Electrostatic Ion Cyclotron Instability Due to an Ion Beam in a Plasma

Experiments were carried out in which a 3–5 keV helium ion beam was injected into a low-density helium plasma along a pulsed magnetic field. Low-frequency waves growing in the direction of the beam flow were observed for the magnetic field strengths of 300–2000 G, which were characterized by the oscillation frequency in excess of the ion cyclotron frequency but less than the ion plasma frequency. The wave mode was axisymmetric and the radial wavenumber was much larger than the axial one. Axial wavelength measurements on launched waves suggested that the phase velocity of the growing wave was nearly equal to the ion beam velocity. Most of the experimental results were found to be consistent with a simplified theory of electrostatic ion cyclotron instability in a slow-beam plasma system, which however predicted larger growth rate than that measured.