OBSERVATION OF BEAM-ENHANCED SHEATH INSTABILITY IN A DOUBLE PLASMA-DEVICE

The effects of ion beams on sheath properties are experimentally inves tigated in a double plasma device. The dispersion relations of the ion beam plasmas are measured by interferometer method. The low-frequency instability due to sheath around the negatively biased grid is found to be controlled by two parameters, namely the grid biasing voltage an d source anode biasing voltage. The instability is caused by the reson ant coupling of the three ion beams that arise due to asymmetry of the sheath potential. The sheath structure follows the Child-Langmuir law and the frequency of the instability is also found to be inversely pr oportional to the sheath thickness. Therefore, the transit time model is considered to explain the observed phenomena. The coupling between the beam and the oscillating component of the ions through the sheath enhances the instability growth which occurs mainly in the presheath r egion. The excitation of the instability occurs within certain range o f velocity ratio of different beam modes. (C) 1996 American Institute of Physics.