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.

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.

The workshop focused on the application of ANAs to particle physics keeping in mind the ultimate goal of a collider at the energy frontier (10 TeV, e^+/e^-, e^-/e^-, or γγ). The development of ANAs is conducted at universities and national laboratories worldwide. The community is thematically broad and diverse, in particular since lasers suitable for ANA research (multi-hundred-terawatt peak power, a few tens of femtosecond-long pulses) and acceleration of electrons to hundreds of mega electron volts to multi giga electron volts became commercially available. The community spans several continents (Europe, America, Asia), including more than 62 laboratories in more than 20 countries. It is among the missions of the ICFA-ANA panel to feature the amazing progress made with ANAs, to provide international coordination and to foster international collaborations towards a future HEP collider. The scope of this edition of the workshop was to discuss the recent progress and necessary steps towards realizing a linear collider for particle physics based on novel-accelerator technologies (laser or beam driven in plasma or structures). Updates on the relevant aspects of the European Strategy for Particle Physics (ESPP) Roadmap Process as well as of the P5 (in the US) were presented, and ample time was dedicated to discussions. The major outcome of the workshop is the decision for ALEGRO to coordinate efforts in Europe, in the US, and in Asia towards a pre-CDR for an ANA-based, 10 TeV CM collider. This goal of this coordination is to lead to a funding proposal to be submitted to both EU and EU/US funding agencies. This document presents a summary of the workshop, as seen by the co-chairs, as well as short 'one-pagers' written by the presenters at the workshop.

Creating extended, highly homogeneous plasma columns like that required by plasma wakefield accelerators can be a challenge. We study the propagation of ultra-short, terawatt 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 D2 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 columns 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.

It is widely accepted that the next lepton collider beyond a Higgs factory would require center-of-mass energy of the order of up to 15 TeV. Since, given reasonable space and cost restrictions, conventional accelerator technology reaches its limits near this energy, high-gradient advanced acceleration concepts are attractive. Advanced and novel accelerators (ANAs) are leading candidates due to their ability to produce acceleration gradients on the order of 1–100 GV/m, leading to compact acceleration facilities. However, intermediate energy facilities (IEF) are required to test the critical technology elements on the way towards multi-TeV-class collliders. Here a science case for a 20–100 GeV center-of-mass energy ANA-based lepton collider that can be a candidate for an intermediate energy facility is presented. The IEF can provide numerous opportunities for high energy physics studies including precision Quantum Chromodynamics and Beyond the Standard Model physics measurements, investigation of charged particle interactions with extreme electromagnetic fields, and exploring muon and proton beam acceleration. Possible applications of this collider include the studies of γγ and electron beam-fixed target/beamdump collider designs. Thus, the goal of the proposed IEF is to both carry out particle physics measurements in the 20-100 GeV ranges as well as to serve as an ANA demonstrator facility.

We show in experiments that a long, underdense, relativistic proton bunch propagating in plasma undergoes the oblique instability, which 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 spatiotemporal nature of the instability. We provide a rough estimate of the amplitude of the magnetic field generated in the plasma by the instability and show that the associated magnetic energy increases with plasma density.

Following the successful Run 1 experiment, the Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) Run2 experiment requires the design and implementation of a compact electron source. The “high-quality Electron Accelerator driven by a Reliable Laser wakefield for Industrial uses” (EARLI) project aims to design a stand-alone high-quality electron injector based on a laser wakefield accelerator (LWFA) as an alternative proposal to AWAKE’s baseline design of an X-band electron gun. This project is currently in the design phase, including simulations and experimental tests. Exhaustive beam physics studies for conventional accelerators are applied to LWFA physics.

The phase, growth rate, and onset of long proton bunch self-modulation in plasma can be controlled by a preceding short charged particle bunch. In this paper, by analyzing the growth rates of the self-modulation obtained from particle-in-cell simulation results, we identify two modes of self-modulation, namely noise-seeded and externally seeded self-modulations, and investigate their onset timings. We find that a uniform onset of the self-modulation at each slice of the long proton bunch is crucial for fine-tuning its phase and amplitude. We then demonstrate that a low-energy and low-current electron seed bunch in overdense plasma generates near-axis radial wakefields similar to those observed in the blowout regime. Consequently, the resultant self-modulation is excited as a single mode simultaneously along the entire long proton bunch.

The Advanced Wakefield Experiment (AWAKE) at CERN is the first plasma wakefield accelerator experiment to use a proton bunch as driver. The long bunch undergoes seeded self-modulation (SSM) in a 10 m-long plasma. SSM transforms the bunch into a train of short micro-bunches that resonantly drive high-amplitude wakefields. We use optical transition radiation (OTR) and a streak camera to obtain time-resolved images of the bunch transverse charge density distribution in a given plane. In this paper we present a method to obtain 3D images of the bunch by scanning the OTR across the entrance slit of the streak camera. Reconstruction of the 3D distribution is possible because with seeding self-modulation is reproducible. The 3D images allow for checking the axi-symmetry of SSM and for detecting the possible presence of the non-axi-symmetric hosing instability (HI).

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.

