THE FERMI FREE ELECTRON LASER: A HIGH GRADIENT SOLUTION FOR INCREASING THE LINEAR ACCELERATOR ENERGY

The principle of operation of the Free-Electron Lasers (FELs) was invented in 1971 but only in recent years it has become clear that these devices are very powerful light sources in the X-ray regime. Their high pulse energy, the femtosecond duration of the X-ray pulses, as well as their coherence, open entirely new fields of research, which were inaccessible in the present third generation light sources. The main components of a FEL are a linear accelerator, providing a bunched relativistic electron beam, and a chain of undulator magnets, where the electrons move along sinusoidal trajectories and emit an undulator radiation concentrated in a narrow angular cone along the undulator axis. The fundamental wavelength emitted is proportional to the relativistic Lorentz factor of the electrons, which typically reaches values of several thousand for X-ray emission. The free-electron laser FERMI is the seeded FEL user facility at the Elettra laboratory in Trieste, operating in the VUV to EUV and soft X-rays spectral range. The electron bunches are produced in a laser-driven photo-injector and accelerated to energies up to 1.5 GeV in the Linac. In order to extend the FEL spectral range to shorter wavelength, a feasibility study aimed to increase the Linac energy from 1.5 GeV to 1.8 GeV is currently ongoing. A proposal to replace the existing Backward Traveling Wave sections with new S-band accelerating structures tailored for high gradient operation, low breakdown rates and low wakefield contribution is presented and discussed in this thesis. The complete radiofrequency (RF) design of the new structure is presented. Particular care has been given to the minimization of the electric and magnetic surface fields, in order to reduce the breakdown probability, and to the reduction of the residual quadrupole component of the fields in the RF coupler. A compact solution for an RF pulse compressor system, aimed to enhance the peak power delivered to the structure, is also discussed. In order to prove the feasibility and the reliability of operating at the high gradient that would be required to achieve 1.8 GeV, a first short prototype of the new accelerating structure is now under construction, in collaboration with the Paul Scherrer Institut (PSI), and will be high power tested in the upgraded FERMI Test Facility starting from May 2018.