Resonant excitation of plasma wakefields with a train of relativistic particle bunches

Resonances play a crucial role in media sustaining oscillatory phenomena, such as plasmas. We show with experimental results and numerical simulations that the wakefields driven by individual successive bunches in overdense plasma superpose linearly, and that their amplitude increases along the train, under optimal coupling with plasma density oscillations. We also demonstrate, for the first time, that a train of bunches with increasing charge can be used to enhance the transformer ratio of the acceleration process, hence improving the energy-transfer efficiency.