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Metre-scale plasma wakefield accelerators have imparted energy gain approaching 10 gigaelectronvolts to single nano-Coulomb electron bunches. To reach useful average currents, however, the enormous energy density that the driver deposits into the wake must be removed efficiently between shots. Yet mechanisms by which wakes dissipate their energy into surrounding plasma remain poorly understood. Here, we report picosecond-time-resolved, grazing-angle optical shadowgraphic measurements and large-scale particle-in-cell simulations of ion channels emerging from broken wakes that electron bunches from the SLAC linac generate in tenuous lithium plasma. Measurements show the channel boundary expands radially at 1 million metres-per-second for over a nanosecond. Simulations show that ions and electrons that the original wake propels outward, carrying 90 percent of its energy, drive this expansion by impact-ionizing surrounding neutral lithium. The results provide a basis for understanding global thermodynamics of multi-GeV plasma accelerators, which underlie their viability for applications demanding high average beam current., Plasma wakefield accelerators promise compact, affordable future particle accelerators, but require deposition of enormous energy into a small volume. Here, the authors measure and simulate how this energy transfers from the wake into surrounding plasma, a process that ultimately governs the accelerator’s repetition rate.

Ongoing progress in laser and accelerator technology opens new possibilities in high-field science, notably for the study of the largely unexplored strong-field QED regime where electron-positron pairs can be created directly from light-matter or even light-vacuum interactions. Laserless strategies such as beam-beam collisions have also been proposed with the prospect of pushing strong-field quantum electrodynamics (SFQED) in the nonpertubative regime. Here we report on an original concept to probe strong-field QED by harnessing the interaction between an electron beam and a solid target. When a high-density, ultrarelativistic beam impinges onto an even denser plasma, the beam self fields are reflected at the plasma boundary. In the rest frame of the beam electrons, these fields can exceed the Schwinger field, leading to strong-field QED effects such as quantum nonlinear inverse Compton scattering and nonlinear Breit-Wheeler electron-positron pair creation. We show that such beam-plasma collisions can produce results similar to beam-beam collisions with the advantage of a much simpler experimental setup including the automatic overlap between the beam and the reflected fields. This scenario opens the way to precision studies of strong-field QED, with measurable clear signatures in terms of gamma-ray photon and pair production, and thus is a very promising milestone on the path towards laserless studies of nonperturbative SFQED.

Plasma waves generated in the wake of intense, relativistic laser1,2 or particle beams3,4 can accelerate electron bunches to gigaelectronvolt energies in centimetre-scale distances. This allows the realization of compact accelerators with emerging applications ranging from modern light sources such as the free-electron laser to energy frontier lepton colliders. In a plasma wakefield accelerator, such multi-gigavolt-per-metre wakefields can accelerate witness electron bunches that are either externally injected5,6 or captured from the background plasma7,8. Here we demonstrate optically triggered injection9–11 and acceleration of electron bunches, generated in a multi-component hydrogen and helium plasma employing a spatially aligned and synchronized laser pulse. This ‘plasma photocathode’ decouples injection from wake excitation by liberating tunnel-ionized helium electrons directly inside the plasma cavity, where these cold electrons are then rapidly boosted to relativistic velocities. The injection regime can be accessed via optical11 density down-ramp injection12–16 and is an important step towards the generation of electron beams with unprecedented low transverse emittance, high current and 6D-brightness17. This experimental path opens numerous prospects for transformative plasma wakefield accelerator applications based on ultrahigh-brightness beams. Electron bunches are generated and accelerated to relativistic velocities by tunnel ionization of neutral gas species in a plasma. This represents a step towards ultra-bright, high-emittance beams in plasma wakefield accelerators. [This summary has been amended from ‘laser-plasma’ to ‘plasma wakefield’ accelerators.]

Sources of high-energy photons have important applications in almost all areas of research. However, the photon flux and intensity of existing sources is strongly limited for photon energies above a few hundred keV. Here we show that a high-current ultrarelativistic electron beam interacting with multiple submicrometer-thick conducting foils can undergo strong self-focusing accompanied by efficient emission of gamma-ray synchrotron photons. Physically, self-focusing and high-energy photon emission originate from the beam interaction with the near-field transition radiation accompanying the beam-foil collision. This near field radiation is of amplitude comparable with the beam self-field, and can be strong enough that a single emitted photon can carry away a significant fraction of the emitting electron energy. After beam collision with multiple foils, femtosecond collimated electron and photon beams with number density exceeding that of a solid are obtained. The relative simplicity, unique properties, and high efficiency of this gamma-ray source open up new opportunities for both applied and fundamental research including laserless investigations of strong-field QED processes with a single electron beam., Comment: 17 pages, 11 figures, matches published article

This paper discusses the properties of electron beams formed in plasma wakefield accelerators through ionization injection. In particular, the potential for generating a beam composed of co-located multi-colour beamlets is demonstrated in the case where the ionization is initiated by the evolving charge field of the drive beam itself. The physics of the processes of ionization and injection are explored through OSIRIS simulations. Experimental evidence showing similar features are presented from the data obtained in the E217 experiment at the FACET facility of the SLAC National Laboratory. This article is part of the Theo Murphy meeting issue ‘Directions in particle beam-driven plasma wakefield acceleration’.

