High energy particle accelerators have been crucial in providing a deeper understanding of
fundamental particles and the forces that govern their interactions. In order to increase the energy
or reduce the size of the accelerator, new acceleration schemes need to be developed. Plasma
wakefield acceleration [1–5], in which the electrons in a plasma are excited, leading to strong
electric fields, is one such promising novel acceleration technique. Pioneering experiments have
shown that an intense laser pulse [6–9] or electron bunch [10, 11] traversing a plasma, drives electric fields of tens of giga-volts per metre and above. These values are well beyond those achieved in conventional radio frequency (RF) accelerators which are limited to about 0.1 giga-volt per metre. A limitation of laser pulses and electron bunches is their low stored energy, which motivates the use of multiple stages to reach very high energies [5, 12]. The use of proton bunches is compelling, as they have the potential to drive wakefields and accelerate electrons to high energy in a single accelerating stage [13].
The long proton bunches currently available can be used, as they undergo a process called self-modulation [14–16], a particle–plasma interaction which longitudinally splits the bunch into a series of high density microbunches, which then act resonantly to create large wakefields. The Advanced Wakefield (AWAKE) experiment at CERN [17–19] uses intense bunches of protons, each of energy 400 giga-electronvolts (GeV), with a total 2 bunch energy of 19 kilojoules, to drive a wakefield in a 10 metre long plasma. Bunches of electrons are injected into the wakefield formed by the proton microbunches. This paper presents measurements of electrons accelerated up to 2 GeV at the AWAKE experiment. This constitutes the first demonstration of proton-driven plasma wakefield acceleration. The potential for this scheme to produce very high energy electron bunches in a single accelerating stage [20] means that the results shown here are a significant step towards the development of future high energy particle accelerators [21, 22].