Co-excitation Of Normal Modes And Burst Ion Signal In An Ion-beam-plasma System

The study of waves is one of the most basic and most important problems in plas-mas.In the past years,the normal modes in beam-plasma systems and the wave-particle interactions have been widely studied in theory and experiments.However,little s-tudy on the simultaneous excitation of normal modes and pseudo waves in an ion-beam-background plasma system was made.In order to identify the co-excited waves in the ion beam-background plasma,the experiment was conducted in the double plasma device in which two separation grid-s(used to form and control the energy of the ion beam)was inserted in the experi-mental chamber,a small grid,on which a ramp voltage with adjustable rise time and peak-to-peak amplitude was applied,was used to excite waves in plasma.By studying dependence of the received wave signals on the applied ramp signal and the ion-beam energy,we can identify the types of the excited waves.1.As the rise time and peak to peak voltage of the signal applied on the small grid change,the phase velocity of the normal modes is not varied when the rise time and peak to peak voltage of the signal applied on the small grid is changed,the phase velocities of the normal modes do not vary.However,the phase velocity of the pseudo-wave(burst-ion signal)increases with the decrease of the ramp rise time and the increase of the peak-to-peak voltage.Using this property,the burst-ion signal can be identified from the received waves.2.Among the remaining signals,the two of them,of which the phase velocities increase with the increase of the ion beam energy,which is a characteristic of the beam modes,were identified as the fast and slow waves.Finally,one signal with the phase velocity(lowest among the received signals)changes little with the change of the ramp signal and beam energy was identified as the ion-acoustic wave.