arXiv subjects

M. Bergamaschi

Publications and source records attributed to M. Bergamaschi.

16 recordsLinked to original sources

arXiv2026

Demonstration of a 10-metre-long discharge plasma source for plasma wakefield acceleration

The AWAKE experiment at CERN uses high-energy proton bunches to drive plasma wakefields capable of accelerating electron bunches up to hundreds of GeV in a single plasma. Realising this potential requires a plasma source that can extend to tens of metres while maintaining high longitudinal uniformity and reproducibility. Towards this goal, we report on the first experimental characterisation and beam-based validation of a 10-metre-long pulsed-DC discharge plasma source (DPS) used as an alternative to the laser-ionised rubidium vapour plasma source (VPS). The DPS reliably delivered plasma densities in three noble gases (He, Ar, Xe) at electron densities between 10$^{14}$ and 10$^{15}$ cm$^{-3}$. Proton bunch self-modulation was observed with all three gases, and the resulting modulation frequency was used as a beam-based diagnostic of the plasma density, showing good agreement with laboratory interferometry measurements. The event-to-event modulation frequency was demonstrated to be reproducible to within 0.63%, comparable to that of the VPS. These results establish the DPS as a valid alternative plasma source for beam-driven wakefield experiments, and support its further development towards longer plasma lengths. The present design is tailored to AWAKE requirements, but the underlying discharge concept provides a route to long, reproducible plasma sources with straightforward density control through discharge-beam timing for other beam-plasma experiments.

physics.plasm-ph↗

arXiv2026

Seeding of Self-Modulation using Truncated Seed Bunches as a Path to High Gradient Acceleration

This manuscript proposes a method to enable controlled high-gradient particle acceleration when requiring self-modulation of the drive bunch. While electron bunch seeding of self-modulation (eSSM) has been realised at a plasma electron density $n_\mathrm{pe}\cong10^{14}\mathrm{cm}^{-3}$, it has not been demonstrated at higher plasma densities due to limitations of available seed bunch properties. As experimentally shown in this manuscript, truncating available seed bunches with a relativistic ionisation front allows these limitations to be overcome. This seeding method is called truncated electron bunch seeding of self-modulation (teSSM) and experiments confirm that -- when using teSSM -- self-modulation becomes reproducible at $n_\mathrm{pe}=7\times10^{14}\mathrm{cm}^{-3}$. Additionally, the seed wakefield amplitude is also increased, which is known to be advantageous because it shortens the length needed to reach self-modulation saturation. The presented results establish teSSM as a method for achieving controlled, high-gradient particle acceleration with long drivers and available seed bunches.

physics.acc-ph↗

arXiv2026

Measurement of the Saturation Length of the Self-Modulation Instability

The self-modulation (SM) instability transforms a long charged particle bunch traveling in plasma into a train of microbunches that resonantly drives large-amplitude wakefields. We present the first determination of the saturation length of SM using experimental and numerical results. The saturation length is the distance over which wakefields reach their maximum amplitude along the plasma. By varying the plasma length and measuring the radius of the transverse distribution of the bunch, we find that the saturation length of SM decreases with plasma density and initial field amplitude, e.g., when seeding. The saturation length is a fundamental parameter of the instability, and these results are key for understanding SM and designing plasma wakefield accelerators driven by long bunches, such as AWAKE, or by long laser pulses for radiation production.

physics.plasm-ph↗

arXiv2024

Measurement of the emittance of accelerated electron bunches at the AWAKE experiment

The vertical plane transverse emittance of accelerated electron bunches at the AWAKE experiment at CERN has been determined, using three different methods of data analysis. This is a proof-of-principle measurement using the existing AWAKE electron spectrometer to validate the measurement technique. Large values of the geometric emittance, compared to that of the injection beam, are observed ($\sim \SI{0.5}{\milli\metre\milli\radian}$ compared with $\sim \SI{0.08}{\milli\metre\milli\radian}$), which is in line with expectations of emittance growth arising from plasma density ramps and large injection beam bunch size. Future iterations of AWAKE are anticipated to operate in conditions where emittance growth is better controlled, and the effects of the imaging systems of the existing and future spectrometer designs on the ability to measure the emittance are discussed. Good performance of the instrument down to geometric emittances of approximately $\SI{1e-4}{\milli\metre\milli\radian}$ is required, which may be possible with improved electron optics and imaging.

physics.acc-ph↗

arXiv2024

Plasma Light As Diagnostic For Wakefields Driven By Developing Self-Modulation Of A Long Particle Bunch

