Directions in particle beam-driven plasma wakefield acceleration

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.

Dr Edda Gschwendtner, CERN, Switzerland

Dr Edda Gschwendtner, CERN, Switzerland

Edda Gschwendtner obtained her PhD in physics from the Vienna University of Technology, Austria working at CERN on benchmarking the particle background in the LHC experiments. Since 2005 she has been a senior member of the CERN scientific staff. During the last 20 years she was involved in several accelerator and particle physics projects, such as the SPS and LHC accelerators and the ATLAS experiment, she was responsible for the CERN Neutrino Gran Sasso beam-line and is a member of several international scientific advisory committees. 

Since 2012 she has led the AWAKE Project at CERN and she is the AWAKE Technical Coordinator, in charge of the design, integration, commissioning and operation of the experiment. Edda’s area of research involves also Physics Beyond Colliders studies on HEP particle physics applications based on the AWAKE technology. 

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.

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

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

In 2005, Dr Mete Apsimon worked at CERN as a graduate student to work on tests and conditioning of the high frequency, high gradient normal conducting accelerating structures as an underpinning technology for the CLIC Project. She obtained her PhD from École polytechnique fédérale de Lausanne in 2011, she was working at CERN on experimental characterisation and optimisation of the PHIN photoinjector as the CLIC drive beam injector. Soon after, she was awarded a postdoctoral research fellowship at CERN to devise a local orbit bump assisted, graphite based beam halo cleaning system for SPS in order to ultimately improve the integrated luminosity of the LHC. In 2013, she joint the accelerator physics group in the University of Manchester (UoM) as a member of the Cockcroft Institute of Accelerator Science and Technology (CI) to work on plasma based advanced accelerator concepts. In 2016, she has been short-listed and interviewed by the Rutherford Fellowship committee. Currently, she works on the AWAKE Project while developing a capillary-based discharge plasma source after securing the STFC Early Stage Impact Acceleration funding.

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.

Dr Richard D'Arcy, DESY, Germany

Dr Richard D'Arcy, DESY, Germany

Richard D’Arcy has held the position of Scientific Coordinator of the beam-driven plasma wakefield accelerator (PWFA) project FLASHForward at DESY, Germany since early 2017. From 2015 he led investigations into the implementation of an X-band TDS device at FLASHForward for fs-level PWFA bunch characterisation as a DESY Fellow. Prior to joining the field of plasma acceleration Richard completed postdoctoral stays at Fermilab, US and Rutherford Appleton Laboratory, UK working on high intensity proton sources for the next generation of high-energy target and collider experiments. In 2012 he received his PhD from University College London, UK in the field of novel acceleration techniques through contributions to the Fixed-Field Alternating Gradient proof-of-principle accelerator experiments EMMA at Daresbury Laboratory, UK and COMET at KEK, Japan.

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.

Dr Spencer J Gessner, CERN, Switzerland

Dr Spencer J Gessner, CERN, Switzerland

Spencer Gessner received his undergraduate degree in physics from the University of California Santa Barbara in 2009. In 2010 he was admitted to Stanford University, where he received his PhD in physics in 2016. At SLAC, he started his research with the ATLAS group, but in 2011 switched to research at FACET with a focus on theoretical and experimental aspects of positron acceleration in a plasma wake-field accelerator (PWFA). In the course of his work, Spencer covered a very broad range of topics: from developing the energy spread diagnostic at FACET, to theoretical and experimental aspects of creating hollow plasma channels for acceleration of positrons. Spencer developed a highly innovative approach to this challenging problem and found a number of original solutions. The experimental demonstration of positron acceleration in hollow-channel PWFA was an impressive validation of his approach. His research was supervised by Professor Tor Raubenheimer. After receiving his PhD in 2016, Dr Gessner became a fellow at CERN, where he is studying proton beam-driven plasma wakefield acceleration as a member of the AWAKE Collaboration.

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.

