At EuPRAXIA@SPARC_LAB an X-ray FEL user facility is driven by a plasma accelerator in the particle-driven configuration where an ultra-relativistic beam, the driver, through a plasma generates a wake of charge density useful for accelerate a witness beam. The electron bunches are generated through the so-called comb technique in an RF injector that consist of a 1.6 cell S-band gun followed by four S-band TW accelerating structures. The main working point foresees a 30pC witness and a 200pC driver longitudinally compressed in the first accelerating structure operated in the velocity-bunching regime, that allows to accelerate and manipulate the beam to reach proper transverse and longitudinal parameters. The optimization of the witness emittance is performed with additional magnetic field around the gun and the S-band structures and by shaping the laser pulse at the cathode. The paper reports on beam dynamics studies performed also for beams with higher charges to maximize the transformer ratio in the plasma and the beam brightness. In addition, the insertion of an X-band RF cavity after the gun is proposed aiming to shape the beam current distribution as needed and stabilize it with respect to RF jitters.

The development of compact accelerator facilities providing high-brightness beams is one of the most challenging tasks in the field of next-generation compact and cost affordable particle accelerators. Recent results obtained at SPARC_LAB show evidence of the FEL laser by a compact (3 cm) particle beam plasma accelerator. This work is carried out in the framework of the SPARC_LAB activities concerning the R&D on particle-driven plasma wakefield accelerators for the realization of new compact plasma based facilities i.e EuPRAXIA@SPARC_LAB. The work here presented is a theoretical study demonstrating a possible scheme concerning the implementation of an innovative array of discharge capillaries, operating as active-plasma lenses, and one collimator to build an unconventional transport line for bunches outgoing from plasma accelerating module. Taking advantage of the symmetric and linear focusing provided by an active-plasma lens, the witness is captured and transported along the array without affecting its quality at the exit of the plasma module. At the same time the driver, being over-focused in the same array, can be removed by means of a collimator.

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.

Nowadays, Energy Recovery Linacs (ERLs) became really appealing thanks to their low environmental impact and high sustainability. ERLs require a special low energy injector, usually named merger. The energy at merger exit is clearly the energy that can’t be recycled in the ERL machine and is the amount dumped at the end. The lower the injection energy is the more efficient is the energy recovery process. A physiological issue of low energy ERL injection is the presence of space charge in the dispersive section that introduces to dispersion leaks. Worldwide ad hoc solutions for mergers beamlines design have been studied to address this problem. Here we present a different approach that allowed us to exploit a standard dogleg to design a very low energy merger for an ERL. This has been made possible thanks to the application of the GIOTTO AI code that optimizes of the optics setting finding a proper achromatic configuration.

The application of HTS coils as a matching device and a large-aperture L-band linac make it possible to transport a substantial part of positrons generated in a positron production target through a capture linac. It raises a question of how to manage their large phase space to provide bunches matched to the damping ring acceptance. This paper presents the beam dynamics studies of the FCC-ee positron linac consisting of an adiabatic matching device (AMD) with theoretical field distribution combined with constant solenoidal field along $\frac{9}{10}\pi$ large aperture L-band accelerating sections. AMD field drop rate, as well as the RF field phase and accelerating section length, were varied to find features of a bunch formation. It was shown that 5D normalized beam brightness is a useful parameter to optimize the initial part of the capture linac. A higher beam brightness can be obtained for the higher AMD field drop rate. Starting from some accelerating section length, two peak structure appears in the normalized brightness dependence on the RF field phase. The peaks correspond to the acceleration of the head or the tail of the initial positron longitudinal distribution. The last one provides a higher positron yield.

The studies and R&D on the high-intensity positron source for the FCC-ee have been initiated for a while. The positrons are produced by a 6 GeV electron drive-beam incident on a target-converter at 200 Hz. The drive beam comes in 2 bunches spaced by 25 ns with a maximum charge of ~5 nC per bunch. Two scenarios using conventional and hybrid targets are being studied for positron production. According to the FCC CDR, the Flux Concentrator is used as the matching device for the capture system, followed by several accelerating structures embedded in the solenoidal field. Then, the positrons are further accelerated to be injected into the damping ring. Recently, the feasibility study on using a SC solenoid for the positron capture has been started, and the design based on the HTS technology is under investigation. In addition, the large aperture 2 GHz RF structures, which have been specially designed for the FCC-ee positron capture system, are used with the goal of demonstrating accepted positron yield values well beyond the values obtained with state-of-the-art positron sources. The purpose of this paper is to review the current status of the FCC-ee positron source design, highlighting the recent research into the positron production, capture system, primary acceleration, and injection into the damping ring.

The development of compact accelerator facilities providing high-brightness beams is one of the most challenging tasks in the field of next-generation compact and cost affordable particle accelerators. Recent results obtained at SPARC_LAB show evidence of the FEL laser by a compact (3 cm) particle beam plasma accelerator. This work is carried out in the framework of the SPARC_LAB activities concerning the R&D on particle-driven plasma wakefield accelerators for the realization of new compact plasma based facilities i.e EuPRAXIA@SPARC_LAB. The work here presented is a theoretical study demonstrating a possible scheme concerning the implementation of an innovative array of discharge capillaries, operating as active-plasma lenses, and one collimator to build an unconventional transport line for bunches outgoing from plasma accelerating module. Taking advantage of the symmetric and linear focusing provided by an active-plasma lens, the witness is captured and transported along the array without affecting its quality at the exit of the plasma module. At the same time the driver, being over-focused in the same array, can be removed by means of a collimator.

