Efficient operating mode of the plasma wakefield accelerator

A new operating mode of the plasma wakefield accelerator is found at which high efficiency of the driver-to-witness energy exchange can be achieved simultaneously with high transformer ratio and low energy spread. The efficient acceleration is realized in the blowout regime with a high-current moderate-length driver, if most of the driver and the whole witness are inside the cavern, and the beams are shaped to flatten the profile of the longitudinal electric field. The efficient regime can be demonstrated with state-of-the-art electron beams, but requires a longitudinal compression of the drive beam, high density plasma, and precise control of driver and witness shapes.