We have conducted an experimental and theoretical study of the threshold characteristics of a space simulation beam-plasma discharge with emphasis on density profiles and a density-dependent ignition criterion. The study included various beam-plasma conditions covering beam currents from 8 to 85 ma, beam energies from 0.8 to 2.0 keV, and magnetic fields at 0.9 to 1.5 G. The study included experimental determinations of radial profiles of electron density for each of the selected conditions extending from a low-density, pre-beam-plasma discharge state to a strong beam-plasma discharge condition. At beam-plasma discharge threshold it was determined that (&ohgr;p/&ohgr;c) = 5.4 was the density-dependent ignition criterion. The experimental results are shown to agree with detailed model calculations, which consider the beam-plamsa discharge to be produced by large-amplitude electron plasma waves resulting from the beam-plasma interaction.