The energy frontier of particle physics is several trillion electron volts, but colliders capable of reaching this regime (such as the Large Hadron Collider and the International Linear Collider) are costly and time-consuming to build; it is therefore important to explore new methods of accelerating particles to high energies. Plasma-based accelerators are particularly attractive because they are capable of producing accelerating fields that are orders of magnitude larger than those used in conventional colliders. In these accelerators, a drive beam (either laser or particle) produces a plasma wave (wakefield) that accelerates charged particles. The ultimate utility of plasma accelerators will depend on sustaining ultrahigh accelerating fields over a substantial length to achieve a significant energy gain. Here we show that an energy gain of more than 42 GeV is achieved in a plasma wakefield accelerator of 85 cm length, driven by a 42 GeV electron beam at the Stanford Linear Accelerator Center (SLAC). The results are in excellent agreement with the predictions of three-dimensional particle-in-cell simulations. Most of the beam electrons lose energy to the plasma wave, but some electrons in the back of the same beam pulse are accelerated with a field of approximately 52 GV m(-1). This effectively doubles their energy, producing the energy gain of the 3-km-long SLAC accelerator in less than a metre for a small fraction of the electrons in the injected bunch. This is an important step towards demonstrating the viability of plasma accelerators for high-energy physics applications.

High-energy particle accelerators have been crucial in providing a deeper understanding of fundamental particles and the forces that govern their interactions. To increase the energy of the particles or to reduce the size of the accelerator, new acceleration schemes need to be developed. Plasma wakefield acceleration, in which the electrons in a plasma are excited, leading to strong electric fields (so called 'wakefields'), is one such promising acceleration technique. Experiments have shown that an intense laser pulse or electron bunch traversing a plasma can drive electric fields of tens of gigavolts per metre and above-well beyond those achieved in conventional radio-frequency accelerators (about 0.1 gigavolt per metre). However, the low stored energy of laser pulses and electron bunches means that multiple acceleration stages are needed to reach very high particle energies. The use of proton bunches is compelling because they have the potential to drive wakefields and to accelerate electrons to high energy in a single acceleration stage. Long, thin proton bunches can be used because they undergo a process called self-modulation, a particle-plasma interaction that splits the bunch longitudinally into a series of high-density microbunches, which then act resonantly to create large wakefields. The Advanced Wakefield (AWAKE) experiment at CERN uses high-intensity proton bunches-in which each proton has an energy of 400 gigaelectronvolts, resulting in a total bunch energy of 19 kilojoules-to drive a wakefield in a ten-metre-long plasma. Electron bunches are then injected into this wakefield. Here we present measurements of electrons accelerated up to two gigaelectronvolts at the AWAKE experiment, in a demonstration of proton-driven plasma wakefield acceleration. Measurements were conducted under various plasma conditions and the acceleration was found to be consistent and reliable. The potential for this scheme to produce very high-energy electron bunches in a single accelerating stage means that our results are an important step towards the development of future high-energy particle accelerators.

3module. A 28.5 GeV electron beam with 1:8 10 10 electrons is compressed to 20 m longitudinally and focused to a transverse spot size of 10 m at the entrance of a 10 cm long column of lithium vapor with density 2:8 10 17 atoms=cm 3 . The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.Plasmas have extraordinary potential for advancing the energy frontier in high-energy physics due to the large focusing and accelerating fields that are generated.Beam-plasma interactions have demonstrated focusing gradients of MT=m [1] while laser plasma interactions have demonstrated GeV=cm accelerating gradients [2 -7] over distances of a few mm. Beam-driven plasmawakefield accelerators (PWFA) have recently demonstrated acceleration and focusing of both electrons [8,9] and positrons [10,11] in meter scale plasmas.The experiment described in this Letter uses an ultrarelativistic electron bunch to simultaneously create a plasma in lithium vapor and drive a large amplitude plasma wave. When the electron bunch enters the lithium vapor, the electric field of the leading portion of the bunch ionizes the valence electron of each lithium atom in its vicinity leaving fully ionized neutral plasma for the remainder of the bunch [12,13]. The plasma electrons are then expelled from the beam volume and return one-half plasma period later. The returning plasma electrons form density concentrations on axis behind the bunch leading to a large accelerating field for the particles in the back of the bunch.In linear plasma theory [14] the wakefield amplitude increases as N= 2 z , provided the plasma density is increased such that k p z 2 p where N is the number of electrons in the bunch, z is the bunch length, and k p ! p =c is the inverse of the plasma collisionless skin depth. The nonlinear or blowout regime is reached when the electron bunch density n b N= 2 3=2 z 2 r is greater than the plasma density n p and the beam radius satisfies r c=! p . In the blowout regime, for bunch lengths on the order of the plasma wavelength, the plasma electrons are expelled from the beam volume to a radius r c 2 N= 2 3=2 z n p q leaving behind a pure ion column. This experiment is in a regime in which the electron bunch radius, bunch length, ion channel radius, and plasma wavelength are all on the same order. Although the experiments described here are on the edge of the blowout regime, numerical simulations indicate the N= 2 z increase in plasma-wakefield amplitude can still be realized [15]. Verification of the dramatic increase in accelerating gradient predicted for short bunches is a critical milestone for the application of plasma-wakefield accelerators to future high-energy accelerators and colliders.A single 28.5 GeV bunch of 1:8 10 10 electrons from the Stanford Linear Accelerator Center (SLAC) linac enters the Final Focus Test B...

The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36 GV=m, in good agreement with an analytical model and PIC simulations.

This report describes the conceptual steps in reaching the design of the AWAKE experiment currently under construction at CERN. We start with an introduction to plasma wakefield acceleration and the motivation for using proton drivers. We then describe the self-modulation instability -a key to an early realization of the concept. This is then followed by the historical development of the experimental design, where the critical issues that arose and their solutions are described. We conclude with the design of the experiment as it is being realized at CERN and some words on the future outlook. A summary of the AWAKE design and construction status as presented in this conference is given in [1].