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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.

We discuss a two-color SASE free-electron laser (FEL) amplifier where the time and energy separation of two separated radiation pulses are controlled by manipulation of the electron beam phase space. Two electron beamlets with adjustable time and energy spacing are generated in an RF photo-injector illuminating the cathode with a comb-like laser pulse followed by RF compression in the linear accelerator. We review the electron beam manipulation technique to generate bunches with time and energy properties suitable for driving two-color FEL radiation. Experimental measurements at the SPARC-LAB facility illustrate the flexibility of the scheme for the generation of two-color FEL spectra.

Next-generation plasma-based accelerators can push electron beams to GeV energies within centimeter distances. The plasma, excited by a driver pulse, is indeed able to sustain huge electric fields that can efficiently accelerate a trailing witness bunch, which was experimentally demonstrated on multiple occasions. Thus, the main focus of the current research is being shifted towards achieving a high quality of the beam after the plasma acceleration. In this paper we present a beam-driven plasma wakefield acceleration experiment, where initially preformed high-quality witness beam was accelerated inside the plasma and characterized. In this experiment the witness beam quality after the acceleration was maintained on high level, with 0.2% final energy spread and 3.8 μm resulting normalized transverse emittance after the acceleration. In this article, for the first time to our knowledge, the emittance of the plasma wakefield accelerated beam was directly measured.

Trains of ultrashort electron pulses with THz repetition rate, so-called comblike beams, are assuming an ever growing interest in plasma-based acceleration. In particle-driven plasma wakefield acceleration (PWFA), a train of driver bunches with separation of the order of plasma wavelength, i.e., 300 μm, resonantly excites a plasma wake, which accelerates a trailing witness bunch, injected at the accelerating phase. Comblike beams have great potentialities in different fields of applications. In particular, radiation sources, such as free-electron lasers and THz radiation, take advantage from the possibility to tailor electron beams modulated both in time and energy, to customize emission bandwidth and temporal properties. In these scenarios, the manipulation of longitudinal phase space to investigate different bunch configurations, in terms of energy and time separation, is founded on the knowledge of the 6D phase space of each bunch in the train. In this paper we present the methods developed at the SPARC_LAB test facility in order to fulfill the requirements. Starting from conventional diagnostics, therefore applying well-known tools using more than one diagnostic at the same time, we have completely characterized not only the full 6D phase space of a comblike electron beam with THz repetition rate, but also each single bunch within the train. To our knowledge, this is the first time such a measurement has been performed. Experimental results for multibunch trains in different configurations, suitable for PWFA applications, will be shown and discussed.

We have developed, at the SPARC test facility, a procedure for a real time self-amplified spontaneous emission free electron laser (FEL) device performance control. We describe an actual FEL, including electron and optical beam transport, through a set of analytical formulas, allowing a fast and reliable on-line “simulation” of the experiment. The system is designed in such a way that the characteristics of the transport elements and the laser intensity are measured and adjusted, via a real time computation, during the experimental run, to obtain an on-line feedback of the laser performances. The detail of the procedure and the relevant experimental results are discussed.

A reliable transverse emittance measurement for high-brightness electron beams is of utmost importance for the successful development of fourth generation light sources and for the beam transport in plasma-based accelerators. When the beam exhibits a significant energy spread, typical quadrupole scan emittance measurements may be affected depending on the beam properties and on the quadrupoles arrangement. The emittance degradation induced by chromatic effects in measurements involving magnetic lattices is evaluated analytically for different configurations. Analytical and numerical calculations compared with measurements have been used to evaluate the consequent error on the emittance value measured for single and double quadrupole schemes and for typical operating conditions at the SPARC facility.

