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Directions in particle beam-driven plasma wakefield acceleration

Scientific meeting


Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ


Theo Murphy international scientific meeting organised by Professor Bernhard Hidding, Dr Mark Hogan, Professor Patric Muggli, Professor James Rosenzweig and Professor Brian Foster OBE FRS.

Image shows simulated non-linear plasma wakes accelerating a positron bunch (top-left) and an electron bunch (bottom-right). The accelerated bunches head toward each other in a head-on collision evocative of the interaction point in a linear collider. Credit: Frank Tsung, UCLA

Electric fields in plasmas waves, driven by relativistic particle beams, can accelerate particles to tens of Giga-electron-volt energies on metre-scale distances. Recent conceptual and experimental breakthroughs have ushered in a worldwide effort which may be transformative for accelerators and their applications for natural, life and material sciences. This meeting discussed emerging trends and prospects of this new kind of compact accelerators.

Speaker abstracts and biographies can be found below. Recorded audio of the presentations can also be found below. Meeting papers have been published a volume of Philosophical Transactions A.

Enquiries: contact the Scientific Programmes team

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Schedule of talks

04 June

Session 1 09:00-12:50

Linac-driven electron PWFA

5 talks Show detail Hide detail


Dr Mark J Hogan, SLAC National Accelerator Laboratory, USA

09:05-09:35 Plasma wakefield acceleration experiments on FACET II and the DOE's Strategic Plan for Advanced Accelerator R&D

Dr Mark J Hogan, SLAC National Accelerator Laboratory, USA


A dense, ultra-relativistic electron bunch propagating through a uniform plasma can produce a highly nonlinear wake that can be employed for accelerating a second, trailing bunch in a scheme known as the Plasma Wakefield Accelerator (PWFA). Recent work has shown that a PWFA cavity can accelerate a low energy spread electron bunch containing a significant charge at both high gradients and high energy-extraction efficiency - necessary conditions for making future particle accelerators both compact and less expensive. The next important challenge that must be addressed is the high efficiency acceleration of a low emittance and narrow energy spread electron beam while pump depleting the drive beam. Professor Joshi will explain how such an ambitious experiment is well aligned with the U.S. Department of Energy-High Energy Physics Division’s long-range strategic plan for Advanced Accelerator Research and Development. A new facility, FACET II is being constructed at SLAC that will enable such an experiment. Professor Joshi will describe the plans and the progress for the upcoming experimental campaign on FACET II.

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09:35-09:50 Discussion

09:50-10:15 Beam quality preservation challenges and strategies

Professor Michael Litos, University of Colorado Boulder, USA


Plasma-based electron accelerators have now demonstrated the ability to reliably provide a large energy gain with a small final energy spread. Beam emittance preservation, however, remains an outstanding challenge for the field. If the beam size is appropriately matched to the plasma, the beam will not suffer chromatic emittance growth as it traverses the plasma source. Unfortunately, the typically small matched beam sizes present a serious problem for external beam injection and beam extraction. This makes it difficult for plasma accelerators to interface with conventional accelerator components, including electromagnetic beam transport systems and magnetic undulators. In this talk, a solution for beam matching into and out of a plasma accelerator is presented, based on the use of tailored plasma density ramps at the entrance and exit of the plasma source. A simple model for beam transport through the plasma will be discussed, and it will be shown that a plasma ramp of a practical length can perfectly match a high-energy electron beam into or out of a plasma source with experimentally realistic beam and plasma parameter tolerances. Further, empirical generalized scaling laws will be presented that give a prescription for the ideal plasma density profile as a function of the desired beam parameters entering or leaving the plasma source.

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10:15-10:30 Discussion

10:30-10:50 Coffee

10:50-11:15 Hybrid acceleration towards ultrahigh 6D brightness

Dr Grace G Manahan, SCAPA and University of Strathclyde, UK


Novel acceleration mechanism by employing plasma-based structures are increasingly seen as routes for pushing the energy frontier for high-energy physics and as sources for coherent radiations. Today, electron acceleration to GeV energies within few cm is routinely demonstrated. One novel scheme, the so-called 'Trojan Horse' plasma photocathode, allows decoupling of bunch generation and acceleration. This technique is a unique way towards designer bunches with normalised transverse emittance at the few nm-rad level. At kA-level currents, this yields electron 5D-brightness values – many orders of magnitude better than state-of-the-art both in plasma and conventional accelerator technology. Nevertheless, a remaining limitation of plasma-based accelerators is the inherently large correlated energy spread that is an unfortunate by-product of the enormous accelerating field gradients. This is deleterious on many levels, starting with extraction from the plasma stage, and is known to be a showstopper for key applications such as the FEL. In this talk, Dr Manahan will present a technique of reducing the electron beam’s energy spread by an order of magnitude without compromising the normalised emittance. Here, the energy compensation occurs in a single stage at a constant plasma density profile – the same stage where the witness bunch generation and acceleration take place. The talk will also presents proposed designs of plasma-based ultrahigh 6D-brightness electron systems, driven either by conventional linacs or by laser-plasma-accelerators.

