  We demonstrate experimentally the resonant excitation of plasma waves by
trains of laser pulses. We also take an important first step to achieving an
energy recovery plasma accelerator by showing that unused wakefield energy can
be removed by an out-of-resonance trailing laser pulse. The measured laser
wakefields are found to be in excellent agreement with analytical and numerical
models of wakefield excitation in the linear regime. Our results indicate a
promising direction for achieving highly controlled, GeV-scale laser-plasma
accelerators operating at multi-kilohertz repetition rates. This article was
published in Physical Review Letters 119, 044802 on 27 July 2017. DOI:
10.1103/PhysRevLett.119.044802 Copyright 2017 American Physical Society.
