We describe a constant-current variable-voltage (CCVV) accelerator, intended to operate in the MeV energy range but tunable down to a few percent of an MeV without loss of beam current. We present experimental results from OUT 200-keV single-beam prototype system designed to accelerate up to 0.2 A of H+ or H-or the equivalent current of heavier ions. The beam is accelerated by a series of stackable 100-keV modules, the number depending on the maximum beam energy required. We discuss a proposed 1 MeV accelerator and a conceptual design for a multiaperture system accelerating 10 A of Dions to 1-2 MeV for current drive in a fusion reactor. The use of electrostatic quadrupole focusing allows a conservative design with reduced risk of voltage breakdown and allows us to maintain high currents while varying the beam energy. These features are useful for fusion reactor startup and for industrial applications, such as semiconductor processing and surface hardening.