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11:15-11:30 Discussion

11:30-11:55 On the phase space dynamics of high brightness injection and staging in plasma wakefield acceleration

Dr Weiming An, UCLA, USA


In this talk, the current physics understanding of phase space dynamics for different injection methods and matching between stages in Plasma Wakefield Acceleration (PWFA) will be reviewed, with a focus on how to generate and maintain high quality electron beams of ultra-high brightness and low energy spread. Two major injection methods (plasma density modulation and ionization based injection) will be discussed in detail to show their potentials and limitations. Furthermore, phase space manipulation methods on staging and energy chirp reduction to 0.1% level will also be discussed.

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11:55-12:10 Discussion

12:10-12:35 Experimental demonstration of electron bunch generation from a plasma photocathode

Dr Aihua Deng, UCLA, USA


Dr Deng will talk about the experiments performed at the Facility for Advanced Accelerator Experimental Tests (FACET) at the SLAC National Accelerator Laboratory, which is termed 'E210: Trojan Horse Plasma Wakefield Acceleration'. A new technique will be introduced, called the 'plasma photocathode'. It generates high quality electron beams directly within the large fields of a particle beam driven plasma wave by means of an ionizing laser pulse. The talk will demostrate controlled electron bunch generation in a plasma wakefield accelerator with an ionizing laser pulse to liberate electrons from helium gas in a preformed hydrogen plasma. Two injection modes will be introduced in the experiments: all-optical density down ramp injection and the real plasma photocathode injection. Optically-triggered density ramp injection is shown to provide a stepping stone for experimental parameter adjustments that yield beam generation from a plasma photocathode. The 'plasma photocathode' technique opens a path to beam phase space characteristics exceeding those of conventional photocathodes, and thus to combine the ultra-high energy gain of plasma accelerators with ultra-high beam quality. It will open the door to production of 'designer electron beams' with unprecedentedly low emittance, short duration and high brightness based on precision control of injection in extremely high field waves.

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12:35-12:50 Discussion

12:50-13:40 Lunch

Session 2 13:40-17:00

Hybrid LWFA-driven PWFA

4 talks Show detail Hide detail


Professor James B Rosenzweig, UCLA Department of Physics and Astronomy, USA

13:40-14:10 Experiments towards hybrid acceleration: controlled dual-beam injection and beam-driven wake diagnostics

Professor Stefan Karsch, Ludwig-Maximilians-Universität München and Max-Planck-Institut für Quantenoptik, Germany


Several laser-wakefield acceleration (LWFA) experiments by our group and others have shown results that can only be understood if one takes into account plasma wakefields driven by the LWFA-generated electron beam. This immediately raises the question about the parameters of this wake and whether they would be suitable for accelerating particles.

Professor Karsch presents shadowgraphy images of laser- and LWFA-electron-driven plasma waves, discusses their morphological differences and driver-type influence of ion motion. The electron driven waves were observed for electron beams with an energy of approx 300 MeV and a total bunch charge in excess of 250 pC. While a deceleration of the driving electrons in these waves is immediately evident, the experimental proof of successful acceleration of the electron bunch tail or a second bunch is still missing, mainly due to the difficulties to discriminate against competing processes. However, Professor Karsch presents prerequisites towards such an experimental proof, by demonstrating the generation of independently tunable dual electron beams from an LWFA to act as driver and witness. He also discusses scaling laws for the injected charge in an LWFA, which is crucial for driving plasma waves.