Peak current represents a key demand for new generation electron beam photoinjectors. Many beam applications, such as free electron laser, inverse Compton scattering, terahertz radiation generation, have efficiencies strongly dependent on the bunch length and current. A method of beam longitudinal compression (called velocity bunching) has been proposed some years ago, based on beam longitudinal phase space rotation in a rf field potential. The control of such rotation can lead to a compression factor in excess of 10, depending on the initial longitudinal emittance. Code simulations have shown the possibility to fully compensate the transverse emittance growth during rf compression, and this regime has been experimentally proven recently at SPARC. The key point is the control of transverse beam plasma oscillations, in order to freeze the emittance at its lowest value at the end of compression. Longitudinal and transverse phase space distortions have been observed during the experiments, leading to asymmetric current profiles and higher final projected emittances. In this paper we discuss in detail the results obtained at SPARC in the regime of velocity bunching, analyzing such nonlinearities and identifying the causes. The beam degradation is discussed, both for slice and projected parameters. Analytical tools are derived to experimentally quantify the effect of such distortions on the projected emittance.

SPARC (acronym of “Sorgente Pulsata ed Amplificata di Radiazione Coerente”, i.e. Pulsed and Amplified Source of Coherent Radiation) is a single pass free-electron laser designed to obtain high gain amplification at a radiation wavelength of 500 nm. Self-amplified spontaneous emission has been observed driving the amplifier with the high-brightness beam of the SPARC linac. We report measurements of energy, spectra, and exponential gain. Experimental results are compared with simulations from several numerical codes.

The linac of the European project Extreme Light Infrastructure-Nuclear Physics (ELI-NP) foresees the use of 12 traveling wave C-band accelerating structures. The cavities are 1.8 m long, quasiconstant gradient, and have a field phase advance per cell of 2π/3. They operate at 100 Hz repetition rate, and, because of the multibunch operation, they have been designed with a dipole higher-order mode (HOM) damping system to avoid beam breakup. The structures have symmetric input and output couplers and integrate, in each cell, a damping system based on silicon carbide (SiC) rf absorbers coupled to each cell through waveguides. An optimization of the electromagnetic and mechanical design has been done to simplify the fabrication and to reduce the costs. The cavities have been fabricated, and the first full-scale prototype has been also successfully tested at the nominal gradient of 33 MV/m, repetition rate of 100 Hz, and pulse length of 820 ns. It represents, to our knowledge, the first full-scale linac structure with HOM damping waveguides and SiC absorbers tested at this high gradient. In the paper, we illustrate the realization process of such a complicated device together with the low and high power test results.

Generation of a well-aligned bunch train is of vital importance for preserving the beam quality in a plasma wake-field accelerator. The simultaneous measurements of the relative misalignments in both planes, of the individual femtosecond electron bunches with THz repetition rate in a train, are reported and analyzed with simulations. The new method proposed in this paper demonstrates that the measurements can be done quickly by applying a single quadrupole scan together with a radio-frequency deflector (RFD), while the RFD strength and the RFD phase for each bunch do not need to be known.

The new generation of linac injectors driving free electron lasers in the self-amplified stimulated emission (SASE-FEL) regime requires high brightness electron beams to generate radiation in the wavelength range from UV to x rays. The choice of the injector working point and its matching to the linac structure are the key factors to meet this requirement. An emittance compensation scheme presently applied in several photoinjectors worldwide is known as the “Ferrario” working point. In spite of its great importance there was, so far, no direct measurement of the beam parameters, such as emittance, transverse envelope, and energy spread, in the region downstream the rf gun and the solenoid of a photoinjector to validate the effectiveness of this approach. In order to fully characterize the beam dynamics with this scheme, an innovative beam diagnostic device, the emittance meter, consisting of a movable emittance measurement system, has been designed and built. With the emittance meter, measurements of the main beam parameters in both transverse phase spaces can be performed in a wide range of positions downstream the photoinjector. These measurements help in tuning the injector to optimize the working point and provide an important benchmark for the validation of simulation codes. We report the results of these measurements in the SPARC photoinjector and, in particular, the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac.

