The ion-ion instability is excited in an ion beam plasma system. It is
found that the ion-ion instability is effectively suppressed by RF pu
mps with frequencies dose to or higher than the peak frequency of the
instability. These results are compared with the prediction of the Van
der Pol equation to clarify the physical mechanism of the suppression
of the ion-ion instability by the RF pumps. It is shown that the supp
ression mechanism around f(ex)/f(ii) similar or equal to 1 is synchron
ous suppression whose tendency is similar to the prediction of Van der
Pol equation, and the mechanism at f(ex)/f(ii) > 1 is due to the pert
urbation of the ion beam energy caused by the RF pumps.