  We present results of an experiment showing the first successful
demonstration of a cascaded micro-bunching scheme. Two modulator-chicane
pre-bunchers arranged in series and a high power mid-IR laser seed are used to
modulate a 52 MeV electron beam into a train of sharp microbunches phase-locked
to the external drive laser. This configuration allows to increase the fraction
of electrons trapped in a strongly tapered inverse free electron laser (IFEL)
undulator to 96\%, with up to 78\% of the particles accelerated to the final
design energy yielding a significant improvement compared to the classical
single buncher scheme. These results represent a critical advance in
laser-based longitudinal phase space manipulations and find application both in
high gradient advanced acceleration as well as in high peak and average power
coherent radiation sources.
