Nowadays, there is a deep interest in developing more compact user facilities, and plasma technology is one of the most promising techniques, not only for acceleration modules, but also for what is ancillary to the delivery of radiation to users, such as free electron lasers. In this regard, significant efforts have been made to miniaturize diagnostic stations, detection devices, and transfer lines, e.g., based on active plasma lenses. However, conventional undulators are still too cumbersome and expensive to meet the requirements of compactness and sustainability. For the aforementioned reasons, advanced undulator concepts have aroused great interest in pushing the frontier beyond conventional, magnet-based undulators. In this regard, a promising, very compact alternative is the use of the betatron motion of electrons in an ion channel to emulate an undulator device. This paper reports a feasibility study aiming to develop plasma-based undulator devices at SPARC_LAB as the test facility of the EuPRAXIA@SPARC_LAB project. In particular, this work provides a systematic assessment of free-electron-laser amplification in a plasma ion-channel undulator under experimentally realistic beam parameters, delivering quantitative predictions for gain and radiation performance in this configuration.

BACKGROUND:The clinical translation of Minibeam RT (MBRT) has recently started thanks to the first human treatments recently performed. However, despite experimental evidence, the impact of the dose distribution parameters involved on the magnitude of the effect itself and the underlying radiobiological mechanisms are still only partially understood. To address this issue, systematic investigations are needed through the implementation of advanced quantitative experiments with a multidisciplinary approach, which is the one proposed in the framework of the INFN funded MIRO (MInibeam RadiOtherapy) project. PURPOSE:The aim of this work is to report on the on-going main activities recently carried out in the framework of the project, showing some of the main results achieved during the first 2 years of the project, in terms of: (i) facilities development and characterization; (ii) new dosimetric approaches; (iii) biological investigation of the effect; and (iv) development of the first tools for dose planning. The multidisciplinary approach adopted by this national Collaboration to tackle the main challenges of minibeam radiotherapy for a reliable and solid clinical translation is discussed. METHODS:The facilities involved in the project are: (i) two facilities dedicated to low (up to 9 MeV) and medium (up to 18 MeV) energy electron minibeam studies, one of them also equipped with an electron FLASH LINAC to study possible synergistic effects with UHDR beams; (ii) one facility dedicated to proton minibeam studies, with energies from 70 to 140 MeV. Novel dosimetric approaches, are presented, mainly based on scintillators, silicon and silicon carbide detectors. Novel techniques for the analysis of biological samples are described, leveraging both integrated averaged data and spatially resolved analysis. Furthermore, a framework for biological modeling using a multiscale approach is presented. A dedicated dose-planning tool is currently under development to compare "virtual" minibeam plans with conventional ones and, consequently, to quantitatively investigate the potential for clinical translation. RESULTS:All the facilities were dosimetrically characterized, demonstrating the capability of producing controlled and reproducible electron and proton minibeams. These beams exhibited diverse physical parameters, including peak-to-valley ratios between 3 and 30 at the entrance and center-to-center distances between 2 and 3 mm. The developed detectors, specifically scintillators and silicon detectors, successfully measured the minibeam patterns with sub-millimeter resolution, while large-area silicon carbide detectors were used for average dose measurements. First in-vitro biological investigations performed with low energy minibeams clearly showed an enhanced survival fraction in the 16HBE healthy lung cells, while maintaining iso-effective cell killing in A549 cancer cells. Furthermore, a synergy was observed when combining UHDR electron beams with minibeams. CONCLUSIONS:The multidisciplinary approach was consolidated during these first 2 years of the MIRO project, as demonstrated by the results obtained both in terms of dosimetric characterization and biological investigations. The dedicated framework for modelling is being optimized to support the biological findings, allowing for a better interpretation of the results. The tool for dose planning, still under development, will allow the investigation of peculiar configurations to explore new frontiers in the perspective of future human trials.

EuPRAXIA, the "European Plasma Research Accelerator with eXcellence In Applications," represents the next generation of free-electron lasers (FEL). It aims to develop a compact, cost-efficient particle accelerator using innovative wake-field accelerator technology. High-energy physics often demands higher acceleration voltages, and X-band technology offers high gradients in compact structures. The EuPRAXIA@SPARC_LAB LINAC injector, featuring an S-band RF gun, four S-band structures, and sixteen X-band structures, achieves a maximum beam energy of 1 GeV. For femtosecond-level synchronization and stability, Low-Level Radio Frequency (LLRF) systems are essential. However, commercial X-band LLRF solutions are unavailable. This project, in context of the EuPRAXIA - Doctoral Network, develops an X-band LLRF prototype tailored to meet the EuPRAXIA@SPARC_LAB LINAC's stringent requirements. After validation on a testbench, the prototype will enable industrial production and commercialization. This paper presents the Front-End, Back-End analysis, and further evaluation of the prototype.

