A significant limitation for accelerator physics simulation tools is the inability to accurately predict the distribution of particles in a linear accelerator hadron beam. Even state-of-the-art particle-in-cell codes that contain all the relevant physics are only able to reproduce the beam\u27s measured root-mean-square (RMS) parameters. However, characterizing the beam at several standard deviations beyond RMS is necessary to predict beam loss, one of the limiting factors for achievable beam power and performance in high power, high intensity accelerators. The accelerator community agrees that the discrepancy between measurement and simulation stems from a poor understanding of the initial particle phase space distribution entering the linac. This can be credited to the fact that no complete six dimensional measurement of the initial beam distribution entering a linac has ever been accomplished.This dissertation presents the first complete six-dimensional phase space measurement of a particle beam in a hadron accelerator [1]. The measurement was completed at Spallation Neutron Source Beam Test Facility, a functional duplicate of the SNS front-end capable of producing a pulsed 2.5 MeV H- ion beam. The technique coordinated six movable slits to isolate small, specified volumes systematically over the full six-dimensional phase space and measure the charge inside each volume. The measurement revealed previously unknown, intensity dependent correlations in the phase space distribution that are not visible in lower dimensional measurements. Results will also serve as a full and accurate particle distribution for bench-marking simulations and predicting beam dynamics at the level which relates to beam loss
