AWAKE

The AWAKE (Advanced Proton-Driven Plasma Wakefield Acceleration Experiment) international collaboration is the first experiment exploring the use of existing high-energy proton accelerators to power a next-generation of ultra-compact electron accelerators.

Based at CERN near Geneva, Switzerland, AWAKE is one of several TRIUMF-CERN collaborations, along with the major ALPHA and ATLAS collaborations.

High energy particle physics has used ever larger accelerators, like CERN’s 27-kilometer ring Large Hadron Collider. One of the practical and cost challenges for new high-energy particle accelerators is delivering higher energy with a smaller accelerator.

Existing electron accelerators use intense radio frequency waves pumped into cavities to accelerate electrons. AWAKE is prototyping a compact accelerator technology that uses high-energy proton bunches fired into a plasma, a super-heated mix of ions and electrons, to create a plasma wakefield, a series of energetic waves that can accelerate electrons in the same way a surfer is accelerated on an ocean wave.

The advantage of plasma wakefields is that the acceleration force generated per meter of accelerator length is up to 1000-times stronger than in conventional electron accelerators. Thus, the acceleration energy achieved with a ten-kilometer-long conventional linear accelerator might be achieved with just a 100-meter-long plasma wakefield accelerator.

AWAKE began operation in 2016 and its initial 2016-2017 run successfully demonstrated the evidence of strong plasma wakefields created by a proton bunch, followed by the first-ever wakefield acceleration of electrons in May of 2018.

The AWAKE collaboration involves more than 80 engineers and physicists from 17 countries.

AWAKE uses proton beams from CERN’s Super Proton Synchrotron, the last accelerator in the chain that delivers protons to the Large Hadron Collider. The 400 GeV (billion electron volts)

protons are injected into a 10-metre long plasma cell to initiate strong wakefields which boost 20 MeV electrons from an electron injector to multi-GeV energies.

As with ocean surfing, excellent wakefield acceleration depends on a combination of perfect timing and the right conditions.

To start, rubidium vapour is injected into the 10-meter-long, tube-shaped plasma cell from two flasks at either end of the cell. Next, a laser pulse synchronized with the entry of a proton bunch is fired into the plasma cell, singly ionizing the rubidium, creating a plasma of rubidium ions and free electrons.

The interaction between the proton bunch and the plasma causes the proton bunch to split into a series of high density microbunches which act resonantly to create large wakefields.

Immediately after the laser pulse, an electron beam (the “witness beam”) is fired into the wakefield at an angle that maximizes the number of electrons that “catch” a plasma wave.

The accelerated electrons and protons leave the plasma cell via a beamline, and a pair of magnets are used to separate the electrons from the protons. The electrons are stopped on a scintillator screen that when it interacts with charged particles produces light signals. The scintillator signals enable researchers to identify the number of accelerated electrons, and their energy, inferred from the position at which an electron hits the scintillator.

TRIUMF is providing a combination of scientific and technical expertise to AWAKE, with the largest contribution being the beam instrumentation for the electron accelerator and the electron beam transport line.  

As with a human surfer catching an ocean wave, AWAKE’s electron beam must be injected at just the right time so that the electrons catch the crests of the plasma wave. The TRIUMF-designed electron beam position system enables AWAKE researchers to synchronize the electron and proton wave trajectories to a difference of less than 0.05mm. During early AWAKE runs, the TRIUMF-created beam position monitoring system enabled the commissioning of the electron witness beam infrastructure and the success of the world’s first ever proton-driven plasma wakefield acceleration of electrons. 

Similarly, to calculate the percent of injected electrons that are accelerated researchers must measure the electron beam’s electron charge before and after the plasma cell. TRIUMF designed and built the monitor and electronics to precisely measure the electron current, and thus determine the system’s electron capture efficiency.  

TRIUMF, along with University of Victoria researchers, also helped characterize the temperature control of the rubidium plasma.   

TRIUMF is providing a combination of scientific and technical expertise to AWAKE, with the largest contribution being the beam instrumentation for the electron accelerator and the electron beam transport line.

As with a human surfer catching an ocean wave, AWAKE’s electron beam must be injected at just the right time so that the electrons catch the crests of the plasma wave. The TRIUMF-designed electron beam position system enables AWAKE researchers to synchronize the electron and proton wave trajectories to a difference of less than 0.05 mm. During early AWAKE runs, the TRIUMF-created beam position monitoring system enabled the commissioning of the electron witness beam infrastructure and the success of the world’s first ever proton-driven plasma wakefield acceleration of electrons.

Similarly, to calculate the percent of injected electrons that are accelerated researchers must measure the electron beam’s electron charge before and after the plasma cell. TRIUMF designed and built the monitor and electronics to precisely measure the electron current, and thus determine the system’s electron capture efficiency.

TRIUMF, along with University of Victoria researchers, also helped characterize the temperature control of the rubidium plasma.