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3rd European Advanced Accelerator Concepts Workshop, EAAC 2017, La Biodola, Isola d'Elba, Italy, 24 Sep 2017 - 30 Sep 2017; Nuclear instruments & methods in physics research / A 909, 80 - 83 (2018). doi:10.1016/j.nima.2018.02.063, Beam quality preservation in the external injection scheme is one of the key missing milestones towards staging of plasma-wakefield accelerators, a prerequisite for their utilisation in particle physics or other applications requiring high energy beams. This topic will be studied at FLASHForward, a unique beam-driven plasma wakefield acceleration facility currently under construction at DESY (Hamburg, Germany), in the frame of the FLASHForward X-2 experiment. High-quality 1 GeV-class electron beams from the free-electron laser FLASH with m-emittances, kA-scale currents, and less than 100 fs durations will be utilised to generate driver–witness pairs by using a mask in a dispersive section. In this contribution the physics case and the current status of the FLASHForward X-2 experiment are reviewed. The experimental installation is described, with a focus on the electron beamline. Electron beam dynamics and particle-in-cell simulations are presented., Published by Elsevier, Amsterdam

Physical review accelerators and beams 22(8), 081301 (2019). doi:10.1103/PhysRevAccelBeams.22.081301, The transport and acceleration of positron beams is a crucial challengeon the path towards plasma-based particle colliders. This work pro-poses to use a finite radius plasma in a plasma wakefield acceleratoras a way to reduce the restoring force acting on the plasma electrons,resulting in an elongation of their on-axis return point and, hence,creating a long, high-density electron filament. This leads to a regionwith accelerating and focusing fields for positrons, allowing for theacceleration and quality-preserving transport of high current positronbeams. The generation of the finite radius plasma targets as well astheir associated wakefields are investigated, before quality-preservingpositron acceleration is demonstrated., Published by American Physical Society, College Park, MD

International audience; The EuPRAXIA project aims at designing the world’s first accelerator based on plasma-wakefield advanced technique, which can deliver a 5 GeV electron beam with simultaneously high charge, low emittance and low energy spread to user’s communities. Such challenging objectives can only have a chance to be achieved when particular efforts are dedicated to identify the subsequent issues and to find the way to solve them. Many injection/acceleration schemes and techniques have been explored by means of thorough simulations in more than ten European institutes to sort out the most appropriate ones. The specific issues of high charge, high beam quality and beam extraction then transfer to the user’s applications, have been tackled with many innovative approaches*. This article highlights the different advanced methods that have been employed by the EuPRAXIA collaboration and the preliminary results obtained. The needs in terms of laser and plasma parameters for such an accelerator are also summarized.

High energy density physics 32, 31 - 43 (2019). doi:10.1016/j.hedp.2019.04.001, To understand astrophysical magnetic-field amplification, we conducted a feasibility study for a laboratory experiment of a non-resonant streaming instability at the Photo Injector Test Facility at DESY, Zeuthen site (PITZ). This non-resonant streaming instability, also known as Bell’s instability, is generally regarded as a candidate for the amplification of interstellar magnetic field in the upstream region of supernova-remnant shocks, which is crucial for the efficiency of diffusive shock acceleration. In the beam-plasma system composed of a radio-frequency electron gun and a gas-discharge plasma cell, the goal of our experiment is to demonstrate the development of the non-resonant streaming instability and to find its saturation level in the laboratory environment. Since we find that the electron beam will be significantly decelerated on account of an electrostatic streaming instability, which will decrease the growth rate of desired non-resonant streaming instability, we discuss possible ways to suppress the electrostatic streaming instability by considering the characteristics of a field-emission-based quasi continuous-wave electron beam., Published by Elsevier, Amsterdam [u.a.]

Witness beam stability and preservation of its ultra-low emittance have been identified as critical challenges toward realizing a TeV-class, plasma-based linear collider. In fact, the witness bunch parameters required by a future TeV-class collider have been expected to trigger hosing instability and background ion motion, leading to emittance degradation and, potentially, to bunch loss. Recently, it has been shown that ion motion suppresses the hosing instability, and proper longitudinal bunch shaping can eliminate the ion-motion-induced emittance growth. In this paper, we propose and analyze a plasma-based method to generate the shaped bunches that enable emittance preservation in the presence of ion motion. The method is based on an adiabatic matching procedure, where a bunch with an initially untapered profile is injected in the plasma accelerator stage with an energy low enough that ion motion effects are initially small. As the bunch accelerates, it is adiabatically compressed, ion motion is gradually triggered, and the bunch slowly but continuously readjusts itself in the ion motion-perturbed wakefield acquiring the desired taper. The production of tapered witness bunch profiles that minimize energy spread and preserve emittance for collider-relevant parameters could enable the use of plasma-based accelerators for high-energy physics applications.

