Acceleration of electrons in the plasma wakefield of a proton bunch (1808.09759v2)

Abstract: 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, 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 or electron bunch traversing a plasma, drives electric fields of 10s GV/m and above. These values are well beyond those achieved in conventional RF accelerators which are limited to ~0.1 GV/m. 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. 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. The long proton bunches currently available can be used, as they undergo self-modulation, 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 AWAKE experiment at CERN uses intense bunches of protons, each of energy 400 GeV, with a total bunch energy of 19 kJ, to drive a wakefield in a 10 m 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 AWAKE. 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 means that the results shown here are a significant step towards the development of future high energy particle accelerators.

• The paper demonstrates that electrons can be accelerated to 2 GeV using resonant wakefields driven by a 400 GeV proton bunch.
• The study employs a 10-meter rubidium plasma channel with controlled density gradients and precise injection angles to optimize wakefield resonance.
• Results validate proton-driven wakefield acceleration as a scalable, single-stage method for developing compact high-energy accelerators.

Analysis of Electron Acceleration in Plasma Wakefields Generated by Proton Bunches

This investigation explores a promising frontier in particle accelerator technology: proton-driven plasma wakefield acceleration. The paper engages with an innovative application of plasma physics, utilizing proton bunches to generate wakefields capable of accelerating electrons to GeV energy levels. The Advanced Wakefield (AWAKE) experiment at CERN provides an empirical foundation for this research, culminating in the demonstration of electron energies reaching up to 2 GeV.

Objective and Motivation

The central objective of the paper lies in the advancement of high-energy particle physics through the development of novel acceleration techniques. Plasma wakefield acceleration (PWFA), in particular, offers the potential to achieve electric fields vastly exceeding those possible in conventional RF accelerators, thereby propelling particles to higher energies over shorter distances. While scalable methods involving laser or electron drivers have achieved significant experimental success, their limited energy storage necessitates multi-stage approaches to reach higher energies. Conversely, proton bunches present an attractive alternative due to their substantial energy capacity, allowing for single-stage acceleration to potentially reach TeV scales.

Methodology

The AWAKE experiment facilitated this research by configuring a 10-meter-long plasma channel in rubidium vapor, which interacts with a 400 GeV proton bunch from the CERN SPS accelerator. The physical interaction is further enabled by a laser pulse ionizing the rubidium to form a plasma, triggering the self-modulation of the proton bunch into microbunches. These microbunches engage in resonance to drive substantial wakefields.

Injected electron bunches, pre-accelerated to 18.84 MeV, repeatedly cross the proton path and are captured by the plasma wakefield. A key focus was placed on optimizing injection angles and timing in order to maximize electron capture and energy gain through these resonant wakefields, as confirmed through simulations. The paper also delved into the impact of plasma density gradients on accelerating effectiveness, showcasing the adaptability of the plasma configuration.

Results

Measured outcomes include the successful acceleration of electrons to energies as high as 2 GeV within a 10-meter plasma column. The experiments evidenced sustained reliability and stability of the electron acceleration process, with consistent energy gains observed across consecutive electron bunch injections. Notably, the presence of a controlled plasma density gradient significantly optimized the peak electron energies achieved.

Conclusion and Future Directions

This investigation substantiates the viability of using proton-driven wakefields for GeV-level electron acceleration. It contributes to the foundational understanding required for the development of compact, cost-effective high-energy accelerators capable of investigating fundamental physics at unprecedented scales. The pronounced increase in energy achievable within a singular accelerating stage presents a substantial leap toward realizing future collider designs. Future research aims to further refine the parameters influencing electron capture efficiency and energy gain, potentially expanding the limits of this acceleration mechanism.

The experimental work outlined in this paper underscores a pivotal step toward more economically feasible high-energy accelerators, with ongoing efforts likely to extend these baseline achievements to broader applications across various domains of fundamental research.