O. Reimann

Proton-driven plasma wakefield acceleration: a path to the future of high-energy particle physics

New acceleration technology is mandatory for the future elucidation of fundamental particles and their interactions. A promising approach is to exploit the properties of plasmas. Past research has focused on creating large-amplitude plasma waves by injecting an intense laser pulse or an electron bunch into the plasma. However, the maximum energy gain of electrons accelerated in a single plasma stage is limited by the energy of the driver. Proton bunches are the most promising drivers of wakefields to accelerate electrons to the TeV energy scale in a single stage. An experimental program at CERN—the AWAKE experiment—has been launched to study in detail the important physical processes and to demonstrate the power of proton-driven plasma wakefield acceleration. Here we review the physical principles and some experimental considerations for a future proton-driven plasma wakefield accelerator.

Self-modulation of ultra-relativistic SLAC electron and positron bunches

Long (L>>λpe) charged particle bunches traveling in dense plasmas are subject to a transverse two-stream instability, the self-modulation instability (SMI)1. This instability modulates the bunch in the radial direction with a longitudinal period approximately equal to the plasma wavelength (λpe~ne1/2). The SMI can be used to transform a long bunch into a train of short bunches that can resonantly drive wakefields to large amplitudes. An experiment known as AWAKE2 was recently approved at CERN to drive GV/m wakefields with 400GeV, ~12cm-long proton bunches in a 10m plasma with λpe~1mm. In the linear wakefield regime, the plasma response is symmetric for positively and negatively charged bunches. However, once the nonlinear regime is approached the plasma response becomes asymmetric, leading to differences that could be evidenced using the ultra-relativistic electron and positron bunches3. This asymmetry makes it more difficult to accelerate positively than negatively charged particle bunches in plasmas.

Proposed method for high-speed plasma density measurement in proton-driven plasma wakefield acceleration

Recently a proton-bunch-driven plasma wakefield acceleration experiment using the CERN-SPS beam was proposed. Different types of plasma cells are under study, especially laser ionization, plasma discharge, and helicon sources. One of the key parameters is the spatial uniformity of the plasma density profile along the cell that has to be within < 1% of the nominal density (6 × 1014 cm−3). Here a setup based on a photomixing concept is proposed to measure the plasma cut-off frequency and determine the plasma density.