When an external RF field is applied to an ion beam plasma system, ion-ion instability is suppressed at ʄ/ʄii≥l, where ʄ and ʄii are the frequency of the external RF field and the peak frequency of the instability, respectively. It is also observed that the time-averaged energy distribution function of the ion beam broadens when the instability is suppressed. In order to investigate the cause of this broadening, time-resolved energy distribution functions of ions are measured with a boxcar integrator. As a result, it is found that the energy distribution functions periodically change with the external RF field and the above broadening is apparently caused by the time-averaging of the energy distribution functions.
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