Interpretation of ion-acoustic wave propagation experiments by the multiple-water-bag model

Theoretical and experimental results concerning the linear ion‐acoustic wave propagation in the low‐density range ${\text{(∼\hspace{0.17em}10}}^8\hspace{0.17em}{\mathrm{cm}}^{\text{−3}})$ for a single‐ended $Q$ device are presented. For plasma densities less than 10⁹ cm⁻³, the ions have a beam‐like distribution. This allows one to observe two ion waves traveling in the direction of the plasma flow. Observed phase velocities and damping lengths are in good agreement with the theory if the actual ion distributions are taken into account in calculating the plasma response. The multiple‐water‐bag model presented allows one to compute the plasma response using the measured ion distribution. In the high‐frequency range of measurements ${\text{(0.5\hspace{0.17em}≲\hspace{0.17em}ω}}_0{\mathrm{/\omega }}_{\mathrm{pi}}\text{\hspace{0.17em}≲\hspace{0.17em}1)}$ the beating between the fast and slow ion waves is in agreement with the multiple‐water‐bag predictions where the excitation is described classically by oscillating charge sheets. In contrast to the experiments carried out at higher densities, it can be demonstrated that the macroscopic plasma response is clearly collective.