We outline plans to use plasma light emitted as atomic lines radiation as a diagnostic for wakefields driven in plasma by a self-modulating, long proton bunch. This diagnostic is built into the design of a new vapor/plasma source that will also allow for imposing a plasma density step of various height at various locations. Such a step of a few percent in relative density placed at a location where the self-modulation process grows is predicted by numerical simulations to make the bunch drive wakefields with GV/m amplitude over hundreds of meters of plasma.

physics.plasm-ph↗

arXiv2024

Experimental Observation of Motion of Ions in a Resonantly Driven Plasma Wakefield Accelerator

We show experimentally that an effect of motion of ions, observed in a plasma-based accelerator, depends inversely on the plasma ion mass. The effect appears within a single wakefield event and manifests itself as a bunch tail, occurring only when sufficient motion of ions suppresses wakefields. Wakefields are driven resonantly by multiple bunches, and simulation results indicate that the ponderomotive force causes the motion of ions. In this case, the effect is also expected to depend on the amplitude of the wakefields, experimentally confirmed through variations in the drive bunch charge.

physics.plasm-ph↗

arXiv2023

Filamentation of a Relativistic Proton Bunch in Plasma

We show in experiments that a long, underdense, relativistic proton bunch propagating in plasma undergoes the oblique instability, that we observe as filamentation. We determine a threshold value for the ratio between the bunch transverse size and plasma skin depth for the instability to occur. At the threshold, the outcome of the experiment alternates between filamentation and self-modulation instability (evidenced by longitudinal modulation into microbunches). Time-resolved images of the bunch density distribution reveal that filamentation grows to an observable level late along the bunch, confirming the spatio-temporal nature of the instability. We calculate the amplitude of the magnetic field generated in the plasma by the instability and show that the associated magnetic energy increases with plasma density.

physics.plasm-ph↗

arXiv2023

Hosing of a long relativistic particle bunch in plasma

Experimental results show that hosing of a long particle bunch in plasma can be induced by wakefields driven by a short, misaligned preceding bunch. Hosing develops in the plane of misalignment, self-modulation in the perpendicular plane, at frequencies close to the plasma electron frequency, and are reproducible. Development of hosing depends on misalignment direction, its growth on misalignment extent and on proton bunch charge. Results have the main characteristics of a theoretical model, are relevant to other plasma-based accelerators and represent the first characterization of hosing.

physics.plasm-ph↗

arXiv2023

Generation of 10-m-lengthscale plasma columns by resonant and off-resonant laser pulses

Creating extended, highly homogeneous plasma columns like that required by plasma wakefield accelerators can be a challenge. We study the propagation of ultra-short, TW power ionizing laser pulses in a 10-meter-long rubidium vapor and the plasma columns they create. We perform experiments and numerical simulations for pulses with 780 nm central wavelength, which is resonant with the D$_2$ transition from the ground state of rubidium atoms, as well as for pulses with 810 nm central wavelength, some distance from resonances. We measure transmitted energy and transverse width of the pulse and use schlieren imaging to probe the plasma column in the vapor close to the end of the vapor source. We find, that resonant pulses are more confined in a transverse direction by the interaction than off-resonant pulses are and that the plasma channels they create are more sharply bounded. Off-resonant pulses leave a wider layer of partially ionized atoms and thus lose more energy per unit propagation distance. Using experimental data, we estimate the energy required to generate a 20-meter-long plasma column and conclude that resonant pulses are much more suitable for creating a long, homogeneous plasma.

physics.optics↗

arXiv2023

Development of the Self-Modulation Instability of a Relativistic Proton Bunch in Plasma

Self-modulation is a beam-plasma instability that is useful to drive large-amplitude wakefields with bunches much longer than the plasma skin depth. We present experimental results showing that, when increasing the ratio between the initial transverse size of the bunch and the plasma skin depth, the instability occurs later along the bunch, or not at all, over a fixed plasma length, because the amplitude of the initial wakefields decreases. We show cases for which self-modulation does not develop and we introduce a simple model discussing the conditions for which it would not occur after any plasma length. Changing bunch size and plasma electron density also changes the growth rate of the instability. We discuss the impact of these results on the design of a particle accelerator based on the self-modulation instability seeded by a relativistic ionization front, such as the future upgrade of the AWAKE experiment.

physics.plasm-ph↗

arXiv2023

Techniques to seed the self-modulation instability of a long proton bunch in plasma

