Coupling of multiple proton driven wakefield accelerator stages

Abstract

The AWAKE experiment aims to accelerate leptons to the TeV regime. It will accomplish its goal with a highly energetic proton beam as a wakefield driver. However, to increase the magnitude of the acceleration gradients produced by the driver, one must extend its propagation distance. Prolonging the current stage would be experimentally challenging due to the self-consistent evolution of the ionisation front used during this stage.

We propose adding a pre-formed plasma stage, to the existing stage, where the half-modulated beam (result of the first stage) can propagate. This work aims to study how the wake of the unmodulated part interacts with the modulated part of the beam during the second stage. We first analysed the impact of various wakefield parameters on the structure of the modulated beam, and concluded that the wake's phase is the most relevant parameter for the conservation of the beam. Next, we analysed in detail the propagation of a partially modulated beam. The results showed that seeding the modulation of the unmodulated part is the best approach to guarantee higher acceleration gradients during the second stage.

Lastly, we demonstrated that we can adjust the length of the simultaneously accelerating and focusing region of a wakefield produced by two sources, by controlling their width, and the spacing between them. This is an important result that allows us to increase the accelerator's efficiency, by increasing the number of efficiently accelerated particles.