Criteria governing ion-acoustic waves in two-ion plasmas

It has previously been observed that ion-acoustic waves exhibit two types of behaviour as the proportion of light ions in a mixed plasma is varied. Either the principal mode of the heavy ion plasma changes continuously into that of the light ion plasma, or it becomes too heavily damped to be observable while a second wave appears and develops into the principal light ion mode. A similar phenomenon is observed as the wavenumber or the ratio of electron temperature to ion temperature is changed. Criteria determining which behaviour occurs are derived by considering the form of the dielectric response function. The results may also be used to predict which of the higher-order modes of the plasma is involved in the process, and whether degeneracies exist in the dispersion relation.