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. Normally, the frequency of external forces delivering energy is chosen to be out of tune with respect to the natural plasma electron frequency, so as to avoid resonances leading to the development of deleterious instabilities. However, in the context of plasma wakefield acceleration, resonances can be exploited to generate large-amplitude wakefields, using a train of relativistic particle bunches with frequency content close to the plasma electron frequency, to accelerate a trailing bunch. 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. The findings herein open pathways for high-efficiency, high-gradient plasma wakefield acceleration, advancing its potential for future compact accelerators and free-electron lasers.