A current challenge that is facing the plasma wakefield accelerator (PWFA) community is transverse beam emittance preservation. This can be achieved by balancing the natural divergence of the beam against the strong focusing force provided by the PWFA plasma source in a scheme referred to as beam matching. One method to accomplish beam matching is through the gradual focusing of a beam with a plasma density ramp leading into the bulk plasma. Here, the beam dynamics in a Gaussian plasma density ramp are considered, and an empirical formula is identified that gives the ramp length and beam vacuum waist location needed to achieve near-perfect matching. The method uses only the beam vacuum waist beta function as an input. Numerical studies show that the Gaussian ramp focusing formula is robust for beta function demagnification factors spanning more than an order of magnitude with experimentally favourable tolerances for future PWFA research facilities. This article is part of the Theo Murphy meeting issue ‘Directions in particle beam-driven plasma wakefield acceleration’.

An electron beam experiences chromatic emittance growth in a plasma-based accelerator if it is not matched to the focusing force in the plasma wake. A ramped plasma density profile at the entrance and exit of the plasma source can control the focusing of the beam into and out of the plasma accelerator, limiting emittance growth. Here, we present a comprehensive, analytic theory to describe the transverse beam dynamics and emittance growth in a nearly arbitrary plasma ramp profile. For a given incoming beam, this theory can be used to determine the length of the ramp required to correctly focus the electron beam, the optimal location of the beam's vacuum focus, and the chromatic emittance growth in the ramp. In addition, the theory can be used to determine the effect that errors in the beam focusing and plasma profile have on the emittance of the beam. We illustrate two example ramps to demonstrate the theory: one that provides very fast focusing for beam matching, and one that is robust to errors in the plasma density profile.

An electro-optic sampling beam position monitor for installation at the SLAC National Accelerator Laboratory FACET-II facility is described. Simulations of the detector signal were performed using realistic electron beam current profiles, demonstrating that this single-shot, non-destructive, compact detector is capable of resolving the relative transverse offset and longitudinal separation of an ultra-relativistic, two-bunch electron beam to order 1 μ m and 10 fs, respectively. A design study was performed to optimize the detector’s response to the ultra-relativistic two-bunch electron beam utilized in plasma wakefield accelerator experiments. In addition, the expected sensitivity to beam tilt is studied for typical electron beams used in free electron lasers .

Simulations and theory predict that long electron and positron beams may under favorable conditions self-modulate in plasmas. We report on the progress of experiments studying the self-modulation instability in plasma wakefield experiments at FACET. The experimental results obtained so far, while not being fully conclusive, appear to be consistent with the presence of the self-modulation instability.

International audience; We present a laser-ionized, beam-driven, passive thin plasma lens that operates in the nonlinear blowout regime. This thin plasma lens provides axisymmetric focusing for relativistic electron beams at strengths unobtainable by magnetic devices. It is tunable, compact, and it imparts little to no spherical aberrations. The combination of these features make it more attractive than other types of plasma lenses for highly divergent beams. A case study is built on beam matching into a plasma wakefield accelerator at SLAC National Accelerator Laboratory’s FACET-II facility. Detailed simulations show that a thin plasma lens formed by laser ionization of a gas jet reduces the electron beam’s waist beta function to half of the minimum value achievable by the FACET-II final focus magnets alone.

Recent progress in plasma based accelerator technology has demonstrated its ability to deliver high energy (GeV) beams in compact structures (centimeter to meter scale plasmas). Current developments of that technology are oriented toward producing beams with quality and energy spread comparable to those obtained using standard accelerating structures. In plasma based accelerators, the beam energy spread can be improved during the acceleration process through beam loading. To achieve optimum beam loading, the beam has to be shaped such that the superposition of its space charge fields and plasma fields result in a uniform accelerating field. In this work we show how beam-induced-ionization injection can be used to shape and inject a trailing beam suitable for beam loading. Our particle-in-cell numerical simulations done with OSIRIS show the ionization injection of a shaped 340 pC, 13 kA and $3\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ long electron beam accelerated to 900 MeV in less than 3 cm of plasma. The configurations considered numerically were based on the beams and plasmas that have been and will be available at the FACET facility.

