Plasma accelerators are emerging as formidable and innovative technology for the creation of table-top devices thanks to the possibility to sustain several GV/m accelerating gradients at normal conducting temperature. Among others, the particle-driven configuration has been successfully tested at the SPARC_LAB test facility also demonstrating the emission of plasma-based FEL radiation in SASE and seeding operation. Recently we have performed further experimentals devoted to heightening the accelerating gradient in the plasma. The so-called comb beam has been set up with a 500pC driver followed by a 50pC trailing bunch. The maximum measured energy gain in the plasma has been of almost 30 MeV turning in an accelerating gradient of the order of 1.2 GV/m. The result represents a fundamental achievement also looking at the forthcoming EuPRAXIA@SPARC_LAB plasma-based user facility. Further experimental runs are planned for the next year on the measurements of transverse quality of the electron beam and its eventual preservation. The paper reports on the obtained experimental results and on the numerical studies for the next future experiment at the SPARC_LAB test-facility.

The SPARC\textunderscore LAB test facility at the LNF (Laboratori Nazionali di Frascati, Rome) holds a high brightness photo-injector used to investigate advanced beam manipulation techniques. High brightness electron bunch trains (so-called comb beams) can be generated striking on the photo-cathode of a Radio Frequency (RF) photo-injector with a ultra-short UV laser pulse train in tandem with the velocity bunching technique. Beam dynamics studies have been performed with the aim of optimizing the dynamics of the double beam (driver and witness) used to perform particle driven plasma wake field acceleration (PWFA). In this scenario different scans on beam parameters were carried on adopting the ASTRA simulation code, in order to optimize the witness beam quality and improve the plasma booster stage performances. A benchmark of the simulations has been then performed, reproducing the experimental data obtained from the optimization of machine performances, and a good agreement was found.