This paper introduces a semi-analytic numerical approach (SANA) for the rapid computation of the transverse emittance of beams with finite energy spread in plasma wakefield accelerators in the blowout regime. The SANA method is used to model the beam emittance evolution when injected into and extracted from realistic plasma profiles. Results are compared to particle-in-cell simulations, establishing the accuracy and efficiency of the procedure. In addition, it is demonstrated that the tapering of vacuum-to-plasma and plasma-to-vacuum transitions is a viable method for the mitigation of emittance growth of beams during their injection and extraction from and into plasma cells.

25th International Conference on the Application of Accelerators in Research and Industry, CAARI 2018, Texas, USA, 12 Aug 2018 - 17 Aug 2018; AIP conference proceedings 2160(1), 040012-1 - 040012-9 (2019). doi:https://doi.org/10.1063/1.5127692, Plasma accelerators present one of the most suitable candidates for the development of more compact particle acceleration technologies, yet they still lag behind radiofrequency (RF)-based devices when it comes to beam quality, control, stability and power efficiency. The Horizon 2020-funded project EuPRAXIA (“European Plasma Research Accelerator with eXcellence In Applications”) aims to overcome the first three of these hurdles by developing a conceptual design for a first international user facility based on plasma acceleration. In this paper we report on the main features, simulation studies and potential applications of this future research infrastructure., Published by AIP Publishing, Melville, NY

In this work we provide an in-depth analysis of mechanisms which mitigate the hose instability in the blowout regime in plasma-wakefield accelerators. We show by means of theory and three-dimensional particle-in-cell simulations that the mitigation mechanisms related to a beam energy spread are effective for parameters as used in major plasma-wakefield accelerator experiments and for various beam energies, beam current profiles, and types of hosing seed. In addition, we establish the theoretical principles for the reduction of the initial hosing seed in tapered vacuum-to-plasma transitions and derive respective analytic predictions which are successfully benchmarked against particle-in-cell simulations. We also investigate the possibility to facilitate efficient and stable acceleration of witness beams in the blowout regime. This work therefore provides a deepened understanding for the methods that allow for the mitigation of hosing, a crucial prerequisite to facilitate stable acceleration of high quality beams in plasma-wakefield accelerators. info:eu-repo/semantics/publishedVersion

Philosophical transactions of the Royal Society of London / A Mathematical, physical and engineering sciences Series A 377(2151), 20180392 - (2019). doi:10.1098/rsta.2018.0392, The FLASHForward experimental facility is a high-performance test-bed for precision plasma wakefield research, aiming to accelerate high-quality electron beams to GeV-levels in a few centimetres of ionized gas. The plasma is created by ionizing gas in a gas cell either by a high-voltage discharge or a high-intensity laser pulse. The electrons to be accelerated will either be injected internally from the plasma background or externally from the FLASH superconducting RF front end. In both cases, the wakefield will be driven by electron beams provided by the FLASH gun and linac modules operating with a 10 Hz macro-pulse structure, generating 1.25 GeV, 1 nC electron bunches at up to 3 MHz micro-pulse repetition rates. At full capacity, this FLASH bunch-train structure corresponds to 30 kW of average power, orders of magnitude higher than drivers available to other state-of-the-art LWFA and PWFA experiments. This high-power functionality means FLASHForward is the only plasma wakefield facility in the world with the immediate capability to develop, explore and benchmark high-average-power plasma wakefield research essential for next-generation facilities. The operational parameters and technical highlights of the experiment are discussed, as well as the scientific goals and high-average-power outlook., Published by Soc., London

Mitigation of the beam hose instability in plasma-based accelerators is required for the realization of many applications, including plasma-based colliders. The hose instability is analyzed in the blowout regime including plasma ion motion, and ion motion is shown to suppress the hose instability by inducing a head-to-tail variation in the focusing force experienced by the beam. Hence, stable acceleration in plasma-based accelerators is possible, while, by use of proper bunch shaping, minimizing the energy spread and preserving the transverse beam emittance.

