Multi-gigaelectronvolt acceleration of positrons in a self-loaded plasma wakefield

A particle accelerator that is two orders of magnitude more efficient than conventional radio-frequency accelerators is described in which positrons (rather than electrons) at the front of a bunch transfer their energy to a substantial number of positrons at the rear of the same bunch by exciting a wakefield in the plasma. Accelerated positrons from wakefield plasma Conventional radio-frequency particle accelerators are approaching the limits of development in terms of size and cost. The plasma wakefield accelerator is a promising alternative. In this device an electron bunch is accelerated as it 'surfs' on a plasma wave in the wake created by a second electron bunch. Previously, promising results have been obtained in accelerating electrons but making an electron–positron collider also requires accelerated positrons. Here, Sebastien Corde et al . demonstrate a new accelerating regime in which particles in the front of a positron bunch transfer energy to its rear. The positrons gain five gigaelectronvolts in the process, with a low energy spread, over a distance of 1.3 metres. This implies that the accelerating electric fields are two orders of magnitude stronger than in radio-frequency particle accelerators. Electrical breakdown sets a limit on the kinetic energy that particles in a conventional radio-frequency accelerator can reach. New accelerator concepts must be developed to achieve higher energies and to make future particle colliders more compact and affordable. The plasma wakefield accelerator (PWFA) embodies one such concept, in which the electric field of a plasma wake excited by a bunch of charged particles (such as electrons) is used to accelerate a trailing bunch of particles. To apply plasma acceleration to electron–positron colliders, it is imperative that both the electrons and their antimatter counterpart, the positrons, are efficiently accelerated at high fields using plasmas 1 . Although substantial progress has recently been reported on high-field, high-efficiency acceleration of electrons in a PWFA powered by an electron bunch 2 , such an electron-driven wake is unsuitable for the acceleration and focusing of a positron bunch. Here we demonstrate a new regime of PWFAs 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 about a billion posi