Acceleration of electrons in the plasma wakefield of a proton bunch

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 or to reduce the size of the accelerator, new acceleration schemes need to be developed. Plasma wakefield acceleration1–5, 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 pulse6–9 or electron bunch10,11 traversing a plasma drives electric fields of tens of gigavolts per metre and above. These values are well beyond those achieved in conventional radio-frequency accelerators, which are limited to about 0.1 gigavolt 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 energies5,12. 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 stage13. The long proton bunches currently available can be used, as they undergo a process called self-modulation14–16, 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 CERN17–19 uses intense bunches of protons, each of energy 400 gigaelectronvolts (GeV), with a total 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. Here we present 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 stage20 means that the results shown here are a significant step towards the development of future high-energy particle accelerators21,22.