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

Abstract

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 fpi, 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 (f ≳ fpi) 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.