Block diagram illustrating the main idea underlying the TeraFERMI beamline. THz radiation is collected downstream with respect to FEL emission, and transported in the experimental room, to be used as a pump beam. As a probe, one may use either a portion of the TeraFERMI beam itself, or a synchronized femtosecond probe. Present laser based technique allows to cover almost whole infrared range (form THz to visible) as a probe beam. In the ultraviolet and soft x-ray regime, the pulses from the FEL may be employed as an intrinsically self-synchronized probe.
Comparison between CSR and CTR emissions. In both cases a 1 nC pulse at 1.2 GeV and 700 fs length (flat to top) has been considered. Radiation is collected through an aperture of 200 mrad. The calculations for CSR and CTR make use of formulae from Refs. 21,22 , respectively.
Calculated emission for CTR emission. The calculation considers 1 nC relativistic (1.2 GeV) electron bunches, with a rectangular profile of variable length. The radiation is emitted by a CTR screen of radius 3 mm, and over a 200 mrad acceptance angle. Full lines indicate the far-field calculation, while dashed lines correspond to the near-field, with the light being collected at a distance of 5 cm from the radiative screen. In the inset are reported the energy per pulse values, as a function of the bunch length, for far- (full line and symbols) and near- (dashed line and empty symbols) field approximations.
Calculated CTR emission in the case of 500 pC electron bunches, as those presently used by the FERMI FEL. The blue curve corresponds to an electron bunch entering the FERMI FEL, while the red curve refers to the same electron bunch, after being compressed along the MD line, as calculated at the TeraFERMI extraction point (see text). The green curve refers to an electron bunch for which the effects of the energy spread growth due to lasing are also taken into account (see text). The grey area indicates the emission from a 500 pC electron bunch with 50 fs length. (a) The bunch shape before (blue), and after passing through the MD line with (green) and without (red) considering the effects of the energy spread due to lasing. (b) The THz energy per pulse values referred to the three bunch shapes previously discussed (blue, red, and green diamonds). The grey curve shows the potential gain of the TeraFERMI beamline if the electron bunch was further compressed, up to 50 fs.
Relation between incoming Δt i and outcoming Δt f bunch length after passing through the MD line, according to Eq. (6) . The red line corresponds to the presently used R 56 = 7 mm, while the grey area indicates the improved compression values for R 56 up to 20 mm. In the inset are shown the Δt f values, for various Δt i , as a function of R 56.
(a) Pictorial visualization of the TeraFERMI layout. (b) Optical scheme of the TeraFERMI beamline, comprising 5 focusing elements (f1 to f5). Plane mirrors are indicated by the symbols P. All the distances indicated are in mm.
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