Integrated high efficiency bunch shaping techniques for SWFA

Beam-driven wakefield accelerators, including SWFA (structure-based wakefield accelerator) and PWFA (plasma-based wakefield accelerator) are promising candidates for TeV-class high energy colliders. The enabling tool for a high efficiency and high gradient SWFA is a bunch shaper that produces a specifically shaped electron bunch or bunch train. A number of techniques have been explored to implement the bunch shaper. However, none of the existing methods are currently capable of shaping high-charge bunches (nC range, or equivalently kA range in pulsed current), and are far from the requirement of producing high gradient, G, (GV/m) in realistic SWFA machine. We proposed a new Coherent Synchrotron Radiation (CSR)-free longitudinal bunch shaping technique using transverse deflecting cavities. In this method, two deflectors and a quadruple magnet are used to introduces and exchange the z − x correlation, and a transverse mask tailors the longitudinal density profile. This method provides high-quality shaping regardless of charge level and preserves the beam quality. The only drawback of using a mask, which induces beam loss, will be significantly compensated by using the laser shaping technique that we are developing to generate and transport a longitudinally pre-formed bunch [24]. This integrated bunch shaping technique will eventually provide a solution for the previously unsolved issue of producing a high charge (>10 nC, or equivalently >1 kA before compression) shaped bunch. The principal objective of this project is to demonstrate a novel bunch-shaping technique that is capable of producing a high-charge (>10 nC) shaped drive beam for beam-driven wakefield accelerators. The final deliverable will be a structure-based wakefield acceleration experiment that for the first time demonstrates a high transformer ratio (>5) and a high gradient (>200MV/m). One of the key elements of this technique is a short pulse traveling-wave deflector that is powered by a wakefield power extractor, so that the entire scheme can naturally apply to the future SWFA project and facility. Under Phase I of this project, we will focus on two main tasks: 1) completion of the simulation and experimental layout based on the wakefield-driven short- pulse deflectors; 2) development of the short-pulse traveling-wave deflector. The techniques for producing a shaped high-charge drive bunch that we propose in this project can obtain a much higher energy transfer efficiency and a high gradient, leading to a breakthrough in the performance of wakefield accelerators.