Excitation characteristics of ion waves in a negative ion plasma

Excitation and propagation of ion waves in a negative ion plasma are investigated numerically and compared with the experiments which have been performed using Q-machine and multidipole de discharge device. The dispersion relation of the ion waves derived from conventional complex omega-plain procedure cannot explain previously observed simultaneous propagation of the fast and the slow mode. Spatial excitation coefficients of the slow mode, which are calculated from complex k-plain procedure, are comparable to that of the fast mode. Furthermore, if there exists a plasma ion flow which decreases damping of both modes, the spatial excitation coefficients of the slow mode will increase. From this point of view, the simultaneous propagation of both modes may be possible under a certain laboratory condition. Also the direct calculation of wave potential is presented. In the region not too close to the excitation point, the principal mode which propagates in a negative ion plasma is found to be the fast mode. In the case where T+=T-e and there is a finite ion flow, the interference pattern between the fast and the slow mode appears in the vicinity of the excitation point. Finally, we compare the results with laboratory experiments. Our numerical calculations qualitatively explain the previous observations of the slow mode.