  Modern accelerator front ends almost exclusively include radio-frequency
quadrupoles for initial capture and focusing of the low-energy beam. Dynamics
in the RFQ define the longitudinal bunch parameters. Simulation of the SNS RFQ
with PARMTEQ seeded with a realistic LEBT distribution produces a 2.5 MeV, 40
mA H- beam with root-mean-square emittance of 130 deg-keV. In measurement, a
detailed characterization of the longitudinal phase space is made, including a
novel study of the dependence of longitudinal emittance on transverse
coordinates. This work introduces a new virtual slit technique that provides
sub-slit resolution in an energy spectrometer as well as an approach for
visualizing 4D phase space data. Through simulation and measurement, the
RFQ-formed bunch is confirmed to have significant internal correlated
structure. The high-dimensional features are shown to be in qualitative
agreement. However, the measured rms emittances are up to 30% lower than
predicted, closer to the design value of 95 deg-keV.
