  Plasma wakefields can enable very high accelerating gradients for frontier
high energy particle accelerators, in excess of 10 GeV/m. To overcome limits on
total acceleration achievable, specially shaped drive beams can be used in both
linear and nonlinear plasma wakefield accelerators (PWFA), to increase the
transformer ratio, implying that the drive beam deceleration is minimized
relative to acceleration obtained in the wake. In this Letter, we report the
results of a nonlinear PWFA, high transformer ratio experiment using
high-charge, longitudinally asymmetric drive beams in a plasma cell. An
emittance exchange process is used to generate variable drive current profiles,
in conjunction with a long (multiple plasma wavelength) witness beam. The
witness beam is energy-modulated by the wakefield, yielding a response that
contains detailed spectral information in a single-shot measurement. Using
these methods, we generate a variety of beam profiles and characterize the
wakefields, directly observing beam-loaded transformer ratios up to R=7.8.
Furthermore, a spectrally-based reconstruction technique, validated by 3D
particle-in-cell simulations, is introduced to obtain the drive beam current
profile from the decelerating wake data.
