Vacuum-plasma transition effect on positron acceleration in the bubble regime plasma wakefield accelerators

In recent years, the field of positron acceleration in plasma wakefield accelerators has witnessed rapid theoretical advancements, and several effective schemes have been proposed. In these studies, longitudinally uniform plasma is taken into account, with the positron beam located at the start of the second wakefield bubble. The electron filament near the positron beam provides Coulomb attraction force for the guiding of the positron beam. However, the influence of vacuum-plasma transition on positron beams has not been considered yet, where wakefield bubble is larger than that in the density plateau region, and the positron beam is in the defocusing region. In this paper, we study the evolution of positron beam size in the vacuum-plasma transition region and find out that the transition is detrimental to low-energy positron beams. Wide drive beams with relatively large transverse sizes are proposed as a possible solution to this difficulty.