In an ion-beam–inhomogeneous-plasma system, when the drift frequency
ω* exceeds a harmonic of the ion cyclotron frequency nωci, ion cyclotron harmonic
waves are excited via the plasma inhomogeneity. These waves are strongly enhanced
by ion-beam injection parallel to the magnetic field. As the plasma inhomogeneity
is increased, the number of unstable modes increases as does their amplitude. Ion
cyclotron harmonic waves with frequencies up to 4ωci are observed experimentally;
at least five modes become unstable, and these unstable waves have different az-imuthal
mode numbers l = 1 or 2. The dispersion relations of observed multiple ion
cyclotron harmonic waves agree well with the theoretical ones, taking into account
the plasma inhomogeneity and the ion beam.