Theoretical and experimental investigation of single spike regime with the SPARC free-electron laser

Abstract

Free-electron lasers are based on the coherent emission of synchrotron ra- diation of relativistic electrons within an undulator or wiggler. The reso- nant radiation wavelength depends on the electron beam energy and can be tuned over the entire spectrum from micrometer to X-ray radiation. The basic regimes of operation are the FEL ampli er (SEED FEL) and the Self Ampli ed Spontaneous Emission (SASE)-FEL. The SEED-FEL regime is characterized by the ampli cation of a coherent seeding pulse in a single pass through a su ciently long undulator. The Self Ampli ed Spontaneous Emission (SASE)-FEL, on the other hand, uses the broadband signal of the spontaneous emission to start the FEL ampli cation. A FEL pulse generated by SEED scheme is a full coherent pulse while the SASE is a quasi-coherent pulse. The lack in the coherence properties of SASE-FEL is in the longitudinal time structure where the pulse is composed by many regions (spikes) that are independently one from each others. To obtain a full coherent laser pulse a seed pulse is needed or, in SASE mode, it can be done with very short electron bunches, i.e. a bunch length as long as the region of the beam that radiates coherently due to the FEL interaction. The regime is called the Single Spike regime. The pulses generated are fully coherent, moreover given that the laser pulse length is comparable to the electron beam length, its duration can reach femtosecond scale. This PhD thesis regards the theoretical and experimental analysis of the single spike regime at SPARC(Sorgente Pulsata Autoampli cata di Radiazione Coer- ente) at Frascati, Italy. For SPARC FEL lasing in the visible range, the single spike bunch length is in the order of hundreds of fs. The generation of high brightness ultra-short electron beam is achieved at SPARC with the so called Velocity Bunching (VB) technique. The compression mechanism introduces a linear chirp in the electron bunch energy and the compression ratio must be chosen accurately in order to have the maximum brilliance. The resulting bunch is longer than the single spike length yet, but it has a peaked current shape combined with a strong linear chirp along the beam. The short energy chirped electron beams are injected into a tapered undu- lator to compensate the chirp. This scheme allows to select the brightest region of the bunch to operate in the single spike regime. A theoretical investigation on the Single Spike regime at SPARC is presented in the the- sis, where, imposing the single spike condition on beam length, a scaling law of the charge vs. the length of the beam is introduced in order to obtain the single spike pulse. The superradiant properties of the pulse by means of three-dimensional simulation are also investigated. All the FEL simulations are done with GENESIS and PERSEO code. The theory and simulation results are then checked against the Single Spike SASE pulses obtained experimentally at SPARC. In addition SEED regimes with seed pulse length lower than the electron bunch length are investigated for the superradiance evolution of the seed spike injected at the tail of the electron beam and slipping towards the head. The harmonics in the FEL pulses are ultra-short pulses in the UV range with sub-muJ energy. The experimental results are the spectrum and the energy content of the pulses and they are in a good agreement with the simulations and theory.