Advances in Laboratory-based X-Ray Sources, Optics, and Applications VII : [Proceedings] - SPIE, 2019. - ISBN 97815106291349781510629141 - doi:10.1117/12.2530976 Advances in Laboratory-based X-Ray Sources, Optics, and Applications VII, San Diego, United States, 11 Aug 2019 - 15 Aug 2019; SPIE 111100A (2019). doi:10.1117/12.2530976, We provide a pathway to compact ultrabright light sources, based on ultrabright, high energy electron beams emerging from a combination of plasma Wakefield acceleration and plasma photocathodes. While plasma acceleration is known to produce accelerating fields three or four orders of magnitude larger than conventional accelerators, the plasma photocathode allows production of electron beams three or four orders of magnitude brighter than conventional, and thus is suitable to unleash the full potential of plasma accelerators. In particular, this is the case for various types of light sources, which profit enormously from an increased electron beam brightness. Building on the recent first experimental demonstration of the plasma photocathode, in this work we discuss the prospects of plasma photocathodes for key photon source approaches such as x-ray free-electron lasers, betatron radiation, ion-channel lasers and inverse Compton scattering., Published by SPIE

Plasma wakefield accelerators (PWF A) have demonstrated GeV/m scale accelerating gradients; however, current PWFAs tend to increase the particle beam's emittance (volume of the beam in transverse phase-space) as the beam propagates through the plasma. The beam's emittance can be preserved by tailoring the longitudinal density profile of the plasma to properly focus the beam into the plasma. We present optical techniques to create controlled plasma densities via laser ionization. The longitudinal plasma density profile is determined by the evolution of the intensity of a high power laser pulse as it propagates through a gas. Control of the pulse intensity is achieved using a superposition of Bessel beams created by a tandem lens setup. The width of the plasma is limited by the refraction of the laser pulse off of the ionizing gas, but we have developed several optical techniques to create wider plasmas. Our simulations show that suitable plasma filaments with lengths up to a meter and widths up to a millimeter can be generated in an argon gas using a~ 1 0 TW peak-power Ti:sa laser pulse.

Beam-driven, underdense thin plasma lenses are a compact and tunable means of providing strong focusing to relativistic electron beams. It is necessary to focus electron beams to small sizes in a beam-driven plasma wakefield accelerator to preserve the beam emittance, or volume in phase space. The thin plasma lens can be used to achieve these small beam sizes that are otherwise out of reach of conventional electromagnetic beam delivery systems. Ultrashort (~ 30 fs), low energy (~ 10 mJ) laser pulses can be used to ionize a small region of gas to generate the thin plasma lens. The interplay between the ionized gas and the focusing of the laser pulse are modeled in detail to calculate experimentally achievable 3-D plasma lens profiles, which are then used in simulations of beam focusing. Plasma lens focal lengths and minimum beam sizes achievable are estimated, considering the effects of radiation reaction in both the classical and quantum regimes.

Electron beam emittance preservation in a beam-driven, nonlinear plasma wakefield accelerator relies upon matching the beam to the plasma. This occurs when the linear focusing forces of the ion column acting on the electron beam are perfectly balanced against the emittance-driven divergence of the beam. The matching condition is satisfied when the square of the beam size equals the product of the geometric emittance and the inverse betatron wave number of the beam inside the plasma. In practice, however, the matched beam size is too small to achieve with conventional beam delivery systems operating in vacuum. Here, a passive, tapered plasma density ramp at the entrance (exit) of the plasma source is described that can focus the electron beam to (from) the necessary matched size at a rate exceeding the adiabatic plasma focusing limit without diluting the beam's emittance. A numerically derived empirical model predicts both the plasma ramp shape and parameter tolerances required for beam matching as a function of the incoming beam parameters. These results can be easily scaled for the purpose of designing sequential plasma stages in a high-energy linear collider.

An integral part of plasma wakefield acceleration (PWFA) is producing, monitoring, and controlling a plasma source. Understanding the temporal evolution of plasma parameters allows for constraints on experimental lifetime and initial conditions. Recombination factors for low temperature (kBT less than the ionization energy), singly-ionized Argon and Hydrogen plasmas (nominal PWFA plasma sources) are investigated to develop a model of plasma decay. Due to the high density of the plasma sources collisional recombination and ionization dominates radiative processes, and the problem of modelling plasma decay simplifies to finding the recombination coefficients for these plasma sources. This recombination coefficient is presented as a function of plasma temperature to determine plasma decay for a variety of initial plasma conditions. Additionally, plasma temperature is modelled from the ponderomotive force of the ionizing laser pulse. Results are utilized for designing optimal PWFA experiments.

