Observation of High Transformer Ratio from Bunch Shaping Using Emittance Exchange

Argonne Wakefield Accelerator (AWA) group members recently demonstrated transformer ratio enhancement of R=4.8. In the experiment, a triangular drive bunch and a trailing main bunch are passed through a dielectric slab structure (quartz) to generate the wakefield and a subsequent longitudinal phase space measurement system is used to measure R. We generated the drive's triangular current profile with the AWA emittance exchange beamline which allows easy control of the current profile by simple transverse manipulation (e.g. mask). This direct measurement indicated R=4.8 which is the highest measured transformer ratio so far. We present the experimental details in this paper, discuss the status of the high transformer ratio in plasma wakefield acceleration and introduce our future plans.