Professor Sébastien Corde, Ecole Polytechnique, France

Professor Sébastien Corde, Ecole Polytechnique, France

Sébastien Corde started his career in the Laboratoire d'Optique Appliquée at Ecole Polytechnique to work on the development of laser-plasma accelerators and their applications to femtosecond light sources. Within a few years he became an expert in his field, publishing an in-depth review article in Reviews of Modern Physics covering the research area of femtosecond x-ray sources from laser-plasma accelerators. Through a series of groundbreaking experiments, he pushed forward the understanding of the interaction underlying laser-plasma accelerators and demonstrated an innovative all-optical Compton gamma-ray source. Sébastien received four awards for his PhD, including the Outstanding Doctoral Thesis Research in Beam Physics Award from the American Physical Society. He continued his career in the plasma wakefield acceleration group at SLAC. The experiments that he conducted at FACET have led to fast experimental progress in the field of plasma wakefield acceleration, with many important milestones, in particular the demonstration of ultrahigh-field, low-energy spread acceleration of positrons in a plasma. Since then, Sébastien has been appointed as an Assistant Professor at Ecole Polytechnique, a top French engineering school. His research is at the interface between beam-driven and laser-driven plasma accelerators, focusing in particular on the topics of hybrid wakefield acceleration and plasma-based positron acceleration.

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.

Professor Matthew Wing, University College London, UK

Professor Matthew Wing, University College London, UK

Matthew Wing is a particle physicist, having written his PhD on the production of heavy quarks in electron-proton scattering at the HERA collider in Hamburg. He continues to work in this area and is the current Spokesperson of the international ZEUS collaboration. He has also worked on experiments searching for lepton flavour violation and the proposed electron-positron linear collider, both the accelerator design and also detector, and in particular data acquistion, R&D. His main current area of research is proton-driven plasma wakefield acceleration and its application to particle physics, encompassed in the AWAKE experiment at CERN, for which he is the Deputy Spokesperson. Wing has had positions at Bristol University, McGill University and UCL, before becoming a Professor of Physics at UCL in 2010; he is currently a guest Professor at the DESY laboratory in Hamburg.

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.

Associate Professor Erik Adli, University of Oslo, Norway

Associate Professor Erik Adli, University of Oslo, Norway

Erik Adli is an Associate Professor at the Department of Physics, University of Oslo, Norway. He started his career in High-Energy physics in 2004, by contribution to the construction of the ATLAS detector at CERN. He obtained his PhD with the University of Oslo and CERN in 2009, after having performed research on electron-positron collider drive beam energy extraction techniques. In 2011 he moved to California for postdoctoral studies at the Stanford Linear Accelerator Center (SLAC), where he, in collaboration with colleagues from UCLA and Stanford University, built up and performed plasma wakefield acceleration experiments at the FACET test-facility. The collaboration demonstrated the first two-beam plasma wakefield acceleration. Professor Adli is currently leading is leading the Norwegian accelerator activities toward the AWAKE and CLIC projects at CERN, towards FACET at SLAC and toward ESS.

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.

Professor Navid Vafaei-Najafabadi, Stony Brook University, USA

Professor Navid Vafaei-Najafabadi, Stony Brook University, USA

Navid Vafaei-Najafabadi received his PhD degree from UCLA in 2016, after which he joined the Physics and Astronomy Department at Stony Brook University as a junior faculty. Between 2012-2016, he was a primary participant in multiple plasma wakefield accelerator (PWFA) experiments at the FACET facility of SLAC National Accelerator Laboratory. His research areas of interest consist of studying the methods for generating bright electron beams from beam-driven PWFAs as well as the interaction of mid-IR laser pulses with plasma. In particular, Dr Vafaei-Najafabadi is currently dedicated to evaluating the potential use of electron-beam-induced ionization injection technique in beam-driven PWFAs to produce high brightness electron beams suitable for applications such as Free Electron Lasers, and in the long run, linear colliders.

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.  

Dr Massimo Ferrario, INFN Frascati, Italy

Dr Massimo Ferrario, INFN Frascati, Italy

Massimo Ferrario has been working at INFN-LNF since 1991 and he is currently coordinator of the SPARC_LAB facility. In the last 25 years he has been working in the field of high brightness photoinjectors, free electron lasers and advanced accelerator concepts including plasma accelerators. 

He has been chair of the European Advanced Accelerator Workshop held in Italy with more than 300 international attendants. He is author of more than 300 publications and has got several invited talk at international conferences and workshops. He is a member of the CERN Accelerator School (CAS) and the United States Particle Accelerator School (USPAS) where he has given several lectures about the Physics of High Brightness Beams. He is also teaching Accelerator Physics at the University of Roma “La Sapienza”.