AWAKE achieves first ever acceleration of electrons in a proton-driven plasma

Geneva,  In a paper published in the journal Nature, the AWAKE collaboration at CERNreports the first ever successful acceleration of electrons using a wave generated by protons zipping through a plasma. The acceleration obtained over a given distance is already several times higher than that of conventional technologies currently available for particle accelerators. First proposed in the 1970s, the use of plasma waves (or so called wakefields) has the potential to drastically reduce the size of accelerators in the next several decades. 

AWAKE, which stands for “Advanced WAKEfield Experiment”, is a proof-of-principle compact accelerator project for accelerating electrons to very high energies over short distances. Accelerating particles to greater energies over shorter distances is crucial to achieving high-energy collisions that physicists use to probe the fundamental laws of nature, and may also prove to be important in a wide range of industrial and medical applications.

Plasma is a special state of matter that can be induced by ionising a gas – that is, by ejecting electrons from the gas atoms or molecules. In AWAKE, rubidium is heated to convert it into a gas and is then ionised with a laser beam. A proton beam (called the drive beam) is injected along with the laser pulse and causes the plasma to oscillate in a wavelike pattern, much like a ship moving through the water generates oscillations in its wake. AWAKE gets its drive-protons with an energy of 400 GeV (billion electronvolts) from CERN’s Super Proton Synchrotron (SPS), which is the last accelerator in the chain that delivers protons to the LHC. A beam of electrons (called the witness beam) is injected at a slight angle into the oscillating plasma and get accelerated by “surfing” plasma waves.

On 26 May, the AWAKE collaboration successfully accelerated witness-electrons for the first time. Electrons injected into AWAKE at relatively low energies of around 19 MeV (million electronvolts), “rode” the plasma wave, and were accelerated by a factor of around 100, to an energy of almost 2 GeV (billion electronvolts) over a length of 10 metres. 

While previous experiments of wakefield acceleration have relied on using electrons or lasers to drive the wake, AWAKE is the first to use protons. “Drive beams of protons penetrate deeper into the plasma than drive beams of electrons and lasers,” said Allen Caldwell, Spokesperson of the AWAKE collaboration. “Therefore, wakefield accelerators relying on protons for their drive beams can accelerate electrons for a greater distance, consequently allowing them to attain higher energies.” 

“By accelerating electrons to 2 GeV in just 10 metres, AWAKE has demonstrated that it can achieve an average gradient of around 200 MV/m (million volts per metre),” says Technical Coordinator and CERN Project Leader for AWAKE, Edda Gschwendtner. For comparison, the advanced conventional technologies considered for the next generation of electron accelerators promise gradients in the range of 30–100 MV/m. These represent today’s state of the art in particle accelerators for the overall distance over which acceleration can be sustained, on the one hand, and the intensity and quality of accelerated beams, on the other – two important factors required for high-energy physics experiments. The next steps of AWAKE, which aims to achieve 1000 MV/m, include addressing these additional requirements.

AWAKE has made rapid progress since inception. The plasma cell was installed in early 2016. A few months later, the first drive beams of protons were injected into the plasma cell to commission the experimental apparatus, and a proton-driven wakefield was observed for the first time in late 2016.