Ion Motion in Resonantly-Driven Plasma Wakefield Accelerators

Plasma-based accelerators can sustain electric fields orders of magnitude higher than accelerators based on radio-frequency cavities. The AWAKE experiment at CERN drives plasma wakefields over long distances using highly relativistic proton bunches with 10s of kilo-Joules stored energy. This may allow for TeV energy gain in a single acceleration stage. To establish high accelerating fields, the currently available long proton bunch must be self-modulated to resonantly drive a plasma wave and its associated wakefield.

The charged drive bunch deflects plasma electrons, which then oscillate due to the restoring force of the ions. Resonant excitation using equally spaced bunches relies on the restoring force being uniform along the bunch, which may not be the case if the bunch is long enough for ions to respond to the fields. Recent experiments used ion species with different mass to investigate the impact of ion motion on such resonantly driven systems. Lighter ions were found to reduce wakefield amplitudes towards the proton bunch tail. This work utilizes particle-in-cell simulations to study the effect ion motion on the wakefield development and compares them to experimental results. We show and explain the physics of how ion motion can detune the self-modulation resonance. These findings allow limits to be placed on the choice of plasma gas in resonantly driven wakefield accelerators.