FLASH therapy, a novel cancer treatment technique, aims to control tumor growth, sparing the healthy tissue from radiation damage and thus increasing the therapeutic ratio. Translating FLASH therapy into clinical practice, especially for treating deep-seated tumors, necessitates achieving Very High-Energy Electron (VHEE) levels within the 50-250 MeV range. In 2022 Sapienza University, in collaboration with INFN, launched the SAFEST project, a compact C-band 100 MeV Ultra-High Dose Rate (UHRD) radiation source for the treatment of deep-seated tumors, which was partially funded by Italian PNRR (Next Generation EU). A C-band linac prototype at lower energy, with an electron pulse of 100 nC and repetition frequency <200 Hz, is being developed to test the key choices and technology of a VHEE machine. This paper provides insights into the design strategy of the prototype, discussing the optimization of the main RF and electron beam parameters. The expected dose profiles are also shown and discussed. The progress of this innovative linac represents a step forward in the realization of a C-band compact FLASH VHEE source for cancer treatment.

Synchrotron light sources are commonly fed with 100-MeV normal conducting linear accelerators powered by a dc thermionic electron gun for high current applications. Such systems are robust solutions for electron sources; however, they tend to have complex bunching systems that increase the complexity and overall cost. In this work, we propose an approach where nonrelativistic bunches are directly injected into a relativistic accelerating structure that is detuned from the optimal frequency. This detuning causes a phase jump, enabling the achievement of beam parameters, such as transmission and energy spread, comparable to those obtained with conventional multistructure bunching systems. The proposed solution simplifies the longitudinal beam dynamics and avoids the need for complex bunching systems. The simplicity and reduced size of this design make the proposed solution interesting for compact electron sources. The article proposes, as an example, a possible upgrade of the Swiss Light Source (SLS) 100-MeV injection linac.

The duration of incoherent XUV pulses down to the femtoseconds (fs) can be retrieved through a statistical analysis of the modulations on the observed radiation spectrum. Uncorrelated shot-noise fluctuations in the pulse temporal profile result in incoherent radiation showing a multispike spectrum where the spike width is inversely proportional to the pulse length. In this Letter, single-shot temporal characterization of the betatron radiation pulses emitted by fs-long, 100's MeV electron bunches undergoing acceleration, and propagating through a plasma wiggler was performed in the XUV domain. The retrieved pulse lengths agree with independent measurements performed in the THz spectral range and with theoretical predictions.

In particle accelerators, accurate and stable beam parameters are crucial for experimental success. Traditional methods for measuring parameters like beam energy often rely on invasive techniques that disrupt experiments. This paper presents a novel, non-invasive machine learning-based approach to predict beam energy using parasitic measurements, enabling real-time estimation without interference. The method employs a predictive model optimized for one-step-ahead forecasting and uses time-series data decomposition to handle complex beam energy dynamics. Recursive prediction strategies allow the model to anticipate future variations autonomously. Preliminary results from experiments at the CLEAR accelerator demonstrate the model’s ability to capture both slow trends and rapid energy shifts, adapting to diverse experimental needs. These findings showcase the potential of machine learning to measure beam energy, offering a real-time, non-destructive alternative to conventional methods. This approach promises significant advancements in accelerator-based applications, especially where destructive techniques are impractical.

High-gradient accelerating radio frequency (RF) ] cavities are currently being developed in several national laboratories for applications in high-energy physics. Ultra-high accelerating gradients, reaching up to the GV/m range, can be achieved using ultra-compact accelerating structures operating in the sub-terahertz (sub-THz) regime. However, accurately measuring the key RF parameters of such compact structures presents significant experimental challenges, and even minor inaccuracies can lead to substantial errors. Additionally, RF simulations for these cavities often require extensive computational resources. Among the most critical parameters to determine is the reflection coefficient. To provide a fast and accurate analytical estimation, we have developed an electromagnetic theory describing the coupling between a resonant cavity and an RF waveguide. This approach is based on Bethe’s small-aperture polarization method, further developed by Collin. An exact analytical expression for the reflection coefficient is presented, formulated as a function of the physical parameters of the cavity waveguide system and applicable to arbitrary geometries, materials, and frequencies.

The bunch length in a linac driven Free Electron Laser (FEL) is a major parameter to be characterized to optimize the final accelerator performance. In linear machines, this observable is typically determined from the beam imaged on a screen located downstream of a Transverse Deflecting Structure (TDS) used to impinge a time dependent kick along the longitudinal coordinate of the beam. This measurement is typically performed during the machine setup and only sporadically to check the beam duration, but it cannot be continuously repeated because it is time consuming and invasive. A non-invasive method to determine the electron bunch length has already been presented in the past. This method is based on the analysis of the synchrotron radiation light spot emitted by the bunch passing through a magnetic chicane, provided that the energy chirp impinged on the bunch by the upstream radio frequency structures is known. In order to overcome a systematic discrepancy affecting the synchrotron radiation monitor based results compared to the absolute TDS based ones, we implemented and optimized a machine learning approach to predict the bunch length downstream of the two SwissFEL compression stages-from about 10 fs up to about 2 ps-as well as the beam longitudinal profile at the first one.

