L. Boulton, C. A. Lindstrøm, J. Beinortaite, Jonas Björklund Svensson, J. M. Garland, Pau González Caminal, B. Hidding, G. Loisch, F. Peña, K. Põder, S. Schröder, S. Wesch, M. Wing, J. C. Wood, J. Osterhoff, R. D'Arcy
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 the use of the excess light emitted by the plasma after a beam-plasma interaction in a method that yields noninvasive, longitudinally resolved measurements of the local energy-transfer efficiency from the wake to the accelerated bunch. As we show, by diagnosing the acceleration process directly at the plasma location, this method can provide more insight into effects such as trailing bunch charge loss, which typically impose systematic errors on downstream spectrometer measurements. In the future, this method would also be suitable for the online optimization of individual stages in multistage plasma accelerators.