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Description
The injection of the witness electron bunch in the AWAKE Run 2c experiments will occur in a $\sim30\,$cm gap separating two plasmas. In this gap, the $400\,$GeV, self-modulated drive proton bunch and witness bunch will co-propagate in a Rubidium vapor. This vapor will be partially ionized by the protons through impact ionization, and will thus result in the presence of a low-density ($\sim10^{12}\,$cm$^{-3}$) plasma. Under the influence of the space-charge fields induced by the proton bunch, the plasma electrons form an on-axis filament that acts as a defocusing element for the witness electron bunch. This filament may prevent proper matching of the witness bunch in the second plasma, and/or lead to emittance growth before injection. In this work, we investigate, using numerical simulations, the process of impact ionization resulting from the propagation of the protons, the formation of the plasma electron filament, and discuss its potential effect on the witness electron bunch.
| Paper status | Resubmitted proceeding files received and assigned to an editor. Ready for Review. |
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