An envelope equation, based on a self-consistent equilibrium model of a charge- and currentneutralized, ballistically focused ion beam is derived. Focusing of the beam is limited by electrostatic fields within a Debye sheath at the beam edge and residual electrostatic fields within the beam interior. Numerical and approximate analytical solutions indicate that the initial temperature of the neutralizing electrons must be less than about 10 keV in heavy-ion inertial-confinement fusion systems of fewer than ten beams.