SLAC, Menlo Park, California
The Echo-enabled Harmonic Generation (EEHG) FEL uses two modulators in combination with two dispersion sections that allow one to generate in the beam a high-harmonic density modulation starting with a relatively small initial energy modulation of the beam.[1] After presenting the concept of the EEHG, we address several practically important issues, such as the effect of coherent and incoherent synchrotron radiation in the dispersion sections, the beam transverse size effect in the modulator, etc. Using a representative realistic set of beam parameters, we show how the EEHG scheme enhances the FEL performance and allows one to generate a fully (both longitudinally and transversely) coherent radiation.
[1] G. Stupakov, PRL, 102, 074801 (2009).
D. Xiang and G. Stupakov, PRSTAB, 030702 (2009).
USTRAT/SUPA, Glasgow
Istituto Nazionale di Fisica Nucleare, Milano
Short-wavelength Free Electron Lasers (FELs), which have recently produced intense, hard X-rays, are currently based on the concept of classical Self-Amplified Spontaneous Emission (SASE). In order to extend the production of intense, coherent radiation to sub-Å wavelengths then an alternative to the conventional SASE-FEL concept will be necessary, as conventional SASE-FELs require long wigglers (~100m), large accelerators (~ km) and produce radiation which has poor temporal coherence. Recently, we have introduced the concept of the Quantum Free Electron Laser (QFEL). The QFEL is characterised by quantised electron momentum recoil and the emission of monochromatic, coherent radiation from a compact apparatus. This makes it appealing for applications requiring a high degree of temporal coherence. We show that a SASE-QFEL may offer the possibility to produce intense, coherent radiation at sub-Å wavelengths via harmonic generation.
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.
DEEI, Trieste
ELETTRA, Basovizza
SLAC, Menlo Park, California
In a harmonic generation free electron laser (HG FEL), the electron beam entering the undulator can have an initial energy curvature besides an initial energy chirp. Solving the Vlasov-Maxwell equations within the 1D model, we derive an expression for the Green function for the seeded HG FEL process for the case of the electron beam having both an energy chirp and an energy curvature. We give an asymptotic closed form which is a good approximation in the exponential growth regime, and a series expression that allows the evaluation of the field envelope along the undulator in both lethargy and exponential growth regime. The latter is useful to study the HG FEL behavior in the short modulator, like that of the FERMI@Elettra project. The FEL radiation properties such as central frequency shift and frequency chirp are studied considering Gaussian laser seeds of different temporal duration in respect to the Green functions temporal duration. The energy chirp and curvature of the electron bunch result in a time dependent bunching factor for the FEL start-up process in the radiator of the HG FEL like the FERMI@Elettra. The coherence properties of the FEL are examined.
UCLA, Los Angeles, California
ENEA C.R. Frascati, Frascati (Roma)
PSI, Villigen
Narrow bandwidth FEL operation is one of the main challenges for future FEL facilities. Several schemes have been proposed to obtain a narrower bandwidth than that achievable with self amplified spontaneous emission starting from shot-noise. In this work the properties of two such schemese (high gain harmonic generation and self-seeding) are investigated and compared taking into account several non-ideal beam effects, with particular attention paid to the effect of bandwidth broadening associated to non-linear longitudinal phase-space. The comparison between the two schemes has been carried out with numerical simulations performed with GENESIS 1.3 and PERSEO FEL codes using both an idealized beam and realistic beams obtained with start to end simulations.