The TeraFERMI Project

Page 4 of 4

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. 
 
Last Updated on Friday, 01 October 2021 09:33