Physical review letters 122(3), 034801 (2019). doi:10.1103/PhysRevLett.122.034801, A tunable plasma-based energy dechirper has been developed at FLASHForward to remove thecorrelated energy spread of a 681 MeV electron bunch. Through the interaction of the bunch withwakefields excited in plasma the projected energy spread was reduced from a FWHM of 1.31% to 0.33%without reducing the stability of the incoming beam. The experimental results for variable plasma densityare in good agreement with analytic predictions and three-dimensional simulations. The proof-of-principledechirping strength of $1.8$ $ GeV/mm/m$ significantly exceeds those demonstrated for competing state-of-the-art techniques and may be key to future plasma wakefield-based free-electron lasers and high energyphysics facilities, where large intrinsic chirps need to be removed, Published by APS, College Park, Md.

Particle-in-cell modeling of 10 GeV-Class laser-plasma accelerators, where a short and intense laser pulse with an energy of tens of Joules propagates in a plasma with a length of a few tens of cm and a density ~ 1017 cm−3, is a computationally challenging task, requiring several tens of millions of core hours on today's supercomputers. In this paper, we present simulations results, obtained with the computationally-efficient code INF&RNO [8]–[10], concerning the production of 10 GeV-class electron beams in a 20 cm plasma under realistic conditions using the PW BELLA laser at the Lawrence Berkeley National Laboratory (LBNL). The computational savings provided by INF&RNO allows for extensive parameter scans and detailed characterization of the accelerator performance.

Ion motion has a strong impact on the quality preservation of electron beams in plasma-based colliders. While being crucial for the advance of plasma-based accelerators, the modeling of the physics of interest is highly challenging due to the large disparity of the relevant transverse length scales. To correctly model plasma-based collider schemes with ion motion, a high transverse resolution must be employed in particle-in-cell simulations, which, conventionally, entails significant computational costs. By introducing a refined subgrid in the witness beam region in a quasi-static particle-in-cell code, significant computational savings can be achieved. This numerical approach allows for the accurate modeling of plasma-based colliders with ion motion in the blowout regime while allowing for an orders of maznitude computational speedup.

We formulate a new procedure for modelling the transverse dynamics of relativistic electron beams with significant energy spread when injected into plasma-based accelerators operated in the blow-out regime. Quantities of physical interest, such as the emittance, are furnished directly from solution of phase space moment equations formed from the relativistic Vlasov equation. The moment equations are closed by an Ansatz, and solved analytically for prescribed wakefields. The accuracy of the analytic formulas is established by benchmarking against the results of a semi-analytic/numerical procedure which is described within the scope of this work, and results from a simulation with the 3D quasi-static PIC code HiPACE.

Physical review letters 121(6), 064803 (2018). doi:10.1103/PhysRevLett.121.064803, The hose instability of the drive beam constitutes a major challenge for the stable operation of plasma wakefield accelerators (PWFAs). In this work, we show that drive beams with a transverse size comparable to the plasma blowout radius generate a wake with a varying focusing along the beam, which leads to a rapid detuning of the slice-betatron oscillations and suppresses the instability. This intrinsic stabilization principle provides an applicable and effective method for the suppression of the hosing of the drive beam and allows for a stable acceleration process., Published by APS, College Park, Md.

Physics of plasmas 25(5), 056703 (2018). doi:10.1063/1.5017960, Hosing is a major challenge for the applicability of plasma wakefield accelerators and its modeling is therefore of fundamental importance to facilitate future stable and compact plasma-based particle accelerators. In this contribution, we present a new model for the evolution of the plasma centroid, which enables the accurate investigation of the hose instability in the nonlinear blowout regime. It paves the road for more precise and comprehensive studies of hosing, e.g., with drive and witness beams, which were not possible with previous models., Published by American Institute of Physics, Melville, NY

