Generation of VUV ultra-short coherent optical pulses using electron storage rings

The need of coherent and intense pulsed radiation is spread among many research disciplines, such as biology, nanotechnology, physics, chemistry and medicine. The synchrotron light from traditional sources only partially meets these characteristics. A new kind of light source has been conceived and developed in the last decades: the Free-Electron Laser (FEL). The FEL process relies on the interaction between a relativistic electron beam and an electromagnetic wave in presence of a static and periodic magnetic field, produced by a device called undulator. This interaction generates coherent radiation at a fundamental frequency and its higher harmonics. In the standard configuration, the electron beam is generated by a linear accelerator and the interaction occurs in a single passage through one or several undulators. An alternative configuration can be obtained if the electrons are supplied by a storage ring. This work has been carried out at the Elettra laboratory within the ``new light sources'' group. My thesis focuses on both numerical and experimental issues about the generation of coherent harmonics on storage-ring FELs. The Elettra SRFEL has been originally designed to operate in ``oscillator configuration'' where the radiation is stored in an optical cavity (made of two mirrors). This process also drives the emission of radiation in the harmonics. In this work, different experimental methods have been implemented at Elettra to concentrate the power in giant pulses, both for the fundamental wavelength and its harmonics. Using this technique, it has been possible to generate fundamental radiation at 660 nm and 450 nm with (intra-cavity) power of few mJ and third harmonic radiation at 220 nm and 150 nm with few nJ of power. This process has been studied numerically by using a tri-dimensional simulation which also accounts for the re-circulation of the beam. The results of simulations are in good agreement with experimental measurements and allow to investigate the inner structure of the light below the picoseconds scale, where the instrumentation resolution reaches its limit. Structures of hundreds of femtoseconds inside the laser pulse have been found and this implies a higher peak power. Moreover, the numerical results have been confirmed by spectral measurements. By removing the optical cavity and focusing an external laser in the first undulator, a ``seeded single-pass'' configuration has been implemented. In the first undulator, the interaction with the external laser (``seed'') modulates the electron energy which is converted to spatial modulation (``bunching''). A Fourier analysis of the bunched electron-beam shows the presence of components at all harmonics (even and odd) and this explains why electrons in the second undulator can emit at any harmonic. To implement this configuration a design and layout plus tri-dimensional simulations were performed. Followed by the installation of the seed laser (Ti:Sapphire, lambda = 796 nm), the timing and the diagnostics. The commissioning focused on optimizing the spatial overlap and the synchronization between the electrons and the seed laser. Coherent harmonic radiation has been obtained at 265 nm, the third harmonic of the seed laser. After the characterization of this light, the seed frequency has been doubled by means of a nonlinear crystal. With this setup, radiation down to 99.5 nm (the fourth harmonic of the seed) has been generated. The shot-to-shot stability is comparable to the stability of the synchrotron radiation (fluctuations of few %) but the number of photons per pulse (~10^9) is about two-three orders of magnitude bigger than the synchrotron one. Thus this coherent radiation can be used for experiments similar to those suggested for the next generation FELs. Summarizing, the light source developed during my thesis is a unique facility able to generate coherent radiation with variable polarization, variable duration (between 100 fs and 1 ps), with peak power of the order of mega-Watts in a wide spectral VUV range. In the latest implementation, this radiation source has been used for two different kind of experiments, one in gas-phase, the other of solid state. The obtained results demonstrate the appealing of this source for user experiments. In perspective, there is a plan to extend the wavelength range below 100 nm and to improve the tunability of the source.