The Advanced Wakefield Experiment (AWAKE) at CERN relies on the seeded Self-Modulation (SM) of a long relativistic proton bunch in plasma to accelerate an externally injected MeV witness electron bunch to GeV energies. During AWAKE Run 1 (2016-2018) and Run 2a (2021-2022), two seeding methods were investigated experimentally: relativistic ionization front seeding and electron bunch seeding. In the first one, a short laser pulse copropagates within the proton bunch and ionizes the rubidium vapor, generating the plasma. In the second, a short electron bunch propagates in plasma ahead of the proton bunch and drives the seed wakefields. Both seeding methods will be further employed during AWAKE Run 2b (2023-2024) to study their effect on the SM evolution in the presence of a plasma density step. In this contribution, we will show the main experimental results and discuss their impact for the future design of the experiment, in particular for Run 2c (starting in 2028), where the plasma will be split in two sections: one dedicated to SM of the proton bunch, and the other to the electron acceleration process.

physics.acc-ph↗

arXiv2021

Analysis of Proton Bunch Parameters in the AWAKE Experiment

A precise characterization of the incoming proton bunch parameters is required to accurately simulate the self-modulation process in the Advanced Wakefield Experiment (AWAKE). This paper presents an analysis of the parameters of the incoming proton bunches used in the later stages of the AWAKE Run 1 data-taking period. The transverse structure of the bunch is observed at multiple positions along the beamline using scintillating or optical transition radiation screens. The parameters of a model that describes the bunch transverse dimensions and divergence are fitted to represent the observed data using Bayesian inference. The analysis is tested on simulated data and then applied to the experimental data.

physics.acc-ph↗

arXiv2021

Beam halo from Touschek scattering in the KEK Accelerator Test Facility

Beam halo is one of the crucial issues limiting the machine performance and causing radioactivation in high-intensity accelerators. A clear picture of beam-halo formation is of great importance for successful suppression of the undesired beam loss. We present numerical and experimental studies of transverse and longitudinal halos in the KEK Accelerator Test Facility. The fair accordance between predictions and observations in various conditions indicates that the Touschek scattering is the dominant mechanism forming the horizontal and momentum halos.

physics.acc-ph↗

arXiv2021

Simulation and Experimental Study of Proton Bunch Self-Modulation in Plasma with Linear Density Gradients

We present numerical simulations and experimental results of the self-modulation of a long proton bunch in a plasma with linear density gradients along the beam path. Simulation results agree with the experimental results reported in arXiv:2007.14894v2: with negative gradients, the charge of the modulated bunch is lower than with positive gradients. In addition, the bunch modulation frequency varies with gradient. Simulation results show that dephasing of the wakefields with respect to the relativistic protons along the plasma is the main cause for the loss of charge. The study of the modulation frequency reveals details about the evolution of the self-modulation process along the plasma. In particular for negative gradients, the modulation frequency across time-resolved images of the bunch indicates the position along the plasma where protons leave the wakefields. Simulations and experimental results are in excellent agreement.

physics.plasm-ph↗

arXiv2020

Experimental study of extended timescale dynamics of a plasma wakefield driven by a self-modulated proton bunch

Plasma wakefield dynamics over timescales up to 800 ps, approximately 100 plasma periods, are studied experimentally at the Advanced Wakefield Experiment (AWAKE). The development of the longitudinal wakefield amplitude driven by a self-modulated proton bunch is measured using the external injection of witness electrons that sample the fields. In simulation, resonant excitation of the wakefield causes plasma electron trajectory crossing, resulting in the development of a potential outside the plasma boundary as electrons are transversely ejected. Trends consistent with the presence of this potential are experimentally measured and their dependence on wakefield amplitude are studied via seed laser timing scans and electron injection delay scans.

physics.plasm-ph↗

arXiv2020

Proton beam defocusing in AWAKE: comparison of simulations and measurements

In 2017, AWAKE demonstrated the seeded self-modulation (SSM) of a 400 GeV proton beam from the Super Proton Synchrotron (SPS) at CERN. The angular distribution of the protons deflected due to SSM is a quantitative measure of the process, which agrees with simulations by the two-dimensional (axisymmetric) particle-in-cell code LCODE. Agreement is achieved for beam populations between $10^{11}$ and $3 \times 10^{11}$ particles, various plasma density gradients ($-20 ÷20\%$) and two plasma densities ($2\times 10^{14} \text{cm}^{-3}$ and $7 \times 10^{14} \text{cm}^{-3}$). The agreement is reached only in the case of a wide enough simulation box (at least five plasma wavelengths).

physics.acc-ph↗