Positron Acceleration in the Elongated Bubble Regime

A new concept is proposed for accelerating positrons in a plasma wakefield accelerator. By loading the nonlinear wakefield (back of the bubble) with a short electron bunch, an elongated area of plasma electron accumulation is created after the first bubble, resulting in a favorable positron accelerating region with simultaneous focusing and accelerating fields. The structure of the focusing field is ideal for emittance preservation: linear in the radial direction and uniform in the longitudinal direction. Scaling laws for the optimal loading parameters are obtained through extensive parameter scans. Owing to the good quality of the focusing field, positron acceleration with emittance preservation can be achieved in this new regime and has been demonstrated in the three-dimensional particle-in-cell simulations.