Energy Recovery linear accelerator (ERL) light source facilities based on superconducting radiofrequency (SRF) are deemed of the most resplendent techniques in the future of accelerator physics. Running in a continuous waves mode with a high repetition rate for a long timescale, we discuss High order modes (HOMs) analysis in a two-pass two-way ERL scheme where acceleration and deceleration of electron bunches are supported by a standing wave structure of the RF cavity. The analysis reported in this paper is based on differential equations that describe the beam dynamics (BD) to overcome the limitations imposed by high currents and insure energy recuperation over millions of interactions.

The interest in plasma-based accelerators as drivers of user facilities is growing worldwide thanks to their compactness and reduced costs. The EuPRAXIA@SPARC_LAB collaboration is preparing a technical design report for a multi-GeV plasma-based accelerator with outstanding electron beam quality to pilot an X-ray FEL, the most demanding in terms of beam brightness. The beam dynamics has been studied aiming to a reliable operation of the RF injector to generate a so-called comb-beam with 500 MeV energy suitable as driver of the Beam-driven Plasma Wakefield Accelerator. A case of interest is the generation of a trailing bunch with 1 GeV energy, less than 1 mm-mrad transverse emittance and up to 2 kA peak current at the undulator entrance. The comb-beam is generated through the velocity bunching technique, an RF compression tool that enables high brightness beams within relatively compact machine. Since it is based on a rotation of the beam phase space inside the external RF fields, it could be particularly sensitive to amplitude and phase jitters in the RF injector. The electron beam dynamics and the machine sensitivity to the possible jitters are presented in terms of effect on the beam quality so to provide the basis for the alignment procedure and jitter tolerances. Numerical studies have been consolidated with experimental results obtained at SPARC_LAB, a test facility currently oriented to plasma acceleration physics where the velocity bunching scheme is routinely applied.

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 realization of a plasma based user facility on the model of EuPRAXIA@SPARC_LAB requires to design a working point for the operation that allows to get an high accelerating gradient preserving a low emittance and low energy spread of the accelerated beam. Such beam is supposed to pilot a soft x-ray free electron laser, a device with very challenging requirements in terms of brightness and energy spread. The external injection beam driven scheme by means of an RF photoinjector allows a fine tuning of the working point parameters at the injection, but the high beam current dictates the maximum accelerating gradient that can be obtained while preserving energy spread. These parameters are mostly connected to each other depending on the plasma wavelength and on the separation phase between driver and witness. In this work several simulation scans are presented, varying at the same time the plasma density and driver-witness separation in order to show that, in a realistic working point for EuPRAXIA@SPARC_LAB, it is possible to find an ideal compromise for a witness with a peak current >1kA that allows to preserve the energy spread of the core (80% of the charge) below 0.1%, while maintaining an accelerating gradient of the order of GV/m. The study is completed with a parametric analysis with the aim of establishing the stability requirements of the RF working point and the plasma channel in order to preserve the energy jitter at the same level of the energy spread.

We present the conceptual design of a compact light source named BriXSinO. BriXSinO was born as a demonstrator of the Marix project, but contains also a dual high flux radiation source Inverse Compton Source (ICS) of X-ray and a Free-Electron Laser Oscillator of THz spectral range radiation conceived for medica applications and general applied research. The accelerator is a push-pull CW-SC Energy Recovery Linac (ERL) based on the technology of superconducting cavities and allows to sustain MW-class beam power with al-most just one hundred kW active power dissipation/consumption. ICS line produces 33 keV monochromatic X-Rays via Compton scattering of the electron beam with a laser system in Fabry-Pérot cavity at a repetition rate of 100 MHz. The THz FEL oscillator is based on an undulator imbedded in optical cavity and generates THz wavelengths from 15 to 50 micron. The possibility of generating in the FEL cavity also synchronized X radiation is also shown.

A High Brightness Beams Test Facility has been recently funded at the INFN-LASA laboratory in Segrate (Milan- Italy). The Test Facility will allow to perform developments in ERL construction and design and to carry out experiments with the high current CW electron beam in frontier areas of accelerator physics. The Test Facility setup will comprise a high-performance laser driven DC Gun followed by a normal conducting RF buncher-acceleration section to provide 1 MeV 5 mA CW electron beam. A Superconducting RF booster linac able to increase the electron energies up to 5-10 MeV maintaining beam current up to 2.5 mA is part of the proposal for further funding.

Thomson/Compton scattering is a method to produce high energy photons through the collision of low energy photons in a laser pulse onto relativistic electrons. In the linear (incoherent) Thomson/Compton regime, the flux scales linearly with the number of primary particles and the bandwidth of the produced photons depend, amongst other factors, on the energy spread of them. In general, an increase of the primary particles is connected to a larger energy spread (e.g.non-constant acceleration gradients, collective effects, etc). Therefore their number is restricted by the desired bandwidth, and thus limits the flux. In our previous (theoretical) studies we showed that the ideal Thomson spectrum can be retrieved when an electron bunch with a linear energy correlation of several percent collides with a matched linearly chirped laser pulse. Here we extend the scheme to allow for higher order energy correlations and quantify how the electron distribution influences the bandwidth. Furthermore we discuss the practical viability to maximize the primary particles, with the focus on linear accelerators (LINACS) for the electrons and laser pulses based on the chirped pulse amplification (CPA) scheme. These could potentially provide up to tens of nano-Coulomb electron bunches and tens, or even over a hundred, Joule lasers pulses respectively.

The relativistic interaction of short pulsed lasers or electrons with plasma has recently led to the birth of a new generation of femtosecond X-ray sources. Radiations with properties similar to those that can be observed from a wiggler or undulator, can be generated by the oscillations induced in the exited plasma by electrons (PWFA) or by lasers (LWFA), making plasma an interesting medium both for the acceleration as well as for the radiation source, whit properties of being compact, providing collimated, incoherent, femtosecond radiation, and a lot of effort is being made to understand and improve this new source to make it really competitive. This paper summarizes and shows some theoretical results and numerical simulation of a simplified model called plasma ion column, using as a starting point the parameters expected for EuPRAXIA@SPARC_LAB facility, highlighting strengths, limitations and scaling laws, which allow for a comparison with other types of more consolidated sources of light as Compton, Synchrotron and Free electron lasers.

The transfer line that carries the electron beam from the plasma to the undulators is certainly a critical line in EuPRAXIA@SPARC\_LAB as in all plasma driven Free Electron Lasers. This machine section must serve multiple purposes: capturing the highly divergent bunches at the plasma exit, separating the driver bunch from the witness and finally matching the witness to the FEL undulators. In addition, the line must be as compact as possible so as to best contain the chromatic outbreak of the beam. In this paper we present the results of the design and optimization phase of the transfer line taking into account important collective effects such as space-charge and coherent synchrotron radiation emission in the chicane. Moreover, we show here our evaluations on the expected effect of chromatism after the plasma extraction on the witness and its core and the filtering procedure of the witness halo.

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.