Three-dimensional beam size compression for external injection of plasma wakefield acceleration

Purpose Plasma acceleration is a novel acceleration principle characterized by a high acceleration gradient. This area has garnered extensive research interest from major accelerator laboratories worldwide because of its potential to increase accelerator energy and reduce accelerator size. Currently, using existing conventional accelerators as external injectors for plasma-based accelerators is a promising direction that has attracted notable interest from the research community. However, a critical challenge is matching the three-dimensional beam sizes produced by conventional radio-frequency (RF) accelerators to the plasma accelerating structures. This alignment is crucial for utilizing existing non-state-of-the-art accelerators in plasma-based acceleration research. For instance, the BEPCII linac generates beams with transverse sizes approximately one millimeter and longitudinal sizes around ten picoseconds, while plasma accelerating structures are typically on the order of hundreds of micrometers. Addressing this mismatch is essential for advancing plasma acceleration research. In our recent work at BEPCII, we tackled the challenge of transporting both electron and positron beams from an RF linac into the plasma.
Methods We adopted an innovative design concept that decouples longitudinal and transverse beam size compression, implementing them in separate stages. For transverse beam size compression, we utilized a global optimization method that balances the nominal beta functions with chromatic aberrations at the interaction point and provides self-compensation for chromatic aberrations along the beamline.
Results By using this approach, we developed a beam transport line for BEPCII that achieves a tenfold compression of the three-dimensional beam sizes produced by the conventional accelerator.
Conclusion This paper presents our design concept and the resulting transport line design, demonstrating its effectiveness in addressing the beam size matching challenge for plasma acceleration.