Observation of ion-acoustic waves in two-ion-species plasmas

Coexistence of two modes of ion-acoustic waves in two-ion-species plasmas is investigated both theoretically and experimentally. Two ions are combinations of H₂, He, Ne, Ar and Xe. The boundary of existence of two modes in the plane of an electron–ion temperature ratio and heavy–light ion mass ratio is obtained from the two-species ion-acoustic dispersion relation with a real frequency and a complex wavenumber. The boundary is given approximately by T_e/T_i = 2M_h/M_l, where T_e and T_i are temperatures of electrons and ions, respectively, and M_h and M_l are masses of heavy and light ions, respectively. The experiment is perfomed with a double-plasma device. Ion-acoustic waves are transmitted by applying a sinusoidal pulse to the anode of the source plasma. A single mode is observed for Ne–Ar, He–Ne, Ne–Xe and Ar–Xe plasmas and two modes are detected for H₂–Ar, H₂–Ne and He–Ar plasmas.