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Charged and quasi-neutral beams propagating through an unmagnetised plasma are subject to numerous collisionless instabilities on the small scale of the plasma skin depth. The electrostatic two-stream instability, driven by longitudinal and transverse wakefields, dominates for dilute beams. This leads to modulation of the beam along the propagation direction and, for wide beams, transverse filamentation. A three-dimensional spatiotemporal two-stream theory for warm beams with a finite extent is developed. Unlike the cold beam limit, diffusion due to a finite emittance gives rise to a dominant wavenumber, and a cut-off wavenumber above which filamentation is suppressed. Particle-in-cell simulations with quasineutral electron-positron beams in the relativistic regime give excellent agreement with the theoretical model. This work provides deeper insights into the effect of diffusion on filamentation of finite beams, crucial for comprehending plasma-based accelerators in laboratory and cosmic settings.

The long proton beams present at CERN have the potential to evolve into a train of microbunches through the self-modulation instability process. The resonant wakefield generated by a periodic train of proton microbunches can establish a high acceleration field within the plasma, facilitating electron acceleration. This paper investigates the impact of plasma density on resonant wakefield excitation, thus influencing acceleration of a witness electron bunch and its corresponding betatron radiation within the wakefield. Various scenarios involving different plasma densities are explored through particle-in-cell simulations. The peak wakefield in each scenario is calculated by considering a long pre-modulated proton driver with a fixed peak current. Subsequently, the study delves into the witness beam acceleration in the wakefield and its radiation emission. Elevated plasma density increases both the number of microbunches and the accelerating gradient of each microbunch, consequently resulting in heightened resonant wakefield. Nevertheless, the scaling is disrupted by the saturation of the resonant wakefield due to the nonlinearities. The simulation results reveal that at high plasma densities an intense and broadband radiation spectrum extending into the domain of the hard X-rays and gamma rays is generated. Furthermore, in such instances, the energy gain of the witness beam is significantly enhanced. The impact of wakefield on the witness energy gain and the corresponding radiation spectrum is clearly evident at extremely elevated densities.

Plasma wakefields offer high acceleration gradients, orders of magnitude larger than conventional RF accelerators. However, the achievable luminosity remains relatively low, typically limited by the plasma recovery time and the charge which can be accelerated per shot. In this work, we show that a train of drive bunches can be harnessed to accelerate multiple witness bunches in a single shot. We demonstrate that periodically loading the wakefields removes the limit on the energy transfer from the drive beam to the plasma, which allows the luminosity to be increased. Proof-of-concept simulations for the AWAKE scheme are carried out to demonstrate the technique, achieving a doubling of the accelerated charge while exploiting only a fraction of the drive train., Comment: 5 pages, 4 figures

A Higgs Factory is considered the highest priority next collider project by the high-energy physics community. Very advanced designs based on radio-frequency cavities exist, and variations on this approach are still being developed. Recently, an option based on electron-bunch driven plasma wakefield acceleration has also been proposed. In this article, we discuss a further option based on proton-driven plasma wakefield acceleration. This option has significant potential advantages due to the high energy of the plasma wakefield driver, simplifying the plasma acceleration stage. Its success will depend on further developments in producing compact high-energy proton bunches at a high rate, which would also make possible a broad range of synergistic particle-physics research., Comment: 11 pages, 3 figures

The Fluorescence detector Array of Single-pixel Telescopes (FAST) is one of several proposed designs for a next-generation cosmic-ray detector. Such detectors will require enormous collecting areas whilst also needing to remain cost-efficient. To meet these demands, the FAST collaboration has designed a simplified, low-cost fluorescence telescope consisting of only four photomultiplier tubes (PMTs). Since standard air shower reconstruction techniques cannot be used with so few PMTs, FAST utilises an alternative two-step approach. In the first step, a neural network is used to provide a first estimate of the true shower parameters. This estimate is then used as the initial guess in a minimisation procedure where the measured PMT traces are compared to simulated ones, and the best-fit shower parameters are found. A detailed explanation of these steps is given, with the expected performance of FAST prototypes at the Telescope Array experiment acting as a demonstration of the technique., Comment: 8 pages, 7 figures, 1 table, Proceedings of the 38th International Cosmic Ray Conference 2023

