Franck–Condon energy spread in diagnostic neutral-particle-beam probes

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Abstract
Plasma potentials can be measured in open‐ended magnetic‐fusion experiments with neutral beams injected at angles within the loss cone. Beam atoms that are ionized by the plasma follow magnetic‐field lines and leave the machine with energy equal to their original beam energy plus the potential through which they have dropped. These diagnostic neutral beams often are composed of hydrogen or deuterium, and they can have components with energy less than the full beam energy because of the breakup of diatomic and triatomic molecules. The Franck–Condon energy released in this breakup can create a considerable energy spread in these lower‐energy components. Here experimental results for this spread are summarized, obtained while using neutral beams for plasma‐potential measurements in the Tandem Mirror Experiment‐Upgrade (TMX‐U). These energy‐spread results agree with theoretical estimates. This inherent energy spread is emphasized because it can be important in interpreting plasma‐potential data obtained with neutral‐particle‐beam probes and because its existence is not always anticipated.