Behavior of the fast cyclotron wave in a nonuniform magnetic field

The fast cyclotron wave becomes unstable and is excited in a spiral electron beam-plasma system when the perpendicular energy component of the beam is sufficiently large. When a nonuniform magnetic field is applied to the system, the cyclotron frequency as well as the parallel velocity component of the beam vary spatially. It is confirmed experimentally, that the variation affects the excited wave and results in a spatial variation of its wavenumber in the way predicted by the dispersion relation of the fast cyclotron wave.