Emittance preservation in the presence of ion motion for low-to-high energy stages of a plasma based accelerator

Plasma based acceleration (PBA) is being considered for a next generation linear collider (LC). In typical PBA-LC designs, the extreme beam parameters are expected to trigger background ion motion, which can lead to longitudinally varying nonlinear focusing forces and result in emittance growth of the beam. While various schemes have been proposed to mitigate this at low beam energies, a solution to minimize the emittance growth in the later high energy stages of a multistage electron acceleration arm is yet to be found. In this paper, we propose to use an adiabatic plasma density ramp as a matching section that is able to match the witness electron beam to the low-density plasma entrance, where the beam initially has a large matched spot size so the ion motion effects are relatively small. As the beam propagates in the plasma density upramp (downramp), it is adiabatically focused (defocused) and its distribution maintains an equilibrium distribution throughout the entire process even when severe ion collapse has occurred. Simulation results from qpad show that within a single acceleration stage, this concept can limit the projected emittance growth to only ∼2% for a 25 GeV, 100 nm normalized emittance witness beam and ∼20% for a 100 GeV, 100 nm normalized emittance witness beam.