Experimental Observation of Proton Bunch Modulation in a Plasma at Varying Plasma Densities

Experimental Observation of the 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.

Compact spin-polarized positron acceleration in multilayer microhole-array films

Compact spin-polarized positron accelerators play a major role in promoting significant positron application research, which typically require high acceleration gradients and polarization degrees, both of which, however, are still greatly challenging. Here, we put forward a spin-polarized positron acceleration method, which employs an ultrarelativistic high-density electron beam passing through any hole of multilayer microhole array films to excite strong electrostatic and transition radiation fields. Positrons in the polarized electron-positron pair plasma, filled in the front of the multilayer films, can be captured, accelerated, and focused by the electrostatic and transition radiation fields, while maintaining high polarization degrees of above 90% and high acceleration gradients of about TeV/m. A multilayer design allows for capturing more positrons and achieving cascade acceleration. Our method offers promising solutions to the issues of accelerator miniaturization, positron injection, and polarization preservation, and also can be used to accelerate other charged particles.

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, 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.

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.

The AWAKE Run 2 Programme and Beyond

Plasma wakefield acceleration is a promising technology to reduce the size of particle accelerators. The use of high energy protons to drive wakefields in plasma has been demonstrated during Run 1 of the AWAKE programme at CERN. Protons of energy 400 GeV drove wakefields that accelerated electrons to 2 GeV in under 10 m of plasma. The AWAKE collaboration is now embarking on Run 2 with the main aims to demonstrate stable accelerating gradients of 0.5–1 GV/m, preserve emittance of the electron bunches during acceleration and develop plasma sources scalable to 100s of metres and beyond. By the end of Run 2, the AWAKE scheme should be able to provide electron beams for particle physics experiments and several possible experiments have already been evaluated. This article summarises the programme of AWAKE Run 2 and how it will be achieved as well as the possible application of the AWAKE scheme to novel particle physics experiments.

Controlled Growth of the Self-Modulation of a Relativistic Proton Bunch in Plasma

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.

Transition between Instability and Seeded Self-Modulation of a Relativistic Particle Bunch in Plasma

We use a relativistic ionization front to provide various initial transverse wakefield amplitudes for the self-modulation of a long proton bunch in plasma. We show experimentally that, with sufficient initial amplitude [≥(4.1±0.4)  MV/m], the phase of the modulation along the bunch is reproducible from event to event, with 3%-7% (of 2π) rms variations all along the bunch. The phase is not reproducible for lower initial amplitudes. We observe the transition between these two regimes. Phase reproducibility is essential for deterministic external injection of particles to be accelerated.

Proton Bunch Self-Modulation in Plasma with Density Gradient

We study experimentally the effect of linear plasma density gradients on the self-modulation of a 400 GeV proton bunch. Results show that a positive or negative gradient increases or decreases the number of microbunches and the relative charge per microbunch observed after 10 m of plasma. The measured modulation frequency also increases or decreases. With the largest positive gradient we observe two frequencies in the modulation power spectrum. Results are consistent with changes in wakefields' phase velocity due to plasma density gradients adding to the slow wakefields' phase velocity during self-modulation growth predicted by linear theory.

Transport of intense particle beams in large-scale plasmas

Transport of particle beams in plasmas is widely employed in fundamental research, industry, and medicine. Due to the high inertia of ion beams, their transport in plasmas is usually assumed to be stable. Here we report the focusing and flapping of intense slab proton beams transporting through large-scale plasmas by using a recently developed kinetic particle-in-cell simulation code. The beam self-focusing effect in the simulation is prominent and agrees well with previous experiments and theories. Moreover, the beam can curve and flap like turbulence as the beam density increases. Simulation and analysis indicate that the self-generated magnetic fields, produced by movement of collisional plasmas, are the dominant driver of such behaviors. By analyzing the spatial growth rate of magnetic energy and energy deposition of injected proton beams, it is found that the focusing and flapping are significantly determined by the injected beam densities and energies. In addition, a remarkable nonlinear beam energy loss is observed. Our research might find application in inertial confinement fusion and also might be of interest to the laboratory astrophysics community.

Particle physics experiments based on the AWAKE acceleration scheme

New particle acceleration schemes open up exciting opportunities, potentially providing more compact or higher-energy accelerators. The AWAKE experiment at CERN is currently taking data to establish the method of proton-driven plasma wakefield acceleration. A second phase aims to demonstrate that bunches of about 10⁹ electrons can be accelerated to high energy, preserving emittance and that the process is scalable with length. With this, an electron beam of [Formula: see text](50 GeV) could be available for new fixed-target or beam-dump experiments searching for the hidden sector, like dark photons. The rate of electrons on target could be increased by a factor of more than 1000 compared to that currently available, leading to a corresponding increase in sensitivity to new physics. Such a beam could also be brought into collision with a high-power laser and thereby probe the completely unmeasured region of strong fields at values of the Schwinger critical field. An ultimate goal is to produce an electron beam of [Formula: see text](3 TeV) and collide with an Large Hadron Collider proton beam. This very high-energy electron-proton collider would probe a new regime in which the structure of matter is completely unknown. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

Proton-driven plasma wakefield acceleration in AWAKE

In this article, we briefly summarize the experiments performed during the first run of the Advanced Wakefield Experiment, AWAKE, at CERN (European Organization for Nuclear Research). The final goal of AWAKE Run 1 (2013-2018) was to demonstrate that 10-20 MeV electrons can be accelerated to GeV energies in a plasma wakefield driven by a highly relativistic self-modulated proton bunch. We describe the experiment, outline the measurement concept and present first results. Last, we outline our plans for the future. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.