Ion oscillations in the sheath around a negatively-biased grid separat
ing the source and target chambers of a double plasma machine are mode
led using a one-dimensional hybrid simulation. A strong, coherent osci
llation is found with characteristics matching those seen experimental
ly. The oscillation grows when the frequency of a virtual anode oscill
ation in the target chamber matches the bounce frequency for ion bunch
es in the sheath. (C) 1997 Elsevier Science B.V.