Longitudinally resolved measurement of energy-transfer efficiency in a plasma-wakefield accelerator

Energy-transfer efficiency is an important quantity in plasma-wakefield acceleration, especially for applications that demand high average power. Conventionally, the efficiency is measured using an electron spectrometer; an invasive method that provides an energy-transfer efficiency averaged over the full length of the plasma accelerator. Here, we experimentally demonstrate a novel diagnostic utilizing the excess light emitted by the plasma after a beam-plasma interaction, which yields noninvasive, longitudinally resolved measurements of the local energy-transfer efficiency from the wake to the accelerated bunch; here, as high as (58 $\pm$ 3)%. This method is suitable for online optimization of individual stages in a future multistage plasma accelerator, and enables experimental studies of the relation between efficiency and transverse instability in the acceleration process.