In a study reported in Nature, a team working at the Facility for Advanced Accelerator Experimental Tests (FACET) at SLAC has shown that the high electric-field gradients possible in plasma can be harnessed to accelerate positrons, just as well as they can for electrons.
In 2014, an experiment at FACET, which uses the first 2 km of the famous SLAC linac, was able to demonstrate plasma-wakefield acceleration of electrons, with both a high gradient and a high energy-transfer efficiency – a crucial combination that had not previously been achieved (CERN Courier January/February 2015 p9). However, for positrons, plasma-wakefield acceleration is much more challenging, and it was thought that no matter where a trailing positron bunch was placed in a wake, it would lose its compact, focused shape or even slow down.
In the new study, the team demonstrated a new regime for plasma-wakefield acceleration where particles in the front of a single positron bunch transfer their energy to a substantial number of those in the rear of the same bunch by exciting a wakefield in the plasma. In the process, the accelerating field is altered –”self-loaded” – so that in the tests about a billion positrons gained 5 GeV in energy with a narrow energy spread over a distance of just 1.3 m. Moreover, the positrons extract about 30 per cent of the wake’s energy and form a spectrally distinct bunch with a root-mean-square energy spread as low as 1.8%.
This ability to transfer energy efficiently from the front to the rear within a single positron bunch makes the scheme highly attractive as an energy booster for a future electron–positron collider.