Giant ion sources of neutral-beam injectors for fusion (invited)

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Abstract
All large tokamak fusion experiments today use auxiliary heating by multimegawatt beams of neutral isotopes of hydrogen injected with energies in the neighborhood of 100 keV per atom. This requires reliable operation of large ion souces, each delivering many tens of amperes of protons or deuterons, and soon even tritons. For meaningful experiments these sources must operate with pulse durations measured in seconds, although the duty factor may still be small. It is remarkable that the successful sources developed in Europe, Japan, and the U. S. are all very similar in basic design: the plasma is produced by diffuse low‐pressure high‐current discharges in magnetic multipole ‘‘buckets’’ with distributed thermionically emitting cathodes. This article briefly reviews the principal considerations and the basic physics of these sources, and summarizes the collective experience to‐date and describes the impressive recent performance of the U. S. Common Long Pulse Source, as a specific example.