The coaxial wire method is a common and appreciated technique to assess the beam coupling impedance of an accelerator element from scattering parameters. Nevertheless, the results obtained from wire measurements could be inaccurate due to the presence of the stretched conductive wire that artificially creates the conditions for the propagation of a Transverse ElectroMagnetic (TEM) mode. The aim of this work is to establish a solid technique to obtain the beam coupling impedance from electromagnetic simulations, without modifications of the device under test. In this framework, we identified a new relation to get the longitudinal resistive wall beam coupling impedance of a circular chamber directly from the scattering parameters and demonstrated that it reduces to the exact theoretical expression. Furthermore, a possible generalization of the method to arbitrary cross-section chamber geometries has been studied and validated with numerical simulations.

Recently, considerable work has been directed at the development of an ultracompact X-ray free-electron laser (UCXFEL) based on emerging techniques in high-field cryogenic acceleration, with attendant dramatic improvements in electron beam brightness and state-of-the-art concepts in beam dynamics, magnetic undulators, and X-ray optics. A full conceptual design of a 1 nm (1.24 keV) UCXFEL with a length and cost over an order of magnitude below current X-ray free-electron lasers (XFELs) has resulted from this effort. This instrument has been developed with an emphasis on permitting exploratory scientific research in a wide variety of fields in a university setting. Concurrently, compact FELs are being vigorously developed for use as instruments to enable next-generation chip manufacturing through use as a high-flux, few nm lithography source. This new role suggests consideration of XFELs to urgently address emerging demands in the semiconductor device sector, as identified by recent national need studies, for new radiation sources aimed at chip manufacturing. Indeed, it has been shown that one may use coherent X-rays to perform 10–20 nm class resolution surveys of macroscopic, cm scale structures such as chips, using ptychographic laminography techniques. As the XFEL is a very promising candidate for realizing such methods, we present here an analysis of the issues and likely solutions associated with extending the UCXFEL to harder X-rays (above 7 keV), much higher fluxes, and increased levels of coherence, as well as methods of applying such a source for ptychographic laminography to microelectronic device measurements. We discuss the development path to move the concept to rapid realization of a transformative XFEL-based application, outlining both FEL and metrology system challenges.

. - The EuPRAXIA@SPARC LAB RF injector provides highbrightness electron beams accelerated and longitudinally manipulated in the velocity bunching regime (VB). The RF injector works in the so-called comb configuration. It foresees a 30 pC witness and a 200 pC driver longitudinally compressed in the first two accelerating structures both operated in the VB regime. The beam stability can be improved by adding a High Harmonic Cavity (HHC), interposed between the Gun and the first accelerating structure, to shorten and flatter the charge distribution and manipulate the beams to reach proper transverse and longitudinal parameters. The paper reports on beam dynamics studies performed with the insertion of the X-band RF cavity that is proposed to shape the beam current distribution, linearize the longitudinal phase space, and stabilize it with respect to RF jitters.

We present a relativistic self-consistent theory of the coherent stopping power (CSP) of ultradense charged particle beams propagating in a dense matter. CSP corresponds to a collective inelastic collision which adds to the ordinary stopping power of individual particles. Unlike the latter, which depends only on the particles' energy, CSP depends upon many more parameters such as the total charge of the ensemble and its charge density and shape. This paves the way for a broad variety of novel methods to tailor particle absorption and penetration in a dense matter. CSP losses can be explained both by collective excitations of single or multiple molecules and by the emission of coherent Cherenkov radiation. Here we demonstrate that the former mechanism is more relevant than the latter. We find that the coherent energy absorption of subpicosecond particle bunches in water occurs exciting a broad Debye process in the GHz range and an intermolecular stretching vibration mode in the THz region. We generalize the Bethe-Bloch stopping power formula to coherent effects including self-forces, dielectric screening, and absorption by the dense matter. For the sake of a self-consistent dynamical theory including phase space evolution, we have also generalized the Fermi-Eyges theory of particle diffusion to the presence of forces. Nonlinear dynamics is demonstrated, inducing nonlinear dose release, beam self-focusing, and self-enhancement of coherent losses. Given the advent of ultraintense particle sources and their use for biomedical and other relevant applications, our results may be of paramount importance for contemporary and future developments of science and technology.