International audience; During the past two decades of research, the ultra-relativistic beam-driven plasma wakefield accelerator (PWFA) concept has achieved many significant milestones. These include the demonstration of ultra-high gradient acceleration of electrons over meter-scale plasma accelerator structures, efficient acceleration of a narrow energy spread electron bunch at high-gradients, positron acceleration using wakes in uniform plasmas and in hollow plasma channels, and demonstrating that highly nonlinear wakes in the 'blow-out regime' have the electric field structure necessary for preserving the emittance of the accelerating bunch. A new 10 GeV electron beam facility, Facilities for Accelerator Science and Experimental Test (FACET) II, is currently under construction at SLAC National Accelerator Laboratory for the next generation of PWFA research and development. The FACET II beams will enable the simultaneous demonstration of substantial energy gain of a small emittance electron bunch while demonstrating an efficient transfer of energy from the drive to the trailing bunch. In this paper we first describe the capabilities of the FACET II facility. We then describe a series of PWFA experiments supported by numerical and particle-in-cell simulations designed to demonstrate plasma wake generation where the drive beam is nearly depleted of its energy, high efficiency acceleration of the trailing bunch while doubling its energy and ultimately, quantifying the emittance growth in a single stage of a PWFA that has optimally designed matching sections. We then briefly discuss other FACET II plasma-based experiments including in situ positron generation and acceleration, and several schemes that are promising for generating sub-micron emittance bunches that will ultimately be needed for both an early application of a PWFA and for a plasma-based future linear collider.

Hollow channel plasma wakefield acceleration is a proposed method to provide high acceleration gradients for electrons and positrons alike: a key to future lepton colliders. However, beams which are misaligned from the channel axis induce strong transverse wakefields, deflecting beams and reducing the collider luminosity. This undesirable consequence sets a tight constraint on the alignment accuracy of the beam propagating through the channel. Direct measurements of beam misalignment-induced transverse wakefields are therefore essential for designing mitigation strategies. We present the first quantitative measurements of transverse wakefields in a hollow plasma channel, induced by an off-axis 20 GeV positron bunch, and measured with another 20 GeV lower charge trailing positron probe bunch. The measurements are largely consistent with theory., 5 pages, 4 figures. Submitted to Physical Review Letters

To develop plasma wakefield acceleration into a compact and affordable replacement for conventional accelerators, beams of charged particles must be accelerated at high efficiency in a high electric field; here this is demonstrated for a bunch of charged electrons ‘surfing’ on a previously excited plasma wave. Particle colliders that operate at the high-energy frontier using electric fields generated by radio waves are approaching the limits of feasibility in terms of size and cost, but there are other acceleration techniques that could make less expensive and more compact devices. The plasma wakefield accelerator, in which an electron bunch is accelerated by making it 'surf' on a plasma wave excited by another electron bunch, promises an energy gain in the gigaelectron-volt regime over just a few centimetres — an energy gain that requires hundreds of metres using traditional accelerators. Previously, this technique had only been used to accelerate a very small number of electrons at a time. Now, researchers working at FACET, the Facility for Advanced Accelerator Experimental Tests at SLAC National Accelerator Laboratory, USA, have achieved acceleration of about half a billion electrons at once with an unprecedented efficiency for a plasma accelerator. This achievement could be a milestone in the development of affordable and compact accelerators for applications ranging from high energy physics to medical and industrial applications. High-efficiency acceleration of charged particle beams at high gradients of energy gain per unit length is necessary to achieve an affordable and compact high-energy collider. The plasma wakefield accelerator is one concept1,2,3 being developed for this purpose. In plasma wakefield acceleration, a charge-density wake with high accelerating fields is driven by the passage of an ultra-relativistic bunch of charged particles (the drive bunch) through a plasma4,5,6. If a second bunch of relativistic electrons (the trailing bunch) with sufficient charge follows in the wake of the drive bunch at an appropriate distance, it can be efficiently accelerated to high energy. Previous experiments using just a single 42-gigaelectronvolt drive bunch have accelerated electrons with a continuous energy spectrum and a maximum energy of up to 85 gigaelectronvolts from the tail of the same bunch in less than a metre of plasma7. However, the total charge of these accelerated electrons was insufficient to extract a substantial amount of energy from the wake. Here we report high-efficiency acceleration of a discrete trailing bunch of electrons that contains sufficient charge to extract a substantial amount of energy from the high-gradient, nonlinear plasma wakefield accelerator. Specifically, we show the acceleration of about 74 picocoulombs of charge contained in the core of the trailing bunch in an accelerating gradient of about 4.4 gigavolts per metre. These core particles gain about 1.6 gigaelectronvolts of energy per particle, with a final energy spread as low as 0.7 per cent (2.0 per cent on average), and an energy-transfer efficiency from the wake to the bunch that can exceed 30 per cent (17.7 per cent on average). This acceleration of a distinct bunch of electrons containing a substantial charge and having a small energy spread with both a high accelerating gradient and a high energy-transfer efficiency represents a milestone in the development of plasma wakefield acceleration into a compact and affordable accelerator technology.

Despite the urgent need for a TeV-class linear collider in High-Energy Physics (HEP), a clear path to buildable and affordable accelerator technologies has yet to be realized. Clearly, the identification and advancement of next generation accelerator technologies for a linear collider have been one of the main charges since the inception of the Advanced Accelerator Concepts (AAC) workshop. The fundamental requirements of linear colliders for accelerator technologies are to demonstrate high wall-plug efficiency, high beam quality preservation, high effective gradient, scalability, etc. Within the AAC community, beam-driven wakefield acceleration schemes (the central subject of Working Group 4) are always promising and attractive approaches. Since the last AAC workshop, a few high profile experiments related to beam-driven plasma wakefield acceleration have been conducted at the SLAC National Accelerator Laboratory’s FACET facility. These experiments have successfully answered questions related to obtaining...

