Mach probe diagnostic for determining positive ion fluxes in H- ion sources

Abstract

In NNBI ion sources neutral hydrogen atoms and positive ions impacton low work function grid surfaces where they are converted to negative ions, andsubsequently extracted. Knowledge of the positive ions flux iscrucial for optimizing the negative ion yield, and also for benchmarking numericalmodels. To this end a Mach probe (MP) diagnostic can be used, which determines thedirection and magnitude of the positive ion flux. The plasma in ion sources is typicallyweakly magnetized, rendering the interpretation of Mach probe data difficult. Forthis reason a two step approach is chosen, where at first a previously used Machprobe is re-commissioned and applied in a well diagnosed and versatile laboratoryscale experiment. From these experiments important insights regarding the designof a new combined Langmuir- and Mach probe, its calibration procedure, and datainterpretation are obtained.