Maximization of accelerated charge in multibunch-driven plasma wakefield accelerators

Driving the plasma wakefield with a train of multiple particle bunches has the potential to significantly increase the accelerated charge, provided that the witness bunches also form a train. In this scenario, the driver bunches can replenish the wave energy after the passage of the witness bunches, yielding comparable charges in both trains. However, implementing this method at high wakefield amplitudes is complicated by the reshaping of the driver bunches as they reach radial equilibrium with the wave. The witness bunches cannot preserve their quality during this process; nevertheless, they must load the wave to prevent it from reaching the amplitude at which the wakefield period elongates and the wave destroys the driver. The proposed solution is to form an equilibrium train of initially inefficient driver bunches, which becomes efficient upon the injection of the witness bunches. This concept is demonstrated using the example of a self-modulating driver beam. Several factors favoring a high witness charge are identified: injecting additional ``phaser'' bunches at the zero-accelerating-field points, suppressing the formation of an electron halo around the plasma, and operating at the highest possible wakefield amplitudes.