In a plasma wakefield accelerator (PWFA), using advanced injection techniques such as the density down-ramp injection is a promising approach for generating electron beams with ultralow emittance. In this study, we describe how such a scheme could be realized using the beam from the Linac Coherent Light Source (LCLS) accelerator as a driver. Moreover, we explore the potential applications of such high-brightness e-beams in a facility like LCLS and in novel concepts for compact FELs.

The preservation of emittance of the accelerating beam is the next challenge for plasma-based accelerators envisioned for future light sources and colliders. The field structure of a highly nonlinear plasma wake is potentially suitable for this purpose but has not been yet measured. Here we show that the longitudinal variation of the fields in a nonlinear plasma wakefield accelerator cavity produced by a relativistic electron bunch can be mapped using the bunch itself as a probe. We find that, for much of the cavity that is devoid of plasma electrons, the transverse force is constant longitudinally to within ±3% (r.m.s.). Moreover, comparison of experimental data and simulations has resulted in mapping of the longitudinal electric field of the unloaded wake up to 83 GV m−1 to a similar degree of accuracy. These results bode well for high-gradient, high-efficiency acceleration of electron bunches while preserving their emittance in such a cavity., Extraction of ultra-low emittance bunches is an issue to be addressed for future applications of plasma wakefield accelerators. Here, the authors show that the field structure of the plasma could be suitable for this, by measuring the field's longitudinal variation produced by a relativistic electron bunch.

Plasma accelerators driven by particle beams are a very promising future accelerator technology as they can sustain high accelerating fields over long distances with high energy efficiency. They rely on the excitation of a plasma wave in the wake of a drive beam. To generate the plasma, a neutral gas can be field-ionized by the head of the drive beam, in which case the distance of acceleration and energy gain can be strongly limited by head erosion. Here we overcome this limit and demonstrate that electrons in the tail of a drive beam can be accelerated by up to 27 GeV in a high-ionization-potential gas (argon), boosting their initial 20.35 GeV energy by 130%. Particle-in-cell simulations show that the argon plasma is sustaining very high electric fields, of ∼150 GV m−1, over ∼20 cm. The results open new possibilities for the design of particle beam drivers and plasma sources., Plasma accelerators driven by particle beams are a promising technology, but the acceleration distance and energy gain are strongly limited by head erosion in a high-ionization-potential gas. Here the authors observe up to 130% energy boost in a self-focused electron beam, with limited head erosion.

Plasma wakefield accelerators have been used to accelerate electron and positron particle beams with gradients that are orders of magnitude larger than those achieved in conventional accelerators. In addition to being accelerated by the plasma wakefield, the beam particles also experience strong transverse forces that may disrupt the beam quality. Hollow plasma channels have been proposed as a technique for generating accelerating fields without transverse forces. Here we demonstrate a method for creating an extended hollow plasma channel and measure the wakefields created by an ultrarelativistic positron beam as it propagates through the channel. The plasma channel is created by directing a high-intensity laser pulse with a spatially modulated profile into lithium vapour, which results in an annular region of ionization. A peak decelerating field of 230 MeV m−1 is inferred from changes in the beam energy spectrum, in good agreement with theory and particle-in-cell simulations., Plasma wakefield accelerators produce gradients that are orders of magnitude larger than in conventional particle accelerator, but beams tend to be disrupted by transverse forces. Here the authors create an extended hollow plasma channel, which accelerates positrons without generating transverse forces.

An electron beam has gained a maximum energy of 9 GeV per particle in a 1.3 m-long electron beam-driven plasma wakefield accelerator. The amount of charge accelerated in the spectral peak was 28.3 pC, and the root-mean-square energy spread was 5.0%. The mean accelerated charge and energy gain per particle of the 215 shot data set was 115 pC and 5.3 GeV, respectively, corresponding to an acceleration gradient of 4.0 GeV/m at the spectral peak. The mean energy spread of the data set was 5.1%. These results are consistent with the extrapolation of the previously reported energy gain results using a shorter, 36 cm-long plasma source to within 10%, evincing a non-evolving wake structure that can propagate distances of over a meter in length. Wake-loading effects were evident in the data through strong dependencies observed between various spectral properties and the amount of accelerated charge., Comment: 11 pages, 3 figures, Laser and Plasma Accelerators Workshop (LPAW) 2015 Proceedings

Dark current can spoil witness bunch beam quality and acceleration efficiency in particle beam-driven plasma wakefield accelerators. In advanced schemes, hot spots generated by the drive beam or the wakefield can release electrons from higher ionization threshold levels in the plasma media. These electrons may be trapped inside the plasma wake and will then accumulate dark current, which is generally detrimental for a clear and unspoiled plasma acceleration process. Strategies for generating clean and robust, dark current free plasma wake cavities are devised and analyzed, and crucial aspects for experimental realization of such optimized scenarios are discussed.