Space charge forces represent main induced effects in an RF-injector that degrade the beam quality. In this scenario the laser distribution sent on the photocathode acquires an important role in the emittance compensation process, as the slice analysis shows. Starting from the preliminary studies performed on [1], a novel semi-analytical model of space charge forces is proposed in detail for bunch with arbitrary charge distribution to derive expressions of self-induced forces. The performance of the fields at low energy regime (as the field has not expired RF forces) is under present analysis, we can investigate use of this model in low charge regime. Further, the model has been bench-marked with the behavior of the distributions present in the literature and studied for new ones. It has also been applied for the study of the optimization of a C-band hybrid photoinjector now being commissioned, thus explaining the factor two reduction of the emittance observed at the exit of the gun by changing the initial distribution at the cathode.

Machine learning deals with creating algorithms capable of learning from the provided data. These systems have a wide range of applications and can also be a valuable tool for scientific research, which in recent years has been focused on finding new diagnostic techniques for particle accelerator beams. In this context, SPARC_LAB is a facility located at the Frascati National Laboratories of INFN, where the progress of beam diagnostics is one of the main developments of the entire project. With this in mind, we aim to present the design of two neural networks aimed at predicting the spot size of the electron beam of the plasma-based accelerator at SPARC_LAB, which powers an undulator for the generation of an X-ray free electron laser (XFEL). Data-driven algorithms use two different data preprocessing techniques, namely an autoencoder neural network and PCA. With both approaches, the predicted measurements can be obtained with an acceptable margin of error and most importantly without activating the accelerator, thus saving time, even compared to a simulator that can produce the same result but much more slowly. The goal is to lay the groundwork for creating a digital twin of linac and conducting virtualized diagnostics using an innovative approach.

Particle-driven plasma wakefield acceleration (PWFA) exploits the intense wakefields excited in a plasma by a high-brightness driver beam in order to accelerate a trailing, properly delayed witness electron beam. Such a configuration offers notable advantages in achieving very large accelerating gradients that are suitable for applications in particle colliders and photon production. Moreover, the amplitude of the accelerating fields can be enhanced by resonantly exciting the plasma using a multi-pulse driver beam with a proper time structure. Before the injection into the plasma stage, the pulsed electron beam, conventionally termed the comb beam, is usually produced and pre-accelerated in a radio-frequency (RF) linear accelerator (linac). In this pape, we discuss challenging aspects of the dynamics that comb beams encounter in the RF injector stage preceding the plasma. In particular, the examples we analyze focus on the use of velocity bunching to manipulate the time structure of the beam and the impact of dipole short-range wakefields on the transverse emittances. Indeed, both processes crucially affect the phase space distribution and its quality, which are determinant features for an efficient acceleration in the plasma. In addition, the analyses we present are performed with the custom tracking code MILES, which utilizes semi-analytical models for a simplified evaluation of wakefield effects in the presence of space charge forces.

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 accelerating a witness beam. The electron bunches are generated through the so-called comb technique in an RF injector that consists 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, which 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 velocity bunching S-band structures and by shaping the laser pulse at the cathode. The paper reports on beam dynamics studies performed also with the insertion of an X-band RF cavity after the gun that is proposed to shape the beam current distribution and stabilize it with respect to RF jitters.

In high-energy particle accelerators, circulating particle beams with high intensity interact electromagnetically with their surroundings (i.e. metallic vacuum chambers of various geometries). These electromagnetic interactions, which are typically described through the concept of beam-coupling impedance, can represent a serious issue in terms of stability of the particle beam and heating on the accelerator structures. It is therefore crucial to account for beam-coupling impedance through simulations and measurements during the design phase of an accelerator component. However, standard methods used in the accelerator field have known limitations due to the perturbations that they induce in the device geometry. Thus, the bead-pull method, which is a possible alternative approach, is presented in this study. The longitudinal beam-coupling impedance measurement technique is introduced, and it is further developed for novel use in transverse impedance measurements.

In the framework of the High Luminosity Upgrade of the LHC (HL-LHC) the beam intensity from the injectors must be doubled while keeping longitudinal beam parameters unchanged. As such, high-quality beams with high intensities are required also from the Proton Synchrotron (PS). The beam coupling impedance plays a crucial role and mitigation measures must be taken to remain within a stringent impedance budget. Kicker magnets are important contributors to the overall broadband impedance of the PS. Moreover, the detailed study of kicker impedances revealed additional resonant modes which may be critical for the beam stability. The longitudinal beam coupling impedance for the fast extraction kicker KFA79 is presented in this study, and a solution to reduce the impedance of the critical resonant modes is introduced. Electromagnetic (EM) simulations have been performed to determine the impedance behaviour. Finally, the insertion of transition pieces between magnet modules is presented as a measure for mitigating the low frequency resonant impedance contributions.

The SPARC 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.