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14:10-14:25 Discussion

14:25-14:55 Hybrid LWFA|PWFA staging

Dr Alberto de la Ossa, DESY, Germany


Dr de la Ossa presents a conceptual design for a hybrid laser-to-beam-driven plasma wakefield accelerator (LPWFA). In this setup, the electron beams from a laser-driven plasma wakefield accelerator (LWFA) are used as input beams of a new beam-driven plasma accelerator (PWFA). This scenario explicitly makes use of the advantages unique to each method, particularly exploiting the capabilities of PWFA schemes to provide energy-boosted high-brightness beams, while the LWFA stage inherently fulfils the demand for a compact source of high-current (> 10 kA) electron bunches required as PWFA drivers. PWFA with high-current drivers enables the generation of high-quality and high-brightness witness beam in the second stage, which can be efficiently accelerated up to GeV energies with a low energy spread. Effectively, the PWFA stage operates as a beam brightness and energy booster of the initial LWFA output, aiming to match the demanding beam quality requirements of accelerator based light sources. This design consists in two identical plasma stages coupled to each other with a minimal distance in between, and a thin foil acting as a laser blocker for the PWFA stage. In this way, the actual implementation of this hybrid plasma accelerator is relatively simple and particularly compact. The feasibility and the potential of this promising concept is shown through exemplary particle-in-cell simulations and discussed theoretically. In addition, Dr de la Ossa presents and describes preliminary results from a proof-of-concept experiment in HZDR (Germany).

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14:55-15:10 Discussion

15:10-15:30 Tea

15:30-16:00 Achieving Controlled LWFA beams for studies of high field QED

Professor Matt Zepf, FSU Jena, Germany


The combination of GeV e-ebeams and relativistically intense laser is ideal for studying QED effects in the high field regime. First experiments have already been conducted showing radiation reaction effects and the possibility of gamma/laser collisions was proposed as an Experiment for the Gemini laser as early as 2011. Experiments have shown the stable operation of the LWFA accelerator is essential for the success of such experiments. Here we discuss recent results on injection, pointing stability and collimation/focusing using plasma lenses. All of these are essential stepping stones for precision experiments. Our data shows that pointing stability and self-injection  threshold depend directly on the density uniformity of the gas target and that passive lenses provide strong focusing in a simple experimental arrangement.

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16:00-16:15 Discussion

16:15-16:45 Dr Arie Irman, HZDR, Germany

16:45-17:00 Discussion


Poster session

05 June

Session 3 08:45-12:45

Positron and proton PWFA

5 talks Show detail Hide detail


Professor Patric Muggli, Max-Planck-Institut für Physik, Germany

08:45-09:15 The AWAKE Experiment at CERN

Dr Edda Gschwendtner, CERN, Switzerland


AWAKE, the Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN, pursues the demonstration of electron acceleration in plasma. The 400 GeV/c SPS proton beam, with a rms bunch length of 6 – 15 cm drives strong wakefields in a 10 m long rubidium plasma with an electron density of 10^14 – 10^15 cm3. In a first measurement campaign the AWAKE experiment has proven that the proton beam self modulates into micro-bunches, which resonantly drive the wakefields. In 2018 the experiment aims to show the acceleration of electrons in the plasma to GeV energies. An overview of the AWAKE experiment and its physics as well as the most recent result will be presented. The future experimental program, its challenges and of the technology to HEP, will be discussed.

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09:15-09:30 Discussion

09:30-10:00 AWAKE electron injector and diagnostics

Dr Oznur Mete Apsimon, University of Manchester and the Cockcroft Institute, UK


AWAKE is a unique experiment investigating plasma acceleration driven by 400 GeV protons from CERN’s Super Proton Synchrotron (SPS). Experiment is based on a 10m long plasma section located at the end of an SPS transfer line, which had previously served CERN Neutrinos to Gran Sasso (CNGS) experiment. The first goal of the experiment is observation of a phenomenon called the self–modulation instability which is a transverse instability leading to the modulation of 12cm long proton beam at about plasma frequency. This effect is important as the modulated bunch drives the plasma resonantly resulting into larger plasma wakefields. The second goal of this experiment is to demonstrate acceleration of a probe beam in plasma wakefields driven by modulated proton beam. A probe beam is generated externally by an electron source, which consists of an RF gun and a booster linac. 200pC electron beam reaches to 5MeV at the end of the gun and is boosted up to 16-20MeV by the linac. Electrons generated by this source will be transferred via a transfer line into the experimental area and injected into the plasma cell, synchronised with protons. Shape of the transfer line is a deciding factor on the normalised emittance budget of electrons limiting it to 2 mm mrad. Injector is equipped with pepper pot and quadrupole scan diagnostics, allowing emittance monitoring during beam set-up before the injection. In this talk, after a brief introduction, Dr Mete Apsimon will present the layout of the external electron source and performance of emittance diagnostics.

