Self-modulated wakefield accelerators

Plasma waves operate as energy transformers that convert the energy from the driver to a trailing bunch of accelerated particles. Proton bunches at the Large-Hadron-Collider (LHC), CERN, are the most energetic charged particle bunch ever produced in a particle accelerator. As a result, they could be used to accelerate electrons and positrons beyond to the energy frontier in plasmas, in distances orders of magnitude smaller than conventional accelerators (A. Caldwell et al, Nature Physics, 5, 363 – 367 (2009)).

Unlike any previous plasma based acceleration experiment performed to date, which have employed drivers with lengths that are comparable to or shorter than the plasma wavelength, LHC proton bunches at CERN can be hundreds of plasmas wavelengths long. The physics of a plasma accelerator  driven by such long proton bunches differs, dramatically, from any experiment performed to date.

Long proton (or electron or positron) bunches are subject to the self-modulation instability. This instability results from an unstable coupling between the long driver and the plasma and leads to the formation of a train of smaller particle bunches separated (nearly) by the plasma wavelength. These bunches can excite large amplitude plasma waves that are suitable to accelerate electrons and/or positrons to high energies. Follow the links below to find out more about my involvement in self-modulation experiments performed in Europe and in the United States.

Self-modulation experiments


AWAKE experiment (CERN)

A large international collaboration has been setup (the AWAKE collaboration) in order to design, plan, and execute a proof-of-principle experiment, planned to occur during the next 4 years.

This experiment is be a first step towards the energy frontier with plasma acceleration. It is challenging from a technical, experimental and physics point of view. It requires an integrated effort combining experimental and theoretical advance, complemented by high-fidelity plasma simulations.

I have been collaborating with scientists from Max Planck Institute in Munich (Germany) (Prof. Allen Caldwell and Dr. Patric Muggli) and the Budker Institute of Nuclear Physics (Prof. Konstantin Lotov) in order to maximize chances for a successful endeavor.

More info at the official AWAKE website, and on the official collaboration publication. Concept has also been featured in the Sci-Fi series Flash Forward.

Simplified layout of the AWAKE experiment

The first experimental results of AWAKE demonstrated electron acceleration in the 10 meter long plasma. The experiment using a Rubidium gas cell. The Rubidium is initially in a solid state but it is placed in a heated environment which causes the Rubidium to evaporate. The temperature is heated uniformly, to a fraction of a degree. This ensures high uniformity of the plasma density, a key requirement for the acceleration to take place. The plasma is initiated by an ionising laser that co-propagates with the proton bunch. The recent experiments demonstrated the self-modulation instability and the acceleration of an external electron beam up to nearly 2 GeV. The energy gain is not as high as in other experiments but the peak accelerating gradient was 100x higher than conventional accelerators.

OSIRIS simulation of the proton wakefield acceleration experiment at CERN. The self-modulated proton bunch (yellow) drives a plasma wave (blue) that accelerates electrons (spheres) to high energies.

E-209 self-modulation experiment (SLAC)

E-209 collaboration (J. Vieira et al PoP 9, 063105 (2012)).

The AWAKE experiment at CERN is a medium-long term experiment. The physics can be tested right away, taking advantage from currently available un-compressed electron and positron bunches at SLAC FACET.

The study of the self-modulation instability is the main goal of the E-209 experiment. Self-modulation occurs when the length of a particle bunch (or laser pulse) is much larger than the plasma wavelength. The self-consistent interaction with the plasma leads to the formation of a sequence of shorter beamlets (see Figure below).

Each beamlet is separated by the plasma wavelength, leading to resonant amplification of plasma waves from the front towards the tail of the beam. Resonant amplification of the plasma wave amplitude is like when an harmonic oscillator (plasma wave) is externally driven at its natural frequency.

There is much physics to explore, including the role of the background plasma ion motion and the role of competing instabilities.

Additional info can also be found here.

Self-modulated particle beam consists in a train of smaller beamlets. These beamlets ressonantly excite a plasma wave, that grows from the head to the tail of the beam.