Localized injection of electrons within a relativistic plasma wake can potentially produce an ultrashort, monoenergetic electron bunch. Recent experiments at the FACET facility at SLAC explored the injection of helium electrons at the helium-lithium interface of a lithium heat pipe oven and the subsequent acceleration in the beam-produced plasma wake. Electrons accelerated to over 10 GeV in 30 cm of plasma were observed as a distinct charge bunch.

We have investigated the viability of using plasmas formed by ionization of high Z, low ionization potential element rubidium (Rb) for beam-driven plasma wakefield acceleration. The Rb vapor column confined by argon (Ar) buffer gas was used to reduce the expected limitation on the beam propagation length due to head erosion that was observed previously when a lower Z but higher ionization potential lithium vapor was used. However, injection of electrons into the wakefield due to ionization of Ar buffer gas and nonuniform ionization of ${\mathrm{Rb}}^{1+}$ to ${\mathrm{Rb}}^{2+}$ was a possible concern. In this paper we describe experimental results and the supporting simulations which indicate that such ionization of Ar and ${\mathrm{Rb}}^{1+}$ in the presence of combined fields of the beam and the wakefield inside the wake does indeed occur. Some of this charge accumulates in the accelerating region of the wake leading to the reduction of the electric field---an effect known as beam loading. The beam-loading effect is quantified by determining the average transformer ratio $⟨R⟩$ which is the maximum energy gained divided by the maximum energy lost by the electrons in the bunch used to produce the wake. $⟨R⟩$ is shown to depend on the propagation length and the quantity of the accumulated charge, indicating that the distributed injection of secondary Rb electrons is the main cause of beam loading in this experiment. The average transformer ratio is reduced from 1.5 to less than 1 as the excess charge from secondary ionization increased from 100 to 700 pC. The simulations show that while the decelerating field remains constant, the accelerating field is reduced from its unloaded value of 82 to $46\text{ }\text{ }\mathrm{GeV}/\mathrm{m}$ due to this distributed injection of dark current into the wake.

We report initial results of optical probing experiments (E-224) at the FACET/SLAC user facility. These experiments are being carried out in collaboration with the ongoing e-beam driven wakefield experimental campaign (E-200) at FACET. Their aim is to optically study both the short term and long term plasma structure produced by the e-beam driver. The SLAC plasma wakefield experiments have demonstrated the highest energy gain to date [1] and continue work on further optimization. Direct visualization of the plasma wake structure would aid in the understanding of the dynamics of the beam plasma interaction and acceleration and its optimization. Currently, such understanding is derived only from simulations, which are very time consuming and rely on limited knowledge of initial conditions. We describe the optical probing geometry used in this initial run, a variation of a method previously developed in our group [6], as governed by the unique experimental challenges of the FACET beam driven experiments in t...

International audience; We report on the experimental observation of the attraction of a beam of ultrarelativistic electrons towards a column of neutral plasma. In experiments performed at the FACET test facility at SLAC we observe that an electron beam moving parallel to a neutral plasma column, at an initial distance of many plasma column radii, is attracted into the column. Once the beam enters the plasma it drives a plasma wake similar to that of an electron beam entering the plasma column head-on. A simple analytical model is developed in order to capture the essential physics of the attractive force. The attraction is further studied by 3D particle-in-cell numerical simulations. The results are an important step towards better understanding of particle beam–plasma interactions in general and plasma wakefield accelerator technology in particular.

Electrical breakdown sets a limit on the kinetic energy that particles in a conventional radio-frequency accelerator can reach. New accelerator concepts must be developed to achieve higher energies and to make future particle colliders more compact and affordable. The plasma wakefield accelerator (PWFA) embodies one such concept, in which the electric field of a plasma wake excited by a bunch of charged particles (such as electrons) is used to accelerate a trailing bunch of particles. To apply plasma acceleration to electron-positron colliders, it is imperative that both the electrons and their antimatter counterpart, the positrons, are efficiently accelerated at high fields using plasmas. Although substantial progress has recently been reported on high-field, high-efficiency acceleration of electrons in a PWFA powered by an electron bunch, such an electron-driven wake is unsuitable for the acceleration and focusing of a positron bunch. Here we demonstrate a new regime of PWFAs where particles in the front of a single positron bunch transfer their energy to a substantial number of those in the rear of the same bunch by exciting a wakefield in the plasma. In the process, the accelerating field is altered--'self-loaded'--so that about a billion positrons gain five gigaelectronvolts of energy with a narrow energy spread over a distance of just 1.3 metres. They extract about 30 per cent of the wake's energy and form a spectrally distinct bunch with a root-mean-square energy spread as low as 1.8 per cent. This ability to transfer energy efficiently from the front to the rear within a single positron bunch makes the PWFA scheme very attractive as an energy booster to an electron-positron collider.

