Signatures of the self-modulation instability of relativistic proton bunches in the AWAKE experiment

TL;DR

This paper numerically investigates how the self-modulation instability of relativistic proton bunches in the AWAKE experiment can be detected downstream, revealing the relationship between density structures and beam phase space.

Contribution

It demonstrates that density structures in a virtual detector map the transverse phase space and clarifies the origin of observed asymmetries in the detector signals.

Findings

Density structures map the transverse phase space at plasma exit.

Bunch radius growth in the detector is due to divergence increase.

Asymmetries in the detector are caused by divergence asymmetries, not density asymmetries.

Abstract

We investigate numerically the detection of the self-modulation instability in a virtual detector located downstream from the plasma in the context of AWAKE. We show that the density structures, appearing in the temporally resolving virtual detector, map the transverse beam phase space distribution at the plasma exit. As a result, the proton bunch radius that appears to grow along the bunch in the detector results from the divergence increase along the bunch, related with the spatial growth of the self-modulated wakefields. In addition, asymmetric bunch structures in the detector are a result of asymmetries of the bunch divergence, and do not necessarily reflect asymmetric beam density distributions in the plasma.