Deflected-Beam-Excited Ion-Cyclotron Beam Modes

The electrostatic ion-cylcotron instability driven by a perpendicular (to the axial magnetic field) ion beam was observed in the University of Iowa MagPi II device. The perpendicular ion beam was produced by deflecting the injected parallel ion beam with a sufficiently positively biased Langmuir probe. The characteristic of the wave can be approximately described by a dispersion relation which is based on the fluid theory. The onset voltage of the instability was found to be a good measure of the incoming ion beam energy. A method which utilized the onset of this instability as an ion beam diagnostic also is proposed.