The TeraFERMI Project
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}. 