In early December, the AWAKE collaboration made an important step towards a pioneering accelerator technology that would reduce the size and cost of particle accelerators. Having commissioned the facility with first beam in November, the team has now installed a plasma cell and observed a strong modulation of high-energy proton bunches as they pass through it. This signals the generation of very strong electric fields that could be used to accelerate electrons to high energies over short distances.
AWAKE (Advanced Proton Driven Plasma Wakefield Acceleration Experiment) is the first facility to investigate the use of plasma wakefields driven by proton beams. The experiment involves injecting a “drive” bunch of protons from CERN’s Super Proton Synchrotron (SPS) into a 10 m-long tube containing a plasma. The bunch then splits into a series of smaller bunches via a process called self-modulation, generating a strong wakefield as they move through the plasma. “Although plasma-wakefield technology has been explored for many years, AWAKE is the first experiment to use protons as a driver – which, given the high energy of the SPS, can drive wakefields over much longer distances compared with electron- or laser-based schemes,” says AWAKE spokesperson Allen Caldwell of the Max Planck Institute for Physics in Munich.
While it has long been known that plasmas may provide an alternative to traditional accelerating methods based on RF cavities, turning this concept into a practical device is a major challenge. The next step for the AWAKE collaboration is to inject a second beam of electrons, the “witness” beam, which is accelerated by the wakefield just as a surfer accelerates by riding a wave. “To have observed indications for the first time of proton-bunch self-modulation, after just a few days of tests, is an excellent achievement. It’s down to a very motivated and dedicated team,” says Edda Gschwendtner, CERN AWAKE project leader.