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The possibility to accelerate electron beams to ultra-relativistic velocities over short distances by using plasma-based technology holds the potential for a revolution in the field of particle accelerators 1 – 4 . The compact nature of plasma-based accelerators would allow the realization of table-top machines capable of driving a free-electron laser (FEL) 5 , a formidable tool to investigate matter at the sub-atomic level by generating coherent light pulses with sub-ångström wavelengths and sub-femtosecond durations 6 , 7 . So far, however, the high-energy electron beams required to operate FELs had to be obtained through the use of conventional large-size radio-frequency (RF) accelerators, bound to a sizeable footprint as a result of their limited accelerating fields. Here we report the experimental evidence of FEL lasing by a compact (3-cm) particle-beam-driven plasma accelerator. The accelerated beams are completely characterized in the six-dimensional phase space and have high quality, comparable with state-of-the-art accelerators 8 . This allowed the observation of narrow-band amplified radiation in the infrared range with typical exponential growth of its intensity over six consecutive undulators. This proof-of-principle experiment represents a fundamental milestone in the use of plasma-based accelerators, contributing to the development of next-generation compact facilities for user-oriented applications 9 . Using a compact, particle-beam-driven plasma-based accelerator to accelerate high-quality electron beams that are completely characterized in the six-dimensional phase space, free-electron lasing  is observed with narrow-band amplified radiation in the infrared range.

Next-generation plasma-based accelerators can push electron bunches to gigaelectronvolt energies within centimetre distances1,2. The plasma, excited by a driver pulse, generates large electric fields that can efficiently accelerate a trailing witness bunch3–5, enabling the realization of laboratory-scale applications ranging from high-energy colliders6 to ultrabright light sources7. So far, several experiments have demonstrated large accelerations8–10 but the resulting beam quality, particularly the energy spread, is still far from state-of-the-art conventional accelerators. Here we show the results of a beam-driven plasma acceleration experiment where we used an electron bunch as a driver followed by an ultrashort witness bunch. By setting a positive energy chirp on the witness bunch, its longitudinal phase space is rotated during acceleration, resulting in an ultralow energy spread that is even lower than the spread at the plasma entrance. This result will significantly impact the optimization of the plasma acceleration process and its implementation in forthcoming compact machines for user-oriented applications.In a beam-driven plasma wakefield accelerator, the energy spread of an electron bunch is reduced with respect to the plasma entrance, which is achieved through setting a positive energy chirp that rotates the bunches’ longitudinal phase space.

Applications such as colliders and plasma wake field acceleration require high gradient quadrupoles, in the range of 400-500 T/m and with a bore of few millimeters in diameter. The design of a tunable high gradient permanent magnet quadrupole (PMQ), based on the QUAPEVA design [1] developed for the SOLEIL synchrotron, is presented. The quadrupole has a fixed part made of a Halbach quadrupole array [3, 4] surrounded by four permanent magnet cylinders with a radial orientation of the magnetic momentum. The gradient is regulated by rotating the cylinders, reaching a tunability greater than the 25%. The quadrupole has been designed for the COMB plasma wake field beam driven experiment for the SPARC_LAB test-facility at INFN-LNF [2], one of the candidates to host the EuPRAXIA project [7, 8]. The present layout foresees two triplets where the focusing strength tuning is performed by moving two quadrupoles of each triplet along the beam axis. The new quadrupoles have bigger gradient and less multipolar content than actual ones, moreover it has a tuning system that does not need any shift of the magnet.

The Free-Electron Laser facility of the EuPRAXIA@SPARC_LAB infrastructure is driven by an electron beam with 1 GeV energy, produced by an X-band normal conducting linear accelerator followed by a plasma wakefield acceleration stage. The AQUA beamline aims at delivering variable polarization photons in the 3-4 nm wavelength range by means of out-of-vacuum APPLE-X permanent magnet undulators with 18 mm period length. The main radiator is composed by an array of ten APPLE-X 2 m-long modules. The current AQUA design is investigated and discussed taking into account effects on the FEL performance from off-axis injection as well as from modelling of the resistive wall wakefields in the foreseen vacuum chamber.

