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Why are negative hydrogen ions used during the early stages of some proton accelerators?

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Diagram of a linear proton resonance accelerator, or linac, showing protons entering from a source, passing through a series of labeled drift tubes powered by a radio-frequency source, and exiting as a beam through a cavity. — particle accelerator The accelerator is a large-diameter tube within which an electric field oscillates at a high radio frequency. Within the accelerator tube are smaller diameter metallic drift tubes, which are carefully sized and spaced to shield the protons from decelerating oscillations of the electric field. In the spaces between the drift tubes, the electric field is oriented properly to accelerate the protons in their direction of travel.

In large, high-energy particle accelerators, negative hydrogen ions—which consist of a hydrogen atom with an extra electron—are utilized during the initial stages because they are easier to handle.1

In the context of two-stage tandem accelerators, these ions are particularly useful for achieving higher energy levels. A beam of negative hydrogen ions is accelerated toward a positive high-voltage terminal. Once inside the terminal, the particles pass through a thin carbon foil that strips away the extra electrons, converting the negative ions back into positive protons. Because these protons are then repelled by the positive terminal, they receive a second stage of acceleration, effectively doubling the energy that could be achieved by a single application of the high voltage.2

In larger accelerators, these ions are similarly passed through thin foils to strip off the electrons before the protons proceed to the final stages of acceleration.1