Grid Excitation of Ion Waves at Frequencies above the Ion Plasma Frequency

The excitation of ion waves is discussed on the basis of a classical model and experimentally studied in a cesium plasma produced in a single‐ended $Q$ machine, when a small voltage is applied on a grid immersed in the plasma. For frequencies smaller than the ion plasma frequency $f_{\mathrm{pi}}$ , an ion acoustic wave is observed which is consistent with the predictions of the classical linear kinetic theory for a Maxwellian plasma. For higher frequencies ${\mathrm{(f\hspace{0.17em}\gtrsim \hspace{0.17em}f}}_{\mathrm{pi}})$ two components are observed. The ion acoustic wave, which propagates for frequencies higher than the ion plasma frequency and a faster signal, called a “psuedowave” which is in satisfactory agreement with the ballistic linear contribution of ions to the voltage perturbation in the plasma.