  We report on the first detailed beam test attesting the fundamental principle
behind the development of high-current-efficiency ultrafast electron microscope
systems where a radio-frequency cavity is incorporated as a condenser lens in
the beam delivery system. To allow the experiment to be carried out with a
sufficient resolution to probe the performance at the emittance floor, a new
cascade loop RF controller system is developed to reduce the RF noise floor.
Temporal resolution at 50 femtoseconds in full-width-at-half-maximum and
detection sensitivity better than 1% are demonstrated on exfoliated 1T-TaSe$_2$
layers where the multi-order edge-mode coherent phonon excitation is employed
as the standard candle to benchmark the performance. The high temporal
resolution and the significant visibility to very low dynamical contrast in
diffraction signals give strong support to the working principle of the
high-brightness beam delivery via phase-space manipulation in the electron
microscope system.