9th International Particle Accelerator Conference, IPAC18, Vancouver, Kanada, 29 Apr 2018 - 4 May 2018; Journal of physics / Conference Series Conference Series 1067, 042010 (2018). doi:10.1088/1742-6596/1067/4/042010, Density modulations in plasma caused by a high-intensity laser or a high chargedensity electron pulse can generate extreme acceleration fields. Acceleration of electrons in suchfields may produce ultra-relativistic, quasi-monoenergetic, ultra-short electron bunches overdistances orders of magnitudes shorter than in state-of-the-art radio-frequency accelerators.FLASHForward is a beam-driven plasma wakefield accelerator (PWFA) project at DESY withthe goal of producing, characterizing, and utilizing such beams. Temporal characterization ofthe acceleration process is of crucial importance for improving the stability and control in PWFAbeams. While measurement of the transient field of the femtosecond bunch in a single shot ischallenging, in recent years novel techniques with great promise have been developed [1, 2]. Thiswork discusses the plans and status of the transverse diagnostics at FLASHForward., Published by IOP Publ., Bristol

9th International Particle Accelerator Conference, IPAC18, Vancouver, Kanada, 29 Apr 2018 - 4 May 2018; Journal of physics / Conference Series 1067, 042012 (2018). doi:10.1088/1742-6596/1067/4/042012, The self-modulation instability is fundamental for the plasma wakefield acceleration experiment of the AWAKE (Advanced Wakefield Experiment) collaboration at CERN where this effect is used to generate proton bunches for the resonant excitation of high acceleration fields. Utilizing the availability of flexible electron beam shaping together with excellent diagnostics including an RF deflector, a supporting experiment was set up at the electron accelerator PITZ (Photo Injector Test facility at DESY, Zeuthen site), given that the underlying physics is the same. After demonstrating the effect [1] the next goal is to investigate in detail the self-modulation of long (with respect to the plasma wavelength) electron beams.In this contribution we describe parameter studies on self-modulation of a long electron bunch in an argon plasma. The plasma was generated with a discharge cell with densities in the 10$^{13}$ cm$^{−3}$ to 10$^{15}$ cm$^{−3}$ range. The plasma density was deduced from the plasma wavelength as indicated by the self-modulation period. Parameter scans were conducted with variable plasma density and electron bunch focusing., Published by IOP Publ., Bristol

© 2017 authors. Published by the American Physical Society. Density down-ramp (DDR) injection is a promising concept in beam-driven plasma wakefield accelerators for the generation of high-quality witness beams. We review and complement the theoretical principles of the method and employ particle-in-cell (PIC) simulations in order to determine constrains on the geometry of the density ramp and the current of the drive beam, regarding the applicability of DDR injection. Furthermore, PIC simulations are utilized to find optimized conditions for the production of high-quality beams. We find and explain the intriguing result that the injection of an increased charge by means of a steepened ramp favors the generation of beams with lower emittance. Exploiting this fact enables the production of beams with high charge (∼140 pC), low normalized emittance (∼200 nm) and low uncorrelated energy spread (0.3%) in sufficiently steep ramps even for drive beams with moderate peak current (∼2.5 kA).

We use three-dimensional (3D) simulations with the particle-in-cell (PIC) code OSIRIS to demonstrate the theoretical production of high-quality electron bunches in beam-driven plasma wakefield accelerators (PWFA) by means of field-induced ionization injection. In these simulations, two realistic scenarios for PWFA have been considered: the FLASHForward project at DESY and the FACET experiment at SLAC. These two examples illustrate two different strategies for injection. The first one uses the transverse electric fields of the beam to induce injection, and the second constitutes a new method which utilizes only the wakefields to enable ionization and trapping of high quality electron bunches into beam driven plasma wakes. The produced bunches feature multi-kA peak currents, ~ 1 μ m transverse normalized emittances, uncorrelated energy spreads of ≤ 1 % on a GeV-energy scale, and few femtosecond bunch lengths.

We present simulations of injection of plasma electrons triggered by a negative gradient in the plasma-density profile for beam-driven plasma-wakefield acceleration. Studies show the potential of this trapping technique for the production of narrow-energy bandwidth, femtosecond, high-current electron beams with a small transverse normalised emittance in the hundred nm range. Moreover, the accelerated beams feature strong longitudinal phase-space correlations with sub-percent uncorrelated energy spreads. Tuning of density down-ramp injection is explored by scanning the slope and length of the downward transition. It is found that the mechanism causes particle trapping over a wide experimentally accessible parameter range. All simulations were performed in three spatial dimensions with the particle-in-cell code OSIRIS [1] . The investigated driver-beam properties and the plasma profile resemble expected experimental conditions at the FLASHForward facility at DESY.

