The results of the interaction of a high energy ion beam with its self-created plasma in an axial magnetic field is presented. The authors observed unstable waves propagating in the azimuthal direction and identified it as the pure ion Bernstein waves from a study of its propagation characteristics. The experimental results are compared with the linear theory of electrostatic instabilities in a plasma whose ion energy distribution function is a mixture of a Maxwellian background and a delta -function component. Propagation of the unstable waves in a direction opposite to the direction of ion Larmor rotation is observed which corroborates the theoretical result in presence of a radial density gradient.