A new awakening for accelerator cavities

Imagine: an accelerator unbound by length; one that can bring a beam up to the TeV level in just a few hundred metres. Sounds like a dream? Perhaps not for long. At CERN’s Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE), physicists may soon be working to bring this contemporary fairy-tale to life.


The AWAKE experiment in the CNGS facility.

Wherever you find a modern linear particle accelerator, you’ll find with it a lengthy series of RF accelerating cavities. Although based on technology first developed over half a century ago, RF cavities have dominated the accelerating world since their inception. However, new developments in plasma accelerator systems may soon be bringing a new player into the game. By harnessing the power of wakefields generated by beams in plasma cells, physicists may be able to produce accelerator gradients of many GV/m –  hundreds of times higher than those achieved in current RF cavities.

“Plasma wakefield acceleration has already been demonstrated at other laboratories, where electron-driven systems have generated gradients of 50 GV/m,” says Edda Gschwendtner, CERN AWAKE project leader. “AWAKE plans to achieve very high gradients by using a proton-driven plasma wakefield system. This experiment can only be performed at CERN, as it requires the use of high-energy proton beams available here.”

Integration of the AWAKE experimental components in the experimental area.

The physics behind the AWAKE project can be found in its recently published design report: As a proton beam (known as the “drive” beam) is injected into a plasma cell it attracts free electrons in the plasma. These free electrons “overshoot” the proton beam but are attracted back by free ions also in the plasma. These oscillating electrons create what could be described as a “naturally occurring” RF cavity: an accelerating electric field in the plasma through which the next beam (known as the “witness” beam) passes and gets accelerated.

If approved, the AWAKE experiment will use 400 GeV SPS beams for its drive beam. “We performed detailed studies of the best suited location for the AWAKE experiment, and in the recently published design report we propose the re-use of the CNGS facility,” says Edda Gschwendtner. “We will extract an LHC-type 400 GeV proton bunch from the SPS and send it towards a plasma cell. A laser pulse, coincident with the proton bunch, will ionize the (initially neutral) gas in the plasma cell, form a plasma and also seed the proton bunch self-modulation (see box). An electron beam will serve as the witness beam and will be accelerated in the wake of the proton bunches. Several diagnostic tools will have to be installed in the experimental area in order to measure the proton bunch self-modulation effects. A state of the art magnetic spectrometer is installed downstream of the plasma cell to measure the electron bunch properties.”

While the potential is certainly there, there is still a lot to learn about plasma wakefield technology. Protons have never been used for this type of acceleration (AWAKE would be the first) nor has the effect of the beam self-modulation effect on the creation of the accelerating gradient been measured. “It is important to remember that AWAKE will be an accelerator R&D proof-of-principle experiment,” concludes Edda. “We first aim to verify this novel concept before we consider its applications in accelerator projects.

Self-modulation of proton beams

Generating a high electric field in a plasma wakefield acceleration system requires short, densely packed proton beams (order of ~mm). However, as the SPS generates beams with bunch length of more than 10 cm, AWAKE will have to take advantage of the effect known as “self-modulation”.

Self-modulation occurs when long bunches of particles enter a plasma. Instead of travelling through the plasma as a single bunch, it evolves into several mini-bunches spaced at the wavelength of the plasma. This effect provides AWAKE with the short bunches required to create the accelerating gradient. However, as this is a naturally occurring effect, it needs to be controlled. Consequently, AWAKE will use a laser pulse to seed the self-modulation effect in the proton beam to ensure the witness beam is perfectly in phase with the accelerating electric field in the plasma.


by Katarina Anthony