© 2016 Elsevier B.V. The self-modulation instability of long particle beams was proposed as a new mechanism to produce driver beams for proton driven plasma wakefield acceleration (PWFA). The PWFA experiment at the Photo Injector Test facility at DESY, Zeuthen site (PITZ) was launched to experimentally demonstrate and study the self-modulation of long electron beams in plasma. Key aspects for the experiment are the very flexible photocathode laser system, a plasma cell and well-developed beam diagnostics. In this contribution we report about the plasma cell design, preparatory experiments and the results of the first PWFA experiment at PITZ.

Author(s): Mehrling, TJ; Benedetti, C; Schroeder, CB; Martinez De La Ossa, A; Osterhoff, J; Esarey, E; Leemans, WP | Abstract: We would like to correct a typographical error which was introduced in Eqs. (5), (11), (14), (20a), and (20b) during revision of Ref. 1. The series expansion of the plasmaelectron phase space density to the first order of 〈x〉 and 〈px〉 [Eq. (5) in Ref. 1] correctly reads fp ≈ fp0 - cos θ (〈x〉∂r + 〈px〉∂pr)fp,0 The zeroth-order term, fp,0, was not printed in Ref. 1. Accordingly, the correct expansion of the wakefield potential [Eq. (11) in Ref. 1] is ψ(r,θ) ≈ ψ0(r) -cos θ Xp ∂r ψ0 (r), the correct expansion of the source term of the wakefield potential [Eq. (14) in Ref. 1] is S(r, θ) ≈ S0(r) - cos θ Xp ∂rS0(r), and the correct expansions of the force terms [Eqs. (20a) and (20b) in Ref. 1] are Fr,b ≈ Fr,b,o - cos θ Xb ∂rFr,b,0, Fr,b ≈ Fr,p,o - cos θ 〈x〈 ∂rFr,p,0, The zeroth order terms in Eqs. (5), (11), (14), (20a), and (20b) are missing in the printing of Ref. 1 but were fully taken into account for the calculation of all derived equations of the mathematical model. The developed mathematical model and physical conclusions in Ref. 1 are therefore all unaltered by, and consistent with, the above corrections.

© 2017 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Plasma-based accelerators offer the possibility to drive future compact light sources and high-energy physics applications. Achieving good beam quality, especially a small beam energy spread, is still one of the major challenges. Here, we propose to use a periodically modulated plasma density to shape the longitudinal fields acting on an electron bunch in the linear wakefield regime. With simulations, we demonstrate an on-average flat accelerating field that maintains a small beam energy spread.

We present a detailed analysis of the features and capabilities of Wakefield-Induced Ionization (WII) injection in the blowout regime of beam driven plasma accelerators. This mechanism exploits the electric wakefields to ionize electrons from a dopant gas and trap them in a well-defined region of the accelerating and focusing wake phase, leading to the formation of high-quality witness-bunches [Martinez de la Ossa et al., Phys. Rev. Lett. 111, 245003 (2013)]. The electron-beam drivers must feature high-peak currents ($I_b^0\gtrsim 8.5~\mathrm{kA}$) and a duration comparable to the plasma wavelength to excite plasma waves in the blowout regime and enable WII injection. In this regime, the disparity of the magnitude of the electric field in the driver region and the electric field in the rear of the ion cavity allows for the selective ionization and subsequent trapping from a narrow phase interval. The witness bunches generated in this manner feature a short duration and small values of the normalized transverse emittance ($k_p\sigma_z \sim k_p\epsilon_n \sim 0.1$). In addition, we show that the amount of injected charge can be adjusted by tuning the concentration of the dopant gas species, which allows for controlled beam loading and leads to a reduction of the total energy spread of the witness beams. Electron bunches, produced in this way, fulfil the requirements to drive blowout regime plasma wakes at a higher density and to trigger WII injection in a second stage. This suggests a promising new concept of self-similar staging of WII injection in steps with increasing plasma density, giving rise to the potential of producing electron beams with unprecedented energy and brilliance from plasma-wakefield accelerators.

