Acceleration of electrons in the plasma wakefield of a proton bunch

Abstract

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.

Experimental Observation of Proton Bunch Modulation in a Plasma at Varying Plasma Densities

“…Observing the seeded self-modulation is key for the acceleration of externally injected electrons in the wakefields driven by long proton bunches [31]. The results presented here with a relativistic ionization front (as opposed to a sharp rising charge distribution as in Refs.…”

Electron beam characterization with beam loss monitors

“…AWAKE [1], the Advanced WAKEfield experiment at CERN, recently demonstrated acceleration of externally injected electrons in plasma wakefields resonantly excited by a self-modulated [2] [3] relativistic proton bunch [4].…”

Accelerating field enhancement due to ion motion in plasma wakefield accelerators

“…The effect of wakefield enhancement was first noticed in numerical simulations of the AWAKE experiment. In AWAKE [18][19][20], a long proton bunch undergoes seeded self-modulation in the plasma [21][22][23] and transforms into a train of short micro-binches that resonantly drive the plasma wave [24,25]. The number of micro-bunches depends on the plasma density and is typically about one hundred, so the plasma wave exists sufficiently long to move the ions.…”