Methods and apparatus are provided for efficiently operating an ion implanter which includes a charged particle accelerator in a high energy mode and in a low energy mode. The charged particle accelerator includes a high voltage power supply, an accelerator column coupled to the high voltage power supply and a switching assembly. The accelerator column includes a plurality of accelerator electrodes. The high voltage power supply is disabled from energizing the accelerator column in the low energy mode. The switching assembly includes switching elements for electrically connecting the accelerator electrodes to a reference potential in the low energy mode and for electrically isolating the accelerator electrodes from the reference potential in the high energy mode. The switching assembly prevents positive potentials on the accelerator electrodes and thus minimizes space charge expansion of the beam when transporting positive ion beams in the low energy mode.