Hollow-core dielectric Electromagnetic Band Gap (EBG) microstructures powered by lasers represent a new and promising area of accelerator research since, thanks to the short optical wavelength and to the dielectric's high damage threshold greater accelerating gradients, with respect to the metallic counterparts, can be achieved. In this paper, we present MeV-scale beam-dynamics simulations and fabrication results relative to a silicon, woodpile-based travelling-wave structure operating at the wavelength 𝜆0 = 5 μm. The simulated CST and HFSS electric field has been evaluated and used as input for a space charge tracking code, in order to perform beam-dynamics evaluations on the beam injection and extraction into the proposed structure as well as the evolution of the main beam parameters. We also report on the fabrication of first Si prototypes of the woodpile structure that are obtained by the innovative Two Photon Polymerization fabrication process. This technique allows to reach resolutions down to hundreds of nanometers, offering the possibility to print Si-rich structures, or woodpile skeletons to be infiltrated with Si by CVD technique.

One of the key aspects to provide on chip acceleration in Dielectric Laser Accelerators (DLA) from tens of keV up to MeV energies is the phase velocity tapering. This paper presents the simulated performance of sub-relativistic structures, based on tapered slot waveguides. We engineered channel/defect modification in order to obtain a variable phase velocity matched to the increasing velocity of the accelerated particles. Additionally, we present a hollow-core relativistic electromagnetic band gap (EGB) accelerating waveguide. In DLA structures co-propagating schemes are employed for higher efficiency and smaller footprint compared to the cross-propagating schemes. In this respect, we envisage tapered continuous copropagating structures that simultaneously allow wave launching/coupling, beam acceleration, and transverse focusing. The main figures of merit, such as the accelerating gradient, the total energy gain, and the transverse focusing/defocusing forces, are evaluated and used to guide the optimization of the channel/defect modification. Index terms: Dielectric Laser Accelerators (DLA), Photonic Crystal, Dielectric Waveguides

The Southern European Thomson back-scattering source for Applied Research (STAR) project, based on a collaboration among University of Calabria (UniCal), CNISM, INFN and Sincrotrone Trieste, has the goal to install and test at UniCal a short linear accelerator for high brightness electron beams that will drive a unique advanced X-ray Thomson source. In 2021 INFN was committed to install, test and commission an upgrade of the STAR Linac (STAR High Energy Linac) aiming to increase the X-ray beams energy from 30 keV up to 140 KeV. The new layout foreseen an increase of the electron beam energy from 65 MeV up to 150 MeV by the installation of two additional C-band acceleration cavities and an additional transfer-line where the high energy beam could be delivered to a second interaction point with the laser. The whole machine layout foreseen 43 warm electromagnets (solenoids, dipoles, quadrupoles and steerers) powered by 59 DC power supplies that will cover a wide power range from 90W up to 15 kW. In this paper, an overview of the magnet system is given together with the performed tests, the deliveries status and the future steps needed to finalize the complete machine installation.

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.

In 2021, the Italian Institute for Nuclear Physics (INFN) was awarded the project for installing, testing and commissioning the energy upgrade of the Southern European Thomson back-scattering source for Applied Research (STAR) which is currently installed at the University of Calabria (UniCal). The STAR high-energy Linac, STAR-HEL, consists in a layout comprising RF accelerating structures (linacs), with relative magnetic optics components, in order to boost the electron beam energy from 65 MeV up to 150 MeV. In this paper, we discuss the status of the planning, installation and testing of the RF system (accelerating structures, power, network and LLRF) based on C-band (i.e. 5712 MHz RF frequency) technology. For this purpose, two C-band linacs are installed and are independently powered by two RF power stations, located aside the present S-band RF power station, which will deliver 42 MW (nominal) peak power RF pulses of 1us width and up to 100 Hz repetition rate. Operation in C-band permits acceleration with higher gradients, resulting in a more compact linac footprint.

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.

At EuPRAXIA@SPARC_LAB an X-ray FEL user facility is driven by a plasma accelerator in the particle-driven configuration where an ultra-relativistic beam, the driver, through a plasma generates a wake of charge density useful for accelerate a witness beam. The electron bunches are generated through the so-called comb technique in an RF injector that consist of a 1.6 cell S-band gun followed by four S-band TW accelerating structures. The main working point foresees a 30pC witness and a 200pC driver longitudinally compressed in the first accelerating structure operated in the velocity-bunching regime, that allows to accelerate and manipulate the beam to reach proper transverse and longitudinal parameters. The optimization of the witness emittance is performed with additional magnetic field around the gun and the S-band structures and by shaping the laser pulse at the cathode. The paper reports on beam dynamics studies performed also for beams with higher charges to maximize the transformer ratio in the plasma and the beam brightness. In addition, the insertion of an X-band RF cavity after the gun is proposed aiming to shape the beam current distribution as needed and stabilize it with respect to RF jitters.

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.