Observations are carried out in a double-plasma device where the insta
bilities are excited around the negatively biased grid when there is s
ufficient plasma density difference between the two chambers and an io
n beam is injected from the source to the target section. If the densi
ty difference between the two chambers is slowly decreased, the instab
ilities exhibit oppositely different characteristics after a critical
value. Below the critical value of the density ratio, the energy reson
ance of the background ions and the beam ions is a typical condition f
or the excitation of such instabilities. Chaotic phenomena occur when
there is a hump in the plasma potential profile near the sheath edge.
Above the critical value of the density ratio, the instability occurs
due to the interaction of the three beams that arise due to asymmetry
of the sheath potential. (C) 1997 American Institute of Physics.