High-brightness RF photo-injectors are crucial for generating high peak current and low transverse emittance electron beams, which are necessary for driving plasma Wake-field acceleration in advanced accelerator concepts and novel radiation sources. To enhance the EuPRAXIA@SPARC_LAB photo-injector for future upgrades, it is essential to investigate and assess the feasibility of achieving higher charge and multi-bunch working points, commonly referred to as the driver and witness configuration for particle-driven Wake-field acceleration. A solution to reduce the photo-injector’s footprint while preserving beam quality and brightness is to implement a C-band injector operating at 5.712 GHz. Evaluating the possibility of achieving a working point within the velocity bunching acceleration scheme is critical, as this will determine the degree of compression achievable with a full C-band injector. Start-to-end beam dynamics simulations will be conducted to identify the optimum configuration for the C-band photo-injector dedicated to particle-driven plasma-based acceleration.

A systematic study of the polarization of x-gamma rays produced in Thomson and Compton scattering is presented, in both classical and quantum schemes. Numerical results and analytical considerations let us to establish the polarization level as a function of acceptance, bandwidth and energy. Few sources have been considered: the SPARC_LAB Thomson device, as an example of a x-ray Thomson source, ELI-NP, operating in the gamma range. Then, the typical parameters of a beam produced by a plasma accelerator has been analyzed. In the first case, with bandwidths up to 10%, a contained reduction (<10% ) in the average polarization occurs. In the last case, for the nominal ELI-NP relative bandwidth of 5×10−3 , the polarization is always close to 1. For applications requiring larger bandwidth, however, a degradation of the polarization up to 30% must be taken into account. In addition, an all optical gamma source based on a plasma accelerated electron beam cannot guarantee narrow bandwidth and high polarization operational conditions required in nuclear photonics experiments.

The generation of ultra-short electron bunches with ultra-low timing-jitter relative to the photo-cathode (PC) laser has been experimentally proved for the first time at the SPARC_LAB test-facility (INFN-LNF, Frascati) exploiting a two-stage hybrid compression scheme. The first stage employs RF-based compression (velocity-bunching), which shortens the bunch and imprints an energy chirp on it. The second stage is performed in a non-isochronous dogleg line, where the compression is completed resulting in a final bunch duration below 90 fs (rms). At the same time, the beam arrival timing-jitter with respect to the PC laser has been measured to be lower than 20 fs (rms). The reported results have been validated with numerical simulations.

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 with a wavelength of 2-4 nm. 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 inside the plasma module around of 1 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.

The production of high spectral brilliance radiation from electron beam sources depends critically on the electron beam qualities. One must obtain very high electron beam brightness, implying simultaneous high peak current and low emittance. These attributes are enabled through the use of very high field acceleration in a radio-frequency (rf) photoinjector source. Despite the high fields currently utilized, there is a limit on the achievable peak current in high brightness operation, in the range of tens of Ampere. This limitation can be overcome by the use of a hybrid standing wave/traveling wave structure; the standing wave portion provides acceleration at a high field from the photocathode, while the traveling wave part yields strong velocity bunching. This approach is explored here in a C-band scenario, at field strengths (>100MV/m) at the current state-of-the-art. It is found that one may arrive at an electron beam with many hundreds of Amperes with well-sub-micron normalized emittance. This extremely compact injector system also possesses attractive simplification of the rf distribution system by eliminating the need for an rf circulator. We explore the use of this device in a compact 400 MeV-class source, driving both inverse Compton scattering and free-electron laser radiation sources with unique, attractive properties.

Plasma wakefield acceleration is nowadays very attractive in terms of accelerating gradient, able to overcome conventional accelerators by orders of magnitude. However, this poses very demanding requirements on the accelerator stability to avoid large instabilities on the final beam energy. In this study we analyze the correlation between the driver-witness distance jitter (due to the radio-frequency timing jitter) and the witness energy gain in a plasma wakefied accelerator stage. Experimental measurements are reported by using an electro-optical sampling diagnostics with which we correlate the distance between the driver and witness beams before the plasma accelerator stage. The results show a clear correlation due to such a distance jitter, highlighting the contribution coming from the radio-frequency (RF) compression.