  The interaction of intense particle bunches with plasma can give rise to
plasma wakes capable of sustaining gigavolt-per-metre electric fields, which
are orders of magnitude higher than provided by state-of-the-art
radio-frequency technology. 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. 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.
