Emittance Preservation of Positron Beams Traveling in Hollow Plasma Channels

Emittance preservation of the incoming beam is essential for all accelerators. In the blowout regime of plasma wakefield accelerator (PWFA), the electron bunch propagates in a uniform ion column and its emittance is preserved along the plasma. For a positron bunch propagating in a uniform plasma, the beam suffers emittance growth [P. Muggli et al., Phys. Rev. Lett. 2008]. However, it has been shown that when a positron bunch propagates in a hollow plasma, the axial accelerating field can be enhanced when compared to the uniform plasma case [S. Lee, PRE, 64, 045501(R), 2001]. We explore through numerical simulations the possibility of preserving emittance of positron beams by using a hollow plasma channel instead of a uniform plasma. We consider in particular the case of a drive/witness bunch train, necessary to accelerate the witness bunch with a high gradient and a narrow final energy spread. We explore a range of beam and channel parameters to maximize the acceleration and minimize the final energy spread and emittance of the witness bunch. Detailed simulation results will be presented.