The active plasma lens represents a compact and affordable tool with radially symmetric focusing and field gradients up to several kT/m. In order to be used as a focusing device, its effects on the particle beam distribution must be well characterized. Here, we present the experimental results obtained by focusing an high-brightness electron beam by means of a 3 cm-long discharge-capillary pre-filled with Hydrogen gas. We achieved minimum spot sizes of $24\hspace{0.17em}\mu \mathrm{m}$ (rms) showing that, during plasma lensing, the beam emittance increases due to nonlinearities in the focusing field. The results have been cross-checked with numerical simulations, showing an excellent agreement.

We report on the experimental characterization of the effect that a passive plasma lens in the overdense regime has on high-brightness bunch quality by means of 6D phase-space analysis. The passive lens is generated by confining hydrogen gas with a capillary tube pre-ionized with a high-voltage discharge. We observed that the optimum condition is retrieved at the end of the overdense regime with almost no effect on bunch brightness. The presence of gas jets, leaking from the hollow capillary end-points, extends the lens effects also outside of the capillary, resulting in longer focusing channels. Experimental results are supported with numerical simulations of the complete accelerator line together with the plasma channel section.

The ultra-short electron beams, produced through the velocity bunching compression technique at the SPARC_LAB test Facility (Frascati, Italy), are used to produce Coherent Transition Radiation in the terahertz (THz) range. This paper reports on the main features of this THz source, which have a spectral coverage up to 5 THz, a pulse duration down to 100 fs, and an energy per pulse on the order of tens of μJ. These figures of merits open the possibility to apply this source for nonlinear and THz pump-probe experiments in Solid-State Physics and material science.

Beam stability and timing jitter in modern linear accelerators are becoming increasingly important. In particular, if a magnetic or radio-frequency (RF) compression regime is employed, the beam time of arrival jitter at the end of the linac can be strictly correlated with the phase noise of the accelerating fields of the RF structure working off-crest. For this reason, since 2008, an RF fast-feedback technique, which acts within each RF pulse, has been successfully employed at LNF-INFN (Laboratori Nazionali di Frascati dell’Istituto Nazionale di Fisica Nucleare) in the SPARC_LAB (Sources for Plasma Accelerators and Radiation Compton with Laser And Beam) facility on S-band (2856 MHz) klystrons powered by pulse-forming network (PFN) modulators, as reported in this paper. However, in order to meet the more stringent requirements of plasma wakefield acceleration schemes, some upgrades to this feedback system have been recently carried out. The first prototype has been experimentally tested on a C-band (5712 MHz) klystron, driven by a solid-state modulator, in order to investigate the possibility for additional improvement resulting from the inherently more stable power source. In this paper, the design, realization and the preliminary measurement results obtained at SPARC_LAB after such upgrades will be reviewed.

The SPARC_LAB facility, operating at the LNF-INFN laboratories in Frascati, has been devoted over the past years to R&D related to accelerators, advanced diagnostics, and generation of radiation ranging from THz to extreme UV light. The facility has recently been significantly upgraded and moved toward user-oriented applications, especially after the approval of two additional projects with the goal to develop radiation sources up to the soft x-ray range and in view of the EuPRAXIA project, whose preparatory phase is ongoing with the support of the European Commission. This paper summarizes the electron beam and radiation parameters and describes the layout and main features of the upgraded facility in view of future user-oriented experimental activities and applications.