compact sources, a test experiment is under preparation, to couple an electron beam from a laser driven plasma accelerator, stable and tunable in energy, to an undulator. The electron beam is generated in the colliding laser pulses scheme, by focusing two short and intense laser pulse in an underdense plasma plume. The electron bunch has an energy tunable in up to a few hundreds MeV with 1% energy spread, a length 10 fs, a charge in the 10 pC range, while its radius and divergence are respectively 1μm and 3 mrad. As a first step toward a FEL experiment, the transport and radiation through an undulator of this short and compact electron beam is studied. The spontaneous emission through a 60 cm undulator in the 40-120 nm range is presented, and criteria to reach the threshold of Self-Amplified Spontaneous Emission are discussed.
SOLEIL, Gif-sur-Yvette
Imagine Optic, Orsay
BNL, Upton, Long Island, New York
LOA, Palaiseau
RIKEN/SPring-8, Hyogo
JASRI/SPring-8, Hyogo-ken
The VUV Self-Amplified Spontaneous Emission of the SPring-8 Compact SASE Source (SCSS) Test Accelerator is characterized at different stages of amplification up to saturation [1]. Experimental measurements are performed by use of a VUV Hartmann wavefront sensor. This kind of sensor gives access to both intensity and phase profiles of the incoming beam. We characterize the mode selection when approaching the saturation regime of the FEL. Optical quality of the saturated SASE radiation is measured to be better than Lambda/5 PV and Lambda/22 rms (Lambda = 61.5 nm) depending on the machine optimization. Moreover, pointing of the beam as well as spatial structure, size and position of the source are retrieved and their shot-to-shot fluctuations investigated. Analytical [2] and numerical calculations [3], using SRW and GENESIS codes, show good agreement with the experimental measurements. All these elements are of crucial importance for a better understanding and optimization of the FEL and of course for user applications requiring a stable focused beam on their samples. We are grateful to the SCSS Test Accelerator Operation Group at RIKEN for continuous support in the course of the studies
[1] R. Bachelard et al., "Wavefront and Transverse Structure of the FEL Self-Amplified Spontaneous Emission", to be submitted
[2] M. Xie, NIMA 445, 59(2000)
[3] O. Chubar et al., Proc. EPAC-98, 1177(1998)