Recovery time of a plasma-wakefield accelerator

The interaction of intense particle bunches with plasma can give rise to plasma wakes 1 , 2 capable of sustaining gigavolt-per-metre electric fields 3 , 4 , which are orders of magnitude higher than provided by state-of-the-art radio-frequency technology 5 . Plasma wakefields can, therefore, strongly accelerate charged particles and offer the opportunity to reach higher particle energies with smaller and hence more widely available accelerator facilities. However, the luminosity and brilliance demands of high-energy physics and photon science require particle bunches to be accelerated at repetition rates of thousands or even millions per second, which are orders of magnitude higher than demonstrated with plasma-wakefield technology 6 , 7 . Here we investigate the upper limit on repetition rates of beam-driven plasma accelerators by measuring the time it takes for the plasma to recover to its initial state after perturbation by a wakefield. The many-nanosecond-level recovery time measured establishes the in-principle attainability of megahertz rates of acceleration in plasmas. The experimental signatures of the perturbation are well described by simulations of a temporally evolving parabolic ion channel, transferring energy from the collapsing wake to the surrounding media. This result establishes that plasma-wakefield modules could be developed as feasible high-repetition-rate energy boosters at current and future particle-physics and photon-science facilities. Relaxation of a perturbed plasma back to its initial state over nanosecond timescales establishes that megahertz repetition rates are supported, and high luminosities and brilliances are in principle attainable with plasma-wakefield accelerator facilities.