The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield generation and electron acceleration driven by proton bunches. It is a proof-of-principle R&D experiment at CERN and the world's first proton driven plasma wakefield acceleration experiment. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV/c proton beam bunches from the SPS. The first experiments will focus on the self-modulation instability of the long
(rms ~12 cm) proton bunch in the plasma. These experiments are planned for the
end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected into the sample wakefields and be accelerated beyond 1 GeV.
AWAKE is a proof-of-concept acceleration experiment with the aim to inform a design for high energy frontier particle accelerators and is currently being built at CERN. The AWAKE experiment is the world's first proton driven plasma wakefield acceleration experiment, which will use a high-energy proton bunch to drive a plasma wakefield for electron beam acceleration. A 400 GeV/c proton beam will be extracted from the CERN Super Proton Synchrotron, SPS, and utilized as a drive beam for wakefields in a 10 m long plasma cell to accelerate electrons with amplitudes up to the GV/m level. In order to drive the plasma wakefields efficiently, the length of the drive bunch has to be on the order of the plasma wavelength λpe, which corresponds to ≈1 mm for the plasma density used in AWAKE (1014–1015 electrons/cm3). The proton beam for AWAKE has a bunch length of sigma_z = 12 cm, therefore the experiment relies on the self-modulation instability (SMI), which modulates the proton driver at the plasma wavelength in the first few meters of plasma. The SMI is a transverse instability that arises from the interplay between transverse components of the plasma wakefields and the wakefields being driven by regions of different bunch densities. The modulation period s is approximately λpe and the modulated bunch resonantly drives the plasma wakefields. The occurrence of the SMI can be detected by characterizing the longitudinal structure of the proton beam when exiting the plasma cell.
Physics behind AWAKE
It is the aim of the AWAKE project at CERN to demonstrate the acceleration of electrons in the wake created by a proton beam passing through plasma. The proton beam will be modulated as a result of the transverse two-stream instability into a series of micro bunches that will then drive strong wakefields. The wakefields will then be used to accelerate electrons with GV/m strength fields. The AWAKE experiment is currently being commissioned and first data taking is expected this year.
What is a plasma
Plasma is one of the four fundamental states of matter, the others being solid, liquid, and gas. A plasma has properties unlike those of the other states. Plasma consist of a gas of positively charged ions and negatively charged electrons. One of the important characteristics of a plasma is the plasma frequency which defines rapid oscillations of the electron density.
Self-Modulation and microbunches
The AWAKE experiment intends to use the existing SPS beam to drive a high amplitude plasma wakefields, which will then accelerate a witness electron beam. The SPS beam is, however, too long to effectively transfer energy to a plasma-wave. The interaction of the proton beam with the plasma (self-modulation) creates a microbunch structure which is consistent with the plasma wavelength. The resulting micro-bunched beam can effectively transfer energy to the plasma wave.