Development of new-type negative ion sources using powdery samples

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To develop new-type negative ion sources using powdery samples instead of gases or liquids, compact ion sources designed and manufactured by us were examined with many samples under various conditions. Namely, a small amount (-5 mg) of powder deposited directly on a fine mesh or a metal plate was bombarded by electrons (-100-300 eV, -1-3 mA/0.05 cm^2) emitled from a hot tungsten wire. By this method, such ions as D^-, Li^-, F^-, Na^-, Cl^-, Ag^- and O^- were successfully produced from those powdery samples of LiD, LiI, LiF, NaF, NaCl^-, AgCl and BaO, respectively. Especially, saline hydride (e.g., SrH_2) on a boat-type ribbon heated to -700-900 K was useful for producing H^- by both thermal and election-stimulated desorptions. In this case, electrons were supplied from the heated sample itself without using a special election gun. In other wards, the sample served as a triple source supplying H^-, e^- and SrH_2 (sublimated onto a surrounding slit plate). Both thermal desorption energy of H^- and work function of those active spots created by hydrogen release from the sample were determined by a stepwise or temperature-programmed heating method. The active spots consisting of alkali or alkali-earth metal were destroyed by admission of H_2 or O_2, but recreated by stopping the admission. Theoretical analysis of these results yielded a simple model explaining the mechanism of the thermal desarption. The currents of thee above ions after mass analysis were -10^<-12> - 10^<-9> A/0.05 cm^2 (apparent area of the emitting sample surface) and was constant in time to within *5% for more than -10h.In condusion, our compact ion sources (-2 cm^3) are usable for producing various negative ions with little release of gases during operation. Much further work, however, is needed to find the best powdery sample and experimental condition for producing a more stronger and stable beam for every ion species of interest.