The breakthrough provided by plasma-based accelerators enabled unprecedented accelerating fields by boosting electron beams to gigaelectronvolt energies within a few centimeters [1-4]. This, in turn, allows the realization of ultracompact light sources based on free-electron lasers (FELs) [5], as demonstrated by two pioneering experiments that reported the observation of self-amplified spontaneous emission (SASE) driven by plasma-accelerated beams [6,7]. However, the lack of stability and reproducibility due to the intrinsic nature of the SASE process (whose amplification starts from the shot noise of the electron beam) may hinder their effective implementation for user purposes. Here, we report a proof-of-principle experiment using plasma-accelerated beams to generate stable and reproducible FEL light seeded by an external laser. FEL radiation is emitted in the infrared range, showing the typical exponential growth of its energy over six consecutive undulators. Compared to SASE, the seeded FEL pulses have energies 2 orders of magnitude larger and stability that is 3 times higher.

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 ., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

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, to be used in many fields for industrial, medical, and research applications. The ability to shape the beam longitudinal phase space, in particular, plays a key role in achieving high-peak brightness. Here we present a new approach that allows us to tune the longitudinal phase space of a high-brightness beam by means of plasma wakefields. The electron beam passing through the plasma drives large wakefields that are used to manipulate the time-energy correlation of particles along the beam itself. We experimentally demonstrate that such a solution is highly tunable by simply adjusting the density of the plasma and can be used to imprint or remove any correlation onto the beam. This is a fundamental requirement when dealing with largely time-energy correlated beams coming from future plasma accelerators.

Plasma-based technology promises a tremendous reduction in size of accelerators used for research, medical, and industrial applications, making it possible to develop tabletop machines accessible for a broader scientific community. By overcoming current limits of conventional accelerators and pushing particles to larger and larger energies, the availability of strong and tunable focusing optics is mandatory also because plasma-accelerated beams usually have large angular divergences. In this regard, active-plasma lenses represent a compact and affordable tool to generate radially symmetric magnetic fields several orders of magnitude larger than conventional quadrupoles and solenoids. However, it has been recently proved that the focusing can be highly nonlinear and induce a dramatic emittance growth. Here, we present experimental results showing how these nonlinearities can be minimized and lensing improved. These achievements represent a major breakthrough toward the miniaturization of next-generation focusing devices.

Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials such as graphene and topological insulators. Dirac electrons are characterized by notable properties such as a high mobility, a tunable density and, in topological insulators, a protection against backscattering through the spin-momentum locking mechanism. All those properties make graphene and topological insulators appealing for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behaviour. This should mirror in phenomena such as electromagnetic-induced transparency and harmonic generation. Here we demonstrate that in Bi2Se3 topological insulator, an electromagnetic-induced transparency is achieved under the application of a strong terahertz electric field. This effect, concomitantly determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se3, and opens the road towards tunable terahertz nonlinear optical devices based on topological insulator materials.

We present the experimental evidence of the generation of coherent and statistically stable two-color free-electron laser radiation obtained by seeding an electron beam double peaked in energy with a laser pulse single spiked in frequency. The radiation presents two neat spectral lines, with time delay, frequency separation, and relative intensity that can be accurately controlled. The analysis of the emitted radiation shows a temporal coherence and a shot-to-shot regularity in frequency significantly enhanced with respect to the self-amplified spontaneous emission.

The two-dimensional single shot transverse coherence of the Self-Amplified Spontaneous Emission of the SPARC_LAB Free-Electron Laser was measured through the statistical analysis of a speckle field produced by heterodyning the radiation beam with a huge number of reference waves, scattered by a suspension of particles. In this paper we report the measurements and the evaluation of the transverse coherence along the SPARC_LAB undulator modules. The measure method was demonstrated to be precise and robust, it does not require any a priori assumptions and can be implemented over a wide range of wavelengths, from the optical radiation to the x-rays.

We present the experimental demonstration of a new scheme for the generation of ultrashort pulse trains based on free-electron-laser (FEL) emission from a multipeaked electron energy distribution. Two electron beamlets with energy difference larger than the FEL parameter ρ have been generated by illuminating the cathode with two ps-spaced laser pulses, followed by a rotation of the longitudinal phase space by velocity bunching in the linac. The resulting self-amplified spontaneous emission FEL radiation, measured through frequency-resolved optical gating diagnostics, reveals a double-peaked spectrum and a temporally modulated pulse structure.