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10:00-10:15 Discussion

10:15-10:45 FLASHForward – plasma wakefield accelerator science for high average power applications

Dr Richard D'Arcy, DESY, Germany


The field of particle acceleration in plasma waves has seen remarkable progress in the last two decades. These days, acceleration gradients of more than 10 GV/m can be readily achieved using either ultra-short intense laser pulses or high-current density particle beams as plasma wakefield drivers. With the demonstration of first GeV electron beams and a trend towards improved reproducibility, beam quality and control over the involved plasma processes, plasma-acceleration techniques are drawing considerable interest in the traditional accelerator community. As a consequence, DESY, Germany's leading accelerator centre, has established a research programme for beam-driven plasma-based novel acceleration techniques with the goal to symbiotically combine conventional and new accelerator concepts for applications. This presentation will give an introduction into these emerging activities, show first theoretical and experimental results and outline the DESY PWFA flagship project, FLASHForward.

FLASHForward is a pioneering beam-driven plasma-wakefield experiment that aims to produce, in a few centimetres of ionized hydrogen, electron beams of energies exceeding 1.5 GeV that are of sufficient quality to demonstrate gain in a free-electron laser. The experimental beamline will allow for milestone studies assessing plasma-internal particle injection regimes, external injection, and controlled beam capturing and release for subsequent applications. The facility provides a unique combination of low-emittance GeV-class electrons from the superconducting MHz repetition rate, high-average power accelerator FLASH synchronized to a 25 TW laser interacting in a windowless, optically accessible, versatile plasma target. Experiments will commence in 2018 and are foreseen to run for the next decade, opening up new avenues in this highly dynamic research field.

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10:45-11:00 Discussion

11:00-11:15 Coffee

11:15-11:45 Positron acceleration in plasma: challenges and progress

Dr Spencer J Gessner, CERN, Switzerland


Plasma Wakefield Acceleration (PWFA) is an exciting technology for accelerating particle beams to high energies in short distances. The ultimate application of this technology is an energy-frontier linear collider. A plasma-based linear collider has the potential to be more compact and efficient than RF-based colliders, while being less expensive. Of all the challenges on the path to a plasma-based linear collider, the acceleration of positrons in plasma is by far the most daunting. Plasmas are unique in that they are asymmetric accelerators, with light, mobile plasma electrons responding to the driving beam or laser fields, and the heavy ions remaining at rest. Many features of the plasma wakefield that are attractive for accelerating electrons are absent for positrons. In this talk, Dr Gessner will explain the physics of positron acceleration in plasma and reprise the state of research on the topic. The focus will be on the three main techniques tested so far: the quasilinear regime, nonlinear regime, and hollow channel regime. The next steps in this research will be described by S Corde in a following talk.

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11:45-12:00 Discussion

12:00-12:30 Positron acceleration in plasma: solutions, future experiments, and the path to a linear collider

Professor Sébastien Corde, Ecole Polytechnique, France


The concept of plasma-based particle accelerators is a promising path to overcome the limitations of conventional accelerator techniques. They hold out the promise of more compact, more affordable and higher-energy particle accelerators, and are increasingly considered as a mean to push the energy frontier of particle physics even higher. But for application to high-energy physics and to a linear collider, it is however imperative for plasma-based particle accelerators to also be capable of accelerating positrons, while most of the research effort so far has been on the acceleration of electrons. This is crucially important as positron acceleration in plasma is a much more challenging task than electron acceleration.

In this talk, Professor Corde will give a comprehensive overview of the different challenges of positron acceleration in plasma, highlighting the different routes under study, their problems and potential solutions, and the way forward to address them. Indeed, different variants of plasma-based acceleration are considered for positron acceleration in plasmas, the quasi-linear regime, the nonlinear regime and the hollow plasma channel, and they all have different advantages and flaws. Future experiments on positron acceleration and the path to a linear collider will also be discussed.

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12:30-12:45 Discussion

12:45-13:20 Lunch

Session 4 13:20-17:00

PWFA applications

5 talks Show detail Hide detail


Professor Brian Foster OBE FRS, University of Hamburg/DESY, Germany and University of Oxford, UK

