Investigation in improving the Cs-free negative hydrogen ion production with short-pulse low power in the afterglow of pulse-power-modulated plasma sources

Abstract Pulse plasma ion sources have been shown to be capable of achieving high negative hydrogen ion densities ( n H - ) or currents. The fast cooling of the electrons and reduction in the electron density ( n e ) provide favorable plasma conditions to improve the H − production, with a high density ratio of negative hydrogen ions to electrons during the pulse-off period. The purpose of this research is to further improve the performance of volume-produced H − ion sources based on the pulse power modulation. In this study, a time-modulated negative hydrogen plasma sustained by 10 kHz pulse power at the typical gas pressure of 0.3 Pa was numerically investigated using a global model. The model is compared with measurements obtained in a pulsed radio frequency ion source and shows a reasonable agreement. A steplike low-high power strategy in the active glow period and a short-pulse low power in part of the afterglow period are employed to mitigate overshoot of the electron temperature ( T e ) in the initial stage of the pulse period and to optimize H − production in the afterglow, respectively. The model predicts that a small-magnitude, short-duration low power operation in the afterglow increases n H - and decreases n e from the onset of the low power until the beginning of the next high power pulse, due to a modest temporary increase in the T e (less than 2 eV). It facilitates dissociative electron attachments and thus H − formation. The reduction in the ne indicates that positive ions lost to the walls cannot be compensated by weak ionization. H − production with a low number of co-extracted electrons can be optimized by adjusting the magnitude and duration of the low power in the afterglow.