Propagation of argon- and helium-ion acoustic modes in an argon-helium plasma

Linear, nondispersive ion acoustic wave propagation in an argon-helium plasma is investigated in a double plasma device. The experimental wave patterns show that there exists a range of light ion concentration $\alpha$ where both argon and helium modes can be excited and propagate simultaneously, contrary to a previously published experiment of Nakamura et al. The phase velocity of the mode is only slightly affected by the presence of He ions. In the case of the He wave, it can be described by a reduced mass $M_r=\frac{M_{\mathrm{Ar}}}{[1-\alpha +\alpha (\frac{M_{\mathrm{Ar}}}{M_{\mathrm{He}}}\left)\right]}$, showing that the He and Ar ions follow an adiabatic equation of state. The observed increase in Landau damping of the Ar mode is related to the He resonant ions, whereas the damping of the He mode is mainly due to the reduction in the phase velocity of the wave.