We propose a new and simple strategy for controlled ionization-induced trapping of electrons in a beam-driven plasma accelerator. The presented method directly exploits electric wakefields to ionize electrons from a dopant gas and capture them into a well-defined volume of the accelerating and focusing wake phase, leading to high-quality witness-bunches. This injection principle is explained by example of three-dimensional particle-in-cell (PIC) calculations using the code OSIRIS. In these simulations a high-current-density electron-beam driver excites plasma waves in the blow-out regime inside a fully-ionized hydrogen plasma of density $5\times10^{17} \mathrm{cm^{-3}}$. Within an embedded $100 \mathrm{\mu m}$ long plasma column contaminated with neutral helium gas, the wakefields trigger ionization, trapping of a defined fraction of the released electrons, and subsequent acceleration. The hereby generated electron beam features a $1.5 \mathrm{kA}$ peak current, $1.5 \mathrm{\mu m}$ transverse normalized emittance, an uncorrelated energy spread of 0.3% on a GeV-energy scale, and few femtosecond bunch length.

We present particle-in-cell simulations for future laser-plasma wakefield experiments with external bunch injection at the REGAE accelerator facility at DESY, Hamburg, Germany. Two effects have been studied in detail: emittance evolution of electron bunches externally injected into a wake, and longitudinal bunch compression inside the wakefield. Results show significant transverse emittance growth during the injection process, if the electron bunch is not matched to its intrinsic betatron motion inside the wakefield. This might introduce the necessity to include beam-matching sections upstream of each plasma-accelerator section with fundamental implications on the design of staged laser wakefield accelerators. When externally injected at the zero-field crossing of the laser-driven wake, the electron bunch may undergo significant compression in longitudinal direction and be accelerated simultaneously due to the gradient in the acting force. The mechanism would allow for production of single high-energy, ultra-short (on the order of one femtosecond) bunches at REGAE. The optimal conditions for maximal bunch compression are discussed in the presented studies.

We present a study on the emittance evolution of electron bunches, externally injected into laser-driven plasma waves using the three-dimensional particle-in-cell (PIC) code OSIRIS. Results show order-of-magnitude transverse emittance growth during the injection process, if the electron bunch is not matched to its intrinsic betatron motion inside the wakefield. This behavior is supported by analytic theory reproducing the simulation data to a percent level. The length over which the full emittance growth develops is found to be less than or comparable to the typical dimension of a single plasma module in current multistage designs. In addition, the analytic theory enables the quantitative prediction of emittance degradation in two consecutive accelerators coupled by free-drift sections, excluding this as a scheme for effective emittance-growth suppression, and thus suggests the necessity of beam-matching sections between acceleration stages with fundamental implications on the overall design of staged laser-wakefield accelerators.

We introduce the Highly efficient Plasma Accelerator Emulation (HiPACE) code. It is a relativistic, electromagnetic, three-dimensional and fully parallelized particle-in-cell (PIC) code and uses the quasi-static approximation to efficiently simulate a variety of beam-driven plasma-wakefield acceleration scenarios. HiPACE exploits the disparity of time scales in the interaction of highly relativistic particle beams with plasma to decouple beam and plasma evolution. This enables time steps which are many times greater than those used in full PIC codes. Comparisons to the fully explicit PIC code OSIRIS show the capability of the quasi-static PIC code to consistently simulate problems in beam-driven plasma acceleration while reducing the required number of core hours by orders of magnitude. This work outlines the physical basis, describes the numerical implementation and assesses the parallel performance of the code which in combination lead to high computational efficiency.

Physical review letters 121(6), 064801 (2018). doi:10.1103/PhysRevLett.121.064801, Particle-beam-driven plasma wakefield acceleration (PWFA) enables various novel high-gradient techniques for powering future compact light-source and high-energy physics applications. Here, a driving particle bunch excites a wakefield response in a plasma medium, which may rapidly accelerate a trailing witness beam. In this Letter, we present the measurement of ratios of acceleration of the witness bunch to deceleration of the driver bunch, the so-called transformer ratio, significantly exceeding the fundamental theoretical and thus far experimental limit of 2 in a PWFA. An electron bunch with ramped current profile was utilized to accelerate a witness bunch with a transformer ratio of $4.6^{+2.2}_{−0.7}$ in a plasma with length $∼10 cm$, also demonstrating stable transport of driver bunches with lengths on the order of the plasma wavelength., Published by APS, College Park, Md.