Performance of the x-ray free electron laser Linac Coherent Light Source (LCLS) and the Facility for Advanced Accelerator Experimental Tests (FACET) is determined by the properties of their extremely short electron bunches. Multi-GeV electron bunches in both LCLS and FACET are less than 100 fs long. Optimization of beam properties and understanding of free-electron laser operation require electron beam diagnostics with time resolution of about 10 fs. We designed, built and commissioned a set of high frequency $X$-band deflectors which can measure the beam longitudinal space charge distribution and slice energy spread to better than 10 fs resolution at full LCLS energy (14 GeV), and with 70 fs resolution at full FACET energy (20 GeV). Use of high frequency and high gradient in these devices allows them to reach unprecedented performance. We report on the physics motivation, design considerations, operational configuration, cold tests, and typical results of the $X$-band deflector systems currently in use at SLAC.

2Although there is strong theoretical support that na-ture can be described by a grand uni ed theory (GUT) [1,2], there is currently no direct experimental evidence.One of the most powerful ways to test grand uni cationis to look for proton (or bound neutron) decay. MostGUTs have an unstable proton; in the absence of anobservation, setting experimental limits on the protonlifetime can provide useful constraints on the nature ofgrand uni ed theories. Observation, on the other hand,would be tantalizing evidence of new physics beyond theStandard Model.One of the more simple but interesting candidates forgrand uni cation is SO(10), where the Standard Model’sSU(3), SU(2), and U(1) are contained within the largergauge group. The class of models based on SO(10) uni- cation generally make predictions for neutrino massesand mixing that are broadly in accord with all knownneutrino mixing data [3, 4]. The minimal supersym-metric SO(10) model with a 126 Higgs eld describedin Ref. [3] is the particular motivation for the analysispresented here. In addition to predicting neutrino massand mixing in agreement with observations, it leaves R-parity unbroken, which guarantees the existence of stabledark matter. For some region of its allowed parameterspace, this model predicts that the dominant nucleon de-cay modes will be p! ˇ

International audience; In this article we review the prospects of laser wakefield accelerators as next generation light sources for applications. This work arose as a result of discussions held at the 2013 Laser Plasma Accelerators Workshop. X-ray phase contrast imaging, x-ray absorption spectroscopy, and nuclear resonance fluorescence are highlighted as potential applications for laser–plasma based light sources. We discuss ongoing and future efforts to improve the properties of radiation from plasma betatron emission and Compton scattering using laser wakefield accelerators for these specific applications.

Long (L>>λ pe ) charged particle bunches traveling in dense plasmas are subject to a transverse two-stream instability, the self-modulation instability (SMI)1. This instability modulates the bunch in the radial direction with a longitudinal period approximately equal to the plasma wavelength (λ pe ∼n e 1/2). The SMI can be used to transform a long bunch into a train of short bunches that can resonantly drive wakefields to large amplitudes. An experiment known as AWAKE2 was recently approved at CERN to drive GV/m wakefields with 400GeV, ∼12cm-long proton bunches in a 10m plasma with λ pe ∼1mm. In the linear wakefield regime, the plasma response is symmetric for positively and negatively charged bunches. However, once the nonlinear regime is approached the plasma response becomes asymmetric, leading to differences that could be evidenced using the ultra-relativistic electron and positron bunches3. This asymmetry makes it more difficult to accelerate positively than negatively charged particle bunches in plasmas.

A search for the dinucleon decay $pp\ensuremath{\rightarrow}{K}^{+}{K}^{+}$ has been performed using $91.6\text{ }\text{ }\mathrm{kton}\ifmmode\cdot\else\textperiodcentered\fi{}\text{yr}$ data from Super-Kamiokande-I. This decay provides a sensitive probe of the $R$-parity-violating parameter ${{\ensuremath{\lambda}}_{112}}^{\ensuremath{'}\ensuremath{'}}$. A boosted decision tree analysis found no signal candidates in the data. The expected background was $0.28\ifmmode\pm\else\textpm\fi{}0.19$ atmospheric neutrino induced events and the estimated signal detection efficiency was $12.6%\ifmmode\pm\else\textpm\fi{}3.2%$. A lower limit of $1.7\ifmmode\times\else\texttimes\fi{}{10}^{32}\text{ }\text{ }\text{years}$ has been placed on the partial lifetime of the decay $^{16}\mathrm{O}\ensuremath{\rightarrow}^{14}\mathrm{C}{K}^{+}{K}^{+}$ at 90% C.L. A corresponding upper limit of $7.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ has been placed on the parameter ${{\ensuremath{\lambda}}_{112}}^{\ensuremath{'}\ensuremath{'}}$.

We investigate transverse effects in the plasma-wakefield acceleration experiments planned and ongoing at FACET. We use PIC simulation tools, mainly QuickPIC, to simulate the interaction of the drive electron beam and the plasma. In FACET a number of beam dynamics knobs, including dispersion and bunch length knobs, can be used to vary the beam transverse characteristics in the plasma. We present simulation results and the status of the FACET experimental searches.

