Radial variation of the electrostatic plasma potential in an electron beam - plasma experiment and associated ion cyclotron gradient instability

Plasma instability is studied in very low density regimes in an electron beam - plasma experiment at the ion cyclotron frequency region. The electric field propagates azimuthally and exhibits an axial standing wave structure. The spatial distribution of electric potential was measured by Langmuir probes and its influence on the excitation of the instability was investigated. Potential profiles are very sensitive to the velocity distribution functions of ions and of electrons. The instability is driven by the convective effects associated with a plasma global rotation produced by the radial electric field. (D.Gy.)