13:20-13:50 Particle physics experiments based on the AWAKE acceleration scheme

Professor Matthew Wing, University College London, UK


New particle acceleration schemes open up exciting opportunities, potentially providing more compact or higher energy accelerators. The AWAKE experiment at CERN is currently taking data to establish the method of proton-driven plasma wakefield acceleration. A second phase of the experiment is being prepared to demonstrate that bunches of about 10^9 electrons can be accelerated in about 10 m of plasma and that the emittance of the electron bunch is preserved and the energy gain is scalable with length. With this, an electron beam of O(50 GeV) could be achieved at a much higher rate than currently available anywhere in the world. New and improved fixed-target or beam-dump experiments are possible, such as an upgrade to the NA64 experiment which is searching for hidden sector physics such as dark photons using the secondary SPS electron beam. With the expectation of being able to increase the rate of electrons on target by a factor of 1000 using an AWAKE-like beam, sensitivity to new physics is greatly extended. An electron beam of O(50 GeV) could open up the possibility of an electron-proton collider at the TeV scale, with moderate luminosities, at relatively low cost and focusing on studies of the strong force. An ultimate goal is to produce an electron beam of 3 TeV and collide with an LHC proton beam. This very high energy electron-proton collider would probe a new regime in which the structure of matter is completely unknown. Again, this would be relatively low luminosity, but this is offset by the rapidly rising cross sections in this kinematic region. These ideas are reviewed here.

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13:50-14:05 Discussion

14:05-14:35 Linear colliders: opportunities and challenges

Associate Professor Erik Adli, University of Oslo, Norway


A linear electron-positron collider operating at TeV-scale energies will provide high precision measurements which will allow, for example, precision studies of the Higgs boson as well as searches for physics beyond the standard model. A future linear collider should produce collisions at high energy, with high luminosity and with a good wallplug to beam power transfer efficiency. The luminosity per power consumed is a key metric that can be used to compare linear collider concepts. The plasma wakefield accelerator has demonstrated high-gradient, high-efficiency acceleration of an electron beam, and is therefore a promising technology for a future linear collider. Professor Adli will go through the opportunities of using plasma wakefield acceleration technology for a collider, as well as a few of the collider-specific challenges that must be addressed in order for a high-energy, high luminosity-per-power plasma wakefield collider to become a reality.

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14:35-14:50 Discussion

14:50-15:10 Tea

15:10-15:40 Ionization injection beam applications

Professor Navid Vafaei-Najafabadi, Stony Brook University, USA


Ionization of electrons within an ultra-relativistic plasma wake can lead to the formation of electron beams with desirable qualities. Understanding and optimizing the underlying factors that affect the quality of the beams generated via ionization injection is an active area of research. In this talk, Professor Vafaei-Najafabadi will discuss the properties of the electron beams formed in ionization injection, specifically in the case where ionization is initiated by the field of the drive electron beam as it undergoes betatron oscillations. The physics of the interaction will be explored through OSIRIS simulations, showing the possibility of generating an electron beam of 100s of pC with low emittance and percent-level energy spread by confining impurity region to a single betatron cycle. Moreover, by extending the impurity region, a bunch train of identical electron beams can be generated, which will enable pump/probe experiments. Experimental evidence supporting the physics discussed will be presented from the data obtained at the FACET facility of the SLAC National Laboratory, where a 14 kA, 20 GeV electron beam was used to drive a plasma wakefield in the blowout regime. Electron beams generated via ionization injection at these experiments gained up to 30 GeV of energy in a 130 cm lithium plasma while maintaining an emittance of <5 mm-mrad. The future directions and challenges of applying this technique towards generating high brightness beams will also be discussed.

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15:40-15:55 Discussion

15:55-16:25 Challenges and opportunities of PWFA-driven FEL and recent results at SPARC_Lab

Dr Massimo Ferrario, INFN Frascati, Italy


It is widely accepted by the international accelerator scientific community that a fundamental milestone towards the realization of a plasma driven future Linear Collider will be the integration of a high gradient accelerating plasma modules in a short wavelength Free Electron Laser (FEL) user facility, as proposed in the H2020 Design Study EuPRAXIA. This fundamental goal can be achieved only if the plasma accelerator module will be able to keep the high quality electron beam required to drive a high gain Free Electron Laser: high peak current (~ kA), low normalized emittance (sub-μm) and low relative energy spread (< 10-3).

Dr Ferrario reports in this talk the ongoing efforts on the PWFA-driven FEL design studies in the framework of the EuPRAXIA collaboration, with particular emphasis to the EuPRAXIA@SPARC_LAB project at INFN-Frascati which foresees to use a high gradient X-band RF linac to drive plasma oscillations in the accelerator module. This activity is performed in synergy with the EuPRAXIA and XLS-CompactLight design studies and is supported by an on going experimental activity at the SPARC_LAB test facility.  

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16:25-16:40 Discussion

16:40-17:00 Panel discussion and overview

Directions in particle beam-driven plasma wakefield acceleration

4 - 5 June 2018

Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ
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