A d. c. charged particle accelerator comprises accelerator electrodes separated by insulating spacers defining acceleration gaps between adjacent pairs of electrodes. Individually regulated gap voltages are applied across each adjacent pair of accelerator electrodes. In an embodiment, direct connections are provided to gap electrodes from the stage points of a multistage Cockcroft Walton type voltage multiplier circuit. The described embodiment enables an ion beam to be accelerated to high energies and high beam currents, with good accelerator stability.