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.

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.

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.

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.

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.

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.

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.

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.

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.