Observation of beam-enhanced sheath instability in a double plasma device

The effects of ion beams on sheath properties are experimentally investigated 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 and source anode biasing voltage. The instability is caused by the resonant 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 proportional 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 region. The excitation of the instability occurs within certain range of velocity ratio of different beam modes.