# Conceptual and Technical Design Aspects of Accelerators for External Injection in LWFA
## 4 SINBAD-ARES Accelerator Design
### Photo-Injector Design  
A sketch of the ARES photo-injector is shown in Figure 7. It comprises a modified version of the 2.998 GHz REGAE RF gun [68]. Its RF station is constituted by a Toshiba E37326A Klystron and a Scandinova K1 modulator. 6 MW peak input power in the gun cavity corresponds to an accelerating gradient of almost 117 MV/m, which allows for a final electron energy higher than 5 MeV. The design of the antenna connecting the coupler to the gun cavity has been elongated with respect to the original design in order to accommodate a second solenoid (along with a bucking coil). The additional focusing allows for an improvement of the transverse quality of the beam because of the reduction of the non-linear emittance increase induced by the RF fields [69]. Moreover the increased length of the antenna leads to an improved damping of the parasitic RF kick coming from the coupler [70]. Figure 7: Sketch of the SINBAD-ARES gun. The gun solenoids are also used for achieving emittance compensation when matching the beam with the linac. The first solenoid and the bucking coil have a total minimum focal length of about 18 cm for a 5.1 MeV beam and a second solenoid is installed after the coupler. A load lock system allows for in-vacuum cathode exchange. Both \(Cs_{2}Te\) and metallic cathodes are foreseen to be employed. The \(Cs_{2}Te\) cathodes are more suitable for tens of pC (or higher) charge electron generation because of their high quantum efficiency but slow emission time. On the other hand the metallic cathodes are more suitable for the ultra-low charge working points because they allow for producing electron bunches shorter than 1 ps thanks to the faster emission time. The ARES photo-cathode laser is a commercial one (PHAROS-SP-200). This laser allows working with 3 different wavelengths: 1030 nm (fundamental wavelength, not used so far in our project), 515 nm (second harmonic, with potential application for testing electrons emission quality from green cathodes), 257 nm (fourth harmonic, used for electron emission with metallic and \(Cs_{2}Te\) cathodes). The laser is coupled to the beamline downstream of the injector with about a 90 degree angle with respect to the cathode surface. Opposite to the laser in-coupling mirror, a second mirror will be installed for future possible direct visualization of the laser spot-size on the cathode. The optical beamline is designed to allow flat-top transverse shaping of the beam down to a minimum laser diameter of 80 \(\upmu\)m. The laser pulse is longitudinally Gaussian and its FWHM duration can be tuned between 180 fs and 10 ps. As stressed in the previous paragraphs, the non-linearity in the longitudinal phase space and the initial uncorrelated energy spread of the beam play a crucial role in the optimization of the bunch compression, even when the collimator in the magnetic chicane is present. The flexibility of the ARES photo-injector will allow for producing phase space distributions at the gun exit with different features. In Figures 8 and 9 and in Table 3 three working points for the photo-injector parameters for producing a beam with charge Q = 0.5 pC are compared. Transversely flat-top laser pulses allows for improving the non-linearity of the longitudinal phase space and the beam transverse emittance, whereas working closer to the so-called blow-out regime allows for further reducing the non-linear terms both in the transverse and in the longitudinal phase-space. In Figures 10 and 11 and in Table 4 two working points for the photo-injector parameters for producing a beam with charge Q = 20 pC are compared. By using a cigar-like shaping of the photo-cathode laser the slice emittance and the slice energy spread of the beam is minimized. On the other hand by working in the blow-out regime the longitudinal phase space becomes linear.