Further laboratory measurements of the beam-plasma discharge (BPD) produced when an energetic electron beam traverses an initially neutral gas are presented. The experimental data indicate that the critical beam current I//c at energy V, magnetic field B, and system length L required for ignition obeys the empirical relationship I//c varies directly as V**3**/**2/B**0**. **7 PL at pressures P less than 2 multiplied by 10** minus **5 torr. This relationship appears to describe primarily the accumulation of ambient plasma density, collisionally produced by the beam itself, to a critical value at which ignition occurs. Additional measurements of the narrow-band cyclotron-related waves observed at beam currents below BPD threshold do not clearly establish whether this instability is a necessary precursor to the BPD. The relevance of these experiments to past and planned electron beam experiments in space is briefly discussed.