The linac driven coherent THz radiation source at the SPARC-LAB test facility is able to deliver broadband THz pulses with femtosecond shaping. In addition, high peak power, narrow spectral bandwidth THz radiation can be also generated, taking advantage of advanced electron beam manipulation techniques, able to generate an adjustable train of electron bunches with a sub-picosecond length and with sub-picosecond spacing. The paper reports on the manipulation, characterization, and transport of the electron beam in the bending line transporting the beam down to the THz station, where different coherent transition radiation spectra have been measured and studied with the aim to optimize the THz radiation performances.

We report measurements demonstrating the concept of the free-electron laser (FEL) superradiant cascade. Radiation (λ(rad) = 200 nm) at the second harmonic of a short, intense seed laser pulse (λ(seed) = 400 nm) was generated by the cascaded FEL scheme at the transition between the modulator and radiator undulator sections. The superradiance of the ultrashort pulse is confirmed by detailed measurements of the resulting spectral structure, the intensity level of the produced harmonics, and the trend of the energy growth along the undulator. These results are compared to numerical particle simulations using the FEL code GENESIS 1.3 and show a satisfactory agreement.

Higher order harmonic generation in a free-electron laser amplifier operating in the superradiant regime [R. H. Dicke, Phys. Rev. 93, 99 (1954).] has been observed. Superradiance has been induced by seeding a single-pass amplifier with the second harmonic of a Ti:sapphire laser, generated in a β-Barium borate crystal, at seed intensities comparable to the free-electron laser saturation intensity. Pulse energy and spectral distributions of the harmonics up to the 11th order have been measured and compared with simulations.

The injection of a seed in a free-electron laser (FEL) amplifier reduces the saturation length and improves the longitudinal coherence. A cascaded FEL, operating in the high-gain harmonic-generation regime, allows us to extend the beneficial effects of the seed to shorter wavelengths. We report on the first operation of a high-gain harmonic-generation free-electron laser, seeded with harmonics generated in gas. The third harmonics of a Ti:sapphire laser, generated in a gas cell, has been amplified and up-converted to its second harmonic (λ(rad)=133 nm) in a FEL cascaded configuration based on a variable number of modulators and radiators. We studied the transition between coherent harmonic generation and superradiant regime, optimizing the laser performances with respect to the number of modulators and radiators.

We report the first experimental implementation of a method based on simultaneous use of an energy chirp in the electron beam and a tapered undulator, for the generation of ultrashort pulses in a self-amplified spontaneous emission mode free-electron laser (SASE FEL). The experiment, performed at the SPARC FEL test facility, demonstrates the possibility of compensating the nominally detrimental effect of the chirp by a proper taper of the undulator gaps. An increase of more than 1 order of magnitude in the pulse energy is observed in comparison to the untapered case, accompanied by FEL spectra where the typical SASE spiking is suppressed.

In this Letter we report the first experiments aimed at the simultaneous demonstration of the emittance compensation process and velocity bunching in a high brightness electron source, the SPARC photoinjector in INFN-LNF. While a maximum compression ratio up to a factor 14 has been observed, in a particular case of interest a compression factor of 3, yielding a slice current of 120 A with less than 2 microm slice emittance, has been measured. This technique may be crucial in achieving high brightness beams in photoinjectors aiming at optimized performance of short wavelength single-pass free electron lasers or other advanced applications in laser-plasma accelerators.

In this Letter we report the first experimental observation of the double emittance minimum effect in the beam dynamics of high-brightness electron beam generation by photoinjectors; this effect, as predicted by the theory, is crucial in achieving minimum emittance in photoinjectors aiming at producing electron beams for short wavelength single-pass free electron lasers. The experiment described in this Letter was performed at the SPARC photoinjector site, during the first stage of commissioning of the SPARC project. The experiment was made possible by a newly conceived device, called an emittance meter, which allows a detailed and unprecedented study of the emittance compensation process as the beam propagates along the beam pipe.