Observations are carried out in a double-plasma device where the instabilities are excited around the negatively biased grid when there is sufficient plasma density difference between the two chambers and an ion beam is injected from the source to the target section. If the density difference between the two chambers is slowly decreased, the instabilities exhibit oppositely different characteristics after a critical value. Below the critical value of the density ratio, the energy resonance of the background ions and the beam ions is a typical condition for 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. ©1997 American Institute of Physics.