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 solid-state-physics to biology, medicine, industrial production and homeland security. A new frontier in THz science is now represented by the possibility to produce ultra-short, coherent, poweful pulses suitable to manipulate and control material’s properties. 

The key advantage in using THz photons with respect to the more conventional visible and near-infrared photo-excitations, is that THz allows to directly populate low-energy, single-particle and collective excited states, without "heating" the overall electron bath. This allows to focus only on the dynamics of interest, instead of dealing with dissipation phenomena. THz light couples to electronic, magnetic and vibrational degrees of freedom, thus offering the possibility to address an extremely wide range of excitations.

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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.


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 Ginzburg-Frank 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). 

We simulate here the emission from a relativistic electron beam of 1.2 GeV, with a bunch shape modeled by a rectangular bunch density distribution of variable duration Δt, and constant charge Q=1 nC. Full lines refer to the far-field case, while dashed lines account for the near-field. For a 1 ps long bunch, the THz emission is limited below 1 THz. By decreasing the bunch length (while keeping constant the total charge Q), the energy cut-off is progressively shifted to higher frequencies, with a strong enhancement of the overall intensity. As shown in the picture, under the above described conditions, the calculated energy per pulse ranges from 150 µJ to more than 5 mJ for pulse lengths of 50 fs.



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 start-to-end 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 (R56) of the line with proper adjustment of the quadrupole strengths.

 

The previous simulation of the electron bunch properties, considers as a starting point an electron bunch shape suitable for the production of FEL light. However, the effects on the electron bunch induced by the growth in energy spread due to FEL lasing remain to be taken into account. According to the simulation, only the central part of the bunch is perturbed by the seeding laser, and is therefore experiencing the energy spread growth. Such a portion of the beam is weakly affected by the wakefield induced compression discussed above. On the other hand, the two unperturbed parts of the bunch (head and tail) get progressively compressed down to ~100 fs in the MD section, while their peak current increases by more than a factor of two, as shown by the green curve in the Figure. From the point of view of the THz beamline, these two peaked structures in the electron bunch also contribute to the THz emission by increasing the bandwidth up to ~10 THz, with the FEL parameters presently in use. 
 
Ultima modifica il Venerdì, 01 Ottobre 2021 09:33