Emittance preservation in a single PWFA-LC stage using adiabatic plasma density ramp matching sections in the presence of ion motion

Plasma based acceleration (PBA) is being considered as a building block for a future linear collider (LC). In PBA a short pulse laser or particle beam creates a wakefield and a witness particle beam is accelerated in the wakefield. As the witness beam is accelerated its energy spread must be small and its emittance must be preserved. In some designs the witness beam parameters required by a linear collider are expected to trigger background ion motion which can lead to nonlinear focusing forces which vary along the witness beam. This can lead to emittance growth of the witness beam. To mitigate this, we propose to use an adiabatic plasma density ramp as a matching section. We match the beam to the low density plasma entrance, where the beam has a large matched spot size so the ion motion effects are relatively small. As the beam propagates in the plasma upramp (downramp), it is adiabatically focused (defocused) and its distribution evolves slowly towards an equilibrium distribution including the effects of the adiabatically changing ion motion. We present simulation results using the 3D and quasi-3D quasi-static PIC codes QuickPIC and QPAD. In QPAD the fields are expanded into a small number of azimuthal modes. Preliminary results show that this method can reduce the projected emittance growth of the witness beam to only 10~20% within a single stage which includes adiabatic matching sections at the entrance and exit.