Demonstrating High Transformer Ratio Beam-Driven Plasma Wakefield Acceleration

In this work, plasma wakefield acceleration with high transformer ratios, i.e. highratios between acceleration of witness particles to deceleration of driver particles, is investigated experimentally.Particle-beam-driven plasma wakefield acceleration (PWFA) is one of the primecandidates for future compact accelerator technologies. In this scheme, a highbrightness driver particle bunch enters a plasma and initiates oscillations of plasma electrons by expelling them from their equilibrium positions. Particles trailing the driver bunch can be accelerated in the electric field between regions of negative and positive charge excess formed in this oscillation. Acceleration gradients of up to several tens of GV/m have been demonstrated in experiment, exceeding the gradients of conventional technology by orders of magnitude. PWFA could thus allow to accordingly shrink the size of an accelerator, possibly reducing size and cost of an accelerator facility significantly.One key aspect of a PWFA is the ratio between acceleration gradient behind the driver and deceleration gradient inside the driver bunch. This so-called transformer ratio defines the maximum acceleration achievable for a given driver energy and is closely connected to the achievable efficiency. In linear wakefield theory the transformer ratio is limited to 2 for longitudinally symmetric driver bunches, which typically emerge from conventional accelerators. One proposed method to achieve high transformer ratios (HTR) exceeding this limit is to employ driver bunches with lengths of multiple plasma skin depths and sawtooth-like, “triangular” current profiles. Due to the complexity of shaping such bunches and beam-plasma instabilities that can inhibit stable acceleration, HTRs had thus far not been achieved in PWFA.In the course of the present work, the driving of HTR PWFA has been accomplished at the Photo-Injector Test facility at DESY in Zeuthen (PITZ). Existing bunch shaping capabilities at PITZ, based on photocathode laser pulse shaping,were extended to the production of HTR-capable driver bunches. An argon gasdischarge plasma cell was built and optimised for providing the plasma acceleration medium for the PITZ electron beam parameters. A novel plasma density measurement method based on the self-modulation of long electron bunches was developed and validated, which enabled determination of plasma densities not accessible with established methods. Following these preparations, a transformer ratio of 4.6+2.2−0.7was observed experimentally and reproduced in numerical simulations