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 Advanced WAKefield Experiment.

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.

We show in experiments that a long, relativistic proton bunch is focused by the plasma adiabatic response. The free plasma electrons migrate so as to neutralize the space charge field of the bunch, and the bunch is therefore focused by the azimuthal magnetic field generated by its own current, that is not balanced by the radial electric field. Since the length of the bunch is much longer than the plasma electron wavelength, the bunch also undergoes the self-modulation instability. Thus, the amplitude of the wakefields grows along the bunch and along the plasma, and the defocusing effect of the self-modulation can become dominant over the adiabatic focusing effect. We show that, when seeding the self-modulation with a preceding electron bunch, the transition between the effect of the adiabatic response and that of the self-modulation depends on the amplitude of the seed wakefields.

Plasma-based accelerators have made remarkable progress over the last two decades. Their unique characteristics make them tools that can revolutionize fields of science and applications. AWAKE takes advantage of the availability of high-energy, relativistic proton bunches to drive large amplitude wakefields ($\sim$GV/m) in a single plasma over distances sufficient to produce hundreds of GeV to TeV electron bunches.

A long, narrow, relativistic charged particle bunch propagating in plasma is subject to the self-modulation (SM) instability. We show that SM of a proton bunch can be seeded by the wakefields driven by a preceding electron bunch. SM timing reproducibility and control are at the level of a small fraction of the modulation period. With this seeding method, we independently control the amplitude of the seed wakefields with the charge of the electron bunch and the growth rate of SM with the charge of the proton bunch. Seeding leads to larger growth of the wakefields than in the instability case.

Research on the application of advanced and novel accelerator schemes to high-energy physics requires facilities capable of producing multi-GeV particle beams. We briefly review the challenges faced by advanced accelerators in reaching collider-relevant parameters and give a concise description of relevant European facilities and large scale installations, either in operation or in a state of advanced design, with their main goals. We also emphasize contributions from smaller, mostly university groups or laboratories. These facilities and groups advance the field considerably and address some of the challenges arising in the translation of advanced accelerator concepts to a future high-energy physics machine. We highlight the fact that there is in addition the strong need for a dedicated European facility with a scientific and R&D program specific to the research questions exclusive to a plasma-based e(-)e(+) linear collider.

It is widely accepted that the next lepton collider beyond a Higgs factory would require center-of-mass energy of the order of up to 15 TeV. Since, given reasonable space and cost restrictions, conventional accelerator technology reaches its limits near this energy, high-gradient advanced acceleration concepts are attractive. Advanced and novel accelerators (ANAs) are leading candidates due to their ability to produce acceleration gradients on the order of 1--100~GV/m, leading to compact acceleration structures. Over the last 10-15 years significant progress has been achieved in accelerating electron beams by ANAs. For example, the demonstration of several-GeV electron beams from laser-powered capillary discharge waveguides, as well as the proof-of-principle coupling of two accelerating structures powered by different laser pulses, has increased interest in ANAs as a viable technology to be considered for a compact, TeV-class, lepton linear collider. However, intermediate facilities are required to test the technology and demonstrate key subsystems. A 20-100 GeV center-of-mass energy ANA-based lepton collider can be a possible candidate for an intermediate facility. Apart from being a test beam facility for accelerator and detector studies, this collider will provide opportunities to study muon and proton beam acceleration, investigate charged particle interactions with extreme electromagnetic fields (relevant for beam delivery system designs and to study the physics at the interaction point), as well as precision Quantum Chromodynamics and Beyond the Standard Model physics measurements. Possible applications of this collider include the studies of $\gamma\gamma$ and $e$-ion collider designs.

Introduction Protons traveling in a bunch experience a repulsion due to the Coulomb force. This force is known as space-charge force and it causes a defocusing effect on the bunch. Since protons are moving, they also generate a magnetic field that gives rise to a focusing force. In an azimuthal symmetric system, the electric field has essentially a radial component (Er), while the magnetic field lines are circles around the bunch (Bθ ). The total force experienced by particles in the bunch is: Fr = q(Er− vbBθ ) = qEr/γ, where vb is the velocity of the bunch, and γ its Lorentz factor. When the bunch is relativistic (γ 1), the two contributions compensate for one another and the net force acting on the bunch is close to zero (see Figure 1(a)). When the bunch travels in a neutral plasma, plasma electrons migrate so as to neutralize the space-charge field of the bunch [1]. As a consequence, the focusing force generated by the azimuthal magnetic field of the bunch is no longer balanced by the defocusing force due to the space charge electric field, and the bunch is therefore focused [2, 3, 4] (see Figure 1(b)).

The AWAKE project at CERN successfully demonstrated the use of a proton driver to accelerate an electron witness in plasma [1]. One of the key goals for AWAKE Run 2 is to better control this acceleration, separating the protonbeam-modulation and electron-acceleration stages in order to achieve high energy electrons with high beam quality. Controlled acceleration additionally requires careful tuning of the witness bunch parameters at the injection point. In this work, we use particle-in-cell simulations to study the tolerances for this matching, and discuss techniques to loosen these constraints.