  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.