Ultra-high energy cosmic rays (UHECRs), whose energy are beyond $10^{18}~\mathrm{eV}$, are the most energetic particles we have ever detected. The latest results seem to indicate a heavier composition at the highest energies, complicating the search for their origins. Due to the limited number of UHECR events, we need to build an instrument with an order of magnitude larger effective-exposure to collect UHECRs in future decades. The Fluorescence detector Array of Single-pixel Telescopes (FAST) is a proposed low-cost, easily deployable UHECR detector suitable for a future ground array. It is essential to validate the telescope design and autonomous observational techniques using prototypes located in both hemispheres. Here we report on the current status of observations, recent performance results of prototypes, and developments towards a future mini-array., Comment: 8 pages, 8 figures, Proceedings of the 38th International Cosmic Ray Conference (ICRC2023)

Beam-driven plasma wakefield accelerators typically use the external injection scheme to ensure controllable beam quality at injection. However, the externally injected witness bunch may exhibit a non-Gaussian transverse density distribution. Using particle-in-cell simulations, we show that the common beam quality factors, such as the normalized RMS emittance and beam radius, do not strongly depend on the initial transverse shapes of the witness beam. Nonetheless, a beam with a highly-peaked transverse spatial profile can achieve a higher fraction of the total beam charge in the core. The same effect can be seen when the witness beam's transverse momentum profile has a peaked non-Gaussian distribution. In addition, we find that an initially non-axisymmetric beam becomes symmetric due to the interaction with the plasma wakefield, and so it does not cause a detrimental effect for the beam acceleration.

Plasma wakefield acceleration is a promising technology to reduce the size of particle accelerators. 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., Comment: 21 pages, 8 figures. Submitted to Symmetry journal

The spectroscopy of betatron radiation from the focusing plasma column can work as a powerful non-invasive beam diagnostic method for plasma wakefield acceleration experiments such as the AWAKE. In this paper, the effects of radial size mismatch and off-axis injection on the beam dynamics, as well as the spectral features of the betatron radiation emitted by the witness electron bunch in the quasi-linear proton-driven plasma wakefield are studied. It is shown that the evolution of the critical betatron photon energy and the overall photon angular distribution can effectively reveal the initial injection conditions of the witness electron bunch. The possibility of using this method for the diagnostics of the seed electron bunch in the proton self-modulation stage of AWAKE Run 2 is also discussed.

Particle acceleration in a quasilinear plasma wake provides access to high acceleration gradients while avoiding self-trapping of the background electrons. However, the plasma response to the externally injected witness bunch leads to a variation of the focussing fields along the bunch length, which can lead to a emittance growth. In order to investigate the impact of this emittance growth on the overall beam quality, we develop a single figure of merit based on a potential high-energy application for the AWAKE experiment at CERN. We show that the development of such a figure of merit naturally gives rise to constraints on both the tunability and stability of the initial witness bunch parameters. It is further shown that the unique physics of the quasilinear wake gives rise to broad tolerances for the witness bunch radius at the injection point, as the plasma wakefields self-match to the witness bunch., Comment: 10 pages, 12 figures

The Advanced Wakefield (AWAKE) Experiment is a proof-of-principle experiment demonstrating the acceleration of electron beams via proton-driven plasma wakefield acceleration. AWAKE Run 1 achieved acceleration of electron beams to 2 GeV and the intention for Run 2 is to build on these results by achieving acceleration to ~10 GeV with a higher beam quality. As part of the upgrade to Run 2, the existing proton and electron beamlines will be adapted and a second plasma cell and new 150 MeV electron beamline will be added. This new beamline will be required to inject electron bunches with micron-level beam size and stability into the second plasma cell from within the 1 m gap between the two plasma cells. In this paper we describe the techniques used (e.g. numerical optimizers and genetic algorithms) to produce the design of the 150 MeV electron line in order to meet the challenging experimental specifications. Operational techniques are also studied for both electron transfer lines including steering and alignment methods utilising numerical optimizers and beam measurement techniques employing neural networks.

The origin and nature of ultra-high-energy cosmic rays (UHECRs) remain an open question in astroparticle physics. Motivated by the need for an unprecedented aperture for further advancements, the Fluorescence detector Array of Single-pixel Telescopes (FAST) is a prospective next-generation, ground-based UHECR observatory that aims to cover a huge area by deploying a large array of low-cost fluorescence detectors. The full-scale FAST prototype consists of four 20 cm photomultiplier tubes at the focus of a segmented mirror 1.6 m in diameter. Over the last five years, three prototypes have been installed at the Telescope Array Experiment in Utah, USA, and one prototype at the Pierre Auger Observatory in Mendoza, Argentina, commencing remote observation of UHECRs in both hemispheres. We report on the latest results of these FAST prototypes, including telescope calibrations, atmospheric monitoring, ongoing electronics upgrades, development of sophisticated reconstruction methods, and UHECR detections., Comment: 8 pages, 6 figures, Proceedings of the 37th International Cosmic Ray Conference (ICRC 2021)

