The TeraFERMI Project

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Coherent THz Emission

Short electron bunches naturally emit coherently at wavelengths longer than their bunch length. High-gain, single-pass 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 multi-particle 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 (Ps) writes:  
P/Ps=N{1-f(ω)}+N2f(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. Single-pass accelerators can store charges in the order of 1 nC and beyond in a sub-ps bunch, thus with bunch densities significantly larger than what can be achieved in a storage-ring or recirculating machine. This corresponds to a huge coherent gain in the order of 1010.
The mechanism for coherent THz emission, only depends on the electron charge density distribution thus allowing to relax the constrains on the energy spread of the electron beam that have to be satisfied for producing FEL emission. Because of this important property, one can obtain coherent THz radiation from the same electrons participating into the FEL process. Such a parasitic use of the electron beam after the FEL is possible even if the energy-spread has already been "spoiled" by the lasing process, thus allowing for the downstream (parasitic) extraction of coherent THz light.

Last Updated on Friday, 01 October 2021 09:33