The TeraFERMI Project
Motivation
Thanks to the great advancements occurred during the last two decades, THz spectroscopy is now widely employed in several fields of science and technology, ranging from solidstatephysics to biology, medicine, industrial production and homeland security. A new frontier in THz science is now represented by the possibility to produce ultrashort, coherent, poweful pulses suitable to manipulate and control material’s properties. 
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Coherent THz Emission
Short electron bunches naturally emit coherently at wavelengths longer than their bunch length. Highgain, singlepass FEL's normally make use of electron bunches in the 50 fs to 1 ps range, thus implying that coherent long wavelength radiation is naturally emitted up to frequencies ranging from 1 to 20 THz. More quantitatively, in order to properly take into account multiparticle effects in the emission properties of an electron bunch, one should consider that the ratio between the radiated power produced by the bunch (P) over the power radiated by a single particle (P_{s}) writes:
P/P_{s}=N{1f(ω)}+N^{2}f(ω)
where N is the number of electrons in the bunch, and f(ω) is the form factor given by the Fourier Transform of the longitudinal charge density distribution ρ(z).

The first term accounts for the usual incoherent emission, scaling linearly with the number of particles in the bunch, whereas the second term corresponding to the coherent enhancement factor is proportional to the square of the charge. Singlepass accelerators can store charges in the order of 1 nC and beyond in a subps bunch, thus with bunch densities significantly larger than what can be achieved in a storagering or recirculating machine. This corresponds to a huge coherent gain in the order of 10^{10}. 
Coherent Transition Radiation
Transition Radiation is produced when a thin metallic target screen is inserted in the electron trajectory. The high energy electrons travel across the target while an intense current builds up on the metallic screen which is then radiated both in forward and backward directions. The emission properties can be calculated thanks to the GinzburgFrank formula. Transition Radiation displays circular polarization and cylindrical symmetry along the propagation axis. In the case of an ultrashort electron bunch one should apply the coherent enhancement factor discussed in the previous paragraph. One then talks about Coherent Transition Radiation (CTR).


Simulated emission under FEL operation
If the constraints on the electron beam parameters necessary for seeded FEL operation could be relaxed, the FERMI LINAC would be able in principle to produce ultrashort (10's fs) highly charged (nC) electron bunches suitable for the production of coherent THz pulses as those described in the previous section. This could be achieved through chirped pulse compression schemes. However, one of the most intriguing aspects of the TeraFERMI project is the possibility of producing THz light without affecting FEL operation. To this aim, we have investigated the electron beam dynamics in the FERMI main dump (MD) region, with special attention to the particle longitudinal motion.

An accurate investigation of the electron dynamics in the MD line with starttoend particle tracking codes reveals that the emission of Coherent Synchrotron Radiation (CSR) by the two long dipoles placed before the TeraFERMI extraction point plays an important role in the evolution of the particle longitudinal phase space along the MD line. Our calculation reveals that CSR induces an energy chirp of the order of 0.1%, thus enhancing the compression process in the MD line. As a result, a typical current profile with a length of about 600 fs as the one depicted in blue in the Figure inset, is compressed down to 400 fs, as shown by the red curve. The resulting THz emission spectra are also calculated and reported in the Figure with the same color code. The entire system can be further optimized in order to enhance the compression by shortening the bunch for the FEL and increasing the positive transfer element of the magnetic lattice (R_{56}) of the line with proper adjustment of the quadrupole strengths. 