The origin and nature of ultra-high energy cosmic rays (UHECRs) are hot topics in the astroparticle physics community. The Fluorescence detector Array of Single-pixel Telescopes (FAST) is a design for a next-generation ground-based UHECR observatory, addressing the requirements for a large-area, low-cost detector suitable for measuring the properties of the highest energy cosmic rays with an unprecedented aperture. We have developed a full-scale prototype consisting of four 200 mm photomultiplier tubes at the focus of a segmented mirror of 1.6 m in diameter. Over the last three years, we have installed three prototypes at the Telescope Array Experiment in Utah, USA. These telescopes have been steadily taking data since installation. We report on preliminary results of the full-scale FAST prototypes, including measurements of UHECRs, and distant ultra-violet lasers used to study the atmospheric transparency. Furthermore, we discuss the installation of an additional identical FAST prototype at the Pierre Auger Observatory in Argentina. Possible benefits to the Telescope Array Experiment and the Pierre Auger Observatory include a comparison of the transparency of the atmosphere above both experiments, a study of the systematic uncertainty associated with their existing fluorescence detectors, and a cross-calibration of their energy and Xmax scales., Comment: 8 pages, Proceedings of the 36th International Cosmic Ray Conference (ICRC 2019), July 24th - August 1st, 2019, Madison, WI, USA. arXiv admin note: text overlap with arXiv:1810.06678

The attainable transformer ratio in plasma accelerators is limited by instabilities. Using three-dimensional particle-in-cell simulations, we demonstrate that these can be controlled using a hollow plasma channel with a co-axial plasma filament. The driver scatters electrons from the filament, and the slow pinch of the ions leads to a strong chirp of the effective betatron frequency, preventing beam breakup. We demonstrate the monoenergetic acceleration of an electron bunch to 20 GeV over 4.4 m, achieving a transformer ratio of 10, an energy efficiency of 40% and an emittance of 1.8 um.

Contributions of the Fluorescence detector Array of Single-pixel Telescopes (FAST) to the 35th International Cosmic Ray Conference, 12-20 July 2017, Busan, Korea, Comment: 2 proceedings of FAST will be appeared in PoS (ICRC2017)

We use a combination of spatial distribution and radial velocity to search for halo sub-structures in a sample of 412 RR Lyrae stars (RRLS) that covers a $\sim 525$ square degrees region of the Virgo Overdensity (VOD) and spans distances from the Sun from 4 to 75 kpc. With a friends-of-friends algorithm we identified six high significance groups of RRLS in phase space, which we associate mainly with the VOD and with the Sagittarius stream. Four other groups were also flagged as less significant overdensities. Three high significance and 3 lower significance groups have distances between $\sim 10$ and 20 kpc, which places them with the distance range attributed by others to the VOD. The largest of these is the Virgo Stellar Stream (VSS) at 19 kpc, which has 18 RRLS, a factor of 2 increase over the number known previously. While these VOD groups are distinct according to our selection cirteria, their overlap in position and distance, and, in a few cases, similarity in radial velocity are suggestive that they may not all stem separate accretion events. Even so, the VOD appears to be caused by more than one overdensity. The Sgr stream is a very obvious feature in the background of the VOD at a mean distance of 44 kpc. Two additional high significant groups were detected at distances $>40$ kpc. Their radial velocities and locations differ from the expected path of the Sgr debris in this part of the sky, and they are likely to be remnants of other accretion events., Comment: Accepted for publication in ApJ

The influence of initial position on the acceleration of externally-injected electrons in a plasma wakefield is investigated. Test-particle simulations show previously unobserved complex structure in the parameter space, with quasi-stable injection channels forming for particles injected in narrow regions away from the wake centre. Particles injected into these channels remain in the wake for a considerable time after dephasing, and as a result achieve significantly higher energy than their neighbours. The result is relevant to both the planning and optimisation of experiments making use of external injection., Comment: 5 pages, 5 figures

In regimes far beyond the wavebreaking threshold of Raman amplification, we show that significant amplifcation can occur after the onset of wavebreaking, before phase mixing destroys the coherent coupling between pump, probe and plasma wave. Amplification in this regime is therefore a transient effect, with the higher-efficiency "coherent wavebreaking" (CWB) regime accessed by using a short, intense probe. Parameter scans illustrate the marked difference in behaviour between below wavebreaking, in which the energy-transfer efficiency is high but total energy transfer is low, wavebreaking, in which efficiency is low, and CWB, in which moderate efficiencies allow the highest total energy transfer., Comment: 6 pages, 3 figures