UCLA, Los Angeles, California
An electron beam that is subject to the typical FEL microbunching instability is microbunched longitudinally in both density and velocity, according to the shape of the ponderomotive phase bucket. Higher-order three-dimensional microbunching geometries can be created if the e-beam interacts either with a more complicated resonant field structure, or at higher harmonics of the fundamental resonance. At harmonics inside a helical wiggler, the e-beam interacting with an axisymmetric gaussian laser field becomes microbunched into a helix, or combination of twisted helices, depending on the harmonic number. The twisted e-beam can then be used to emit coherent light with orbital angular momentum in a downstream radiator. An experimental effort to explore the principles of this interaction is discussed.
UCLA, Los Angeles, California
Recent observation of coherent optical transition radiation in FEL injectors has provoked considerable theoretical activity. As this phenomena is provoked by microbunching at or near the level of the mean interparticle spacing in the beam, any description of it should resolve the beam at the particle level. Here we present first simulations that fulfill this requirement, based on a three-dimensional code in which fields are found in the beam frame using Fourier methods. The simulations take into account acceleration, which serves to freeze relative longitudinal motion, transverse focusing, and downstream bending motion. Comparisons are made between code predictions and experimental results, as well as with theoretical models.
INFN/LNF, Frascati (Roma)
Istituto Nazionale di Fisica Nucleare, Milano
Università di Roma II Tor Vergata, Roma
ENEA C.R. Frascati, Frascati (Roma)
INFN-Roma II, Roma
UCLA, Los Angeles, California
INFN-Roma, Roma
The optimization of the beam brightness is one of the main objectives of the research and development efforts in rf-photoinjectors devoted to short wavelength FELs. The velocity bunching experiment at SPARC has recently demonstrated the possibility of increasing the beam current via RF compression at low energies, while compensating the self-fields induced emittance degradation by means of continuous magnetic focusing. The result is an increase of the beam brightness by about one order of magnitude. Stable compression ratio up to a factor 12 has been observed. Characterization of longitudinal phase spaces an measure of projected and slice emittances, as a function of the injection phase in the first accelerating structure and for different solenoids field values are presented. Comparisons with simulations are also reported.
ENEA C.R. Frascati, Frascati (Roma)
INFN/LNF, Frascati (Roma)
Istituto Nazionale di Fisica Nucleare, Milano
SOLEIL, Gif-sur-Yvette
CEA, Gif-sur-Yvette
INFN-Roma II, Roma
UCLA, Los Angeles, California
INFN-Roma, Roma
The SPARC FEL can be operated in both SASE and seeded modes. A major part of the second stage of the commissioning, currently in progress, is dedicated to the characterization of the SASE radiation. Simultaneously, we are finalizing the experimental setup for seeding. We present an in-situ characterization of the two input seeds that are foreseen: both are obtained via harmonic generation, the first one in crystal (400 and 266 nm) and the second in rare gas (Argon). We also describe the specific diagnostics implemented for the electron-seed overlap in the undulator, together with the diagnostics for radiation analysis (2D spectrometer and FROG). The seeding will enable the operation of the SPARC FEL in original cascaded configurations.