For wakefield based acceleration schemes, use of an asymmetric (or linearly ramped) drive bunch current profile has been predicted to enhance the transformer ratio and generate large accelerating wakes. We discuss plans and initial results for producing such bunches using the 20 to 23 GeV electron beam at the FACET facility at SLAC National Accelerator Laboratory and sending them through plasmas and dielectric tubes to generate transformer ratios greater than 2 (the limit for symmetric bunches). The scheme proposed utilizes the final FACET chicane compressor and transverse collimation to shape the longitudinal phase space of the beam.

Head erosion is one of the limiting factors in plasma wakefield acceleration (PWFA). We present a study of head erosion with emittance growth in field-ionized plasma from the PWFA experiments performed at the FACET user facility at SLAC. At FACET, a 20.3 GeV bunch with 1.8×1010 electrons is optimized in beam transverse size and combined with a high density lithium plasma for beam-driven plasma wakefield acceleration experiments. A target foil is inserted upstream of the plasma source to increase the bunch emittance through multiple scattering. Its effect on beamplasma interaction is observed with an energy spectrometer after a vertical bend magnet. Results from the first experiments show that increasing the emittance has suppressed vapor field-ionization and plasma wakefields excitation. Plans for the future are presented.

Ionization injection in a plasma wakefield accelerator was investigated experimentally using two lithium plasma sources of different lengths. The ionization of the helium gas, used to confine the lithium, injects electrons in the wake. After acceleration, these injected electrons were observed as a distinct group from the drive beam on the energy spectrometer. They typically have a charge of tens of pC, an energy spread of a few GeV, and a maximum energy of up to 30 GeV. The emittance of this group of electrons can be many times smaller than the initial emittance of the drive beam. The energy scaling for the trapped charge from one plasma length to the other is consistent with the blowout theory of the plasma wakefield.

Searches for a nucleon decay into a charged anti-lepton (e^+ or {\mu}^+) plus a light meson ({\pi}^0, {\pi}^-, {\eta}, {\rho}^0, {\rho}^-, {\omega}) were performed using the Super-Kamiokande I and II data. Twelve nucleon decay modes were searched for. The total exposure is 140.9 kiloton \cdot years, which includes a 91.7 kiloton \cdot year exposure (1489.2 live days) of Super-Kamiokande-I and a 49.2 kiloton \cdot year exposure (798.6 live days) of Super-Kamiokande-II. The number of candidate events in the data was consistent with the atmospheric neutrino background expectation. No significant evidence for a nucleon decay was observed in the data. Thus, lower limits on the nucleon partial lifetime at 90% confidence level were obtained. The limits range from 3.6 \times 10^31 to 8.2 \times 10^33 years, depending on the decay modes., Comment: 25 pages, 18 figures

Artículo escrito por un elevado número de autores, solo se referencian el que aparece en primer lugar, el nombre del grupo de colaboración, si le hubiere, y los autores pertenecientes a la UAM, A new Super-Kamiokande search for supernova relic neutrinos was conducted using 2853 live days of data. Sensitivity is now greatly improved compared to the 2003 Super-Kamiokande result, which placed a flux limit near many theoretical predictions. This more detailed analysis includes a variety of improvements such as increased efficiency, a lower energy threshold, and an expanded data set. New combined upper limits on supernova relic neutrino flux are between 2.8 and 3.1ν̄e cm-2 s-1 > 16 MeV total positron energy (17.3 MeV Eν), The Super-Kamiokande experiment has been built and operated from funding by the Japanese Ministry of Education, Culture, Sports, Science and Technology, the United States Department of Energy, and the U.S. National Science Foundation. Some of us have been supported by funds from the Korean Research Foundation (BK21), the National Research Foundation of Korea (NRF- 20110024009), the Japan Society for the Promotion of Science, and the National Natural Science Foundation of China (Grants No. 10875062 and No. 10911140109)

We report a measurement of muon-neutrino disappearance in the T2K experiment. The 295-km muon-neutrino beam from Tokai to Kamioka is the first implementation of the off-axis technique in a long-baseline neutrino oscillation experiment. With data corresponding to 1.43 × 10(20) protons on target, we observe 31 fully-contained single μ-like ring events in Super-Kamiokande, compared with an expectation of 104 ± 14(syst) events without neutrino oscillations. The best-fit point for two-flavor νμ → ντ oscillations is sin 2(2θ(23)) = 0.98 and |Δm(2)(32)| = 2.65 × 10(−3) eV2. The boundary of the 90% confidence region includes the points (sin2 (2θ(23)), |Δm(2)(32)|) = (1.0, 3.1 × 10(−3) eV2), (0.84, 2.65 × 10(−3) eV2) and (1.0, 2.2 × 10(−3) eV2)., Physical Review D, 85 (3), ISSN:1550-7998, ISSN:0556-2821, ISSN:1550-2368