Interpretation of Spatially Decaying Ion-Acoustic Waves

Detailed examination is given to circumstances surrounding the excitation and detection of spatially propagating ion-acoustic disturbances in a plasma. For relatively weak damping, a theory is presented to show the major influence on spatial signal evolution engendered by source coupling details, by source geometry, and by three-dimensional ion trajectories. In particular, near-zone and far-zone regions are identified for a disk source. Fresnel interference can dominate the near zone (within which many experiments are performed), so that serious questions can be raised as to the interpretation of observed decays. For relatively strong decays, the transition to the ballistic, three-dimensional, free-streaming solution is presented. Experiments are presented to illustrate some features of the theory, including measurements of Landau damping with 0.04 < ki/kr < 0.2 for 0.6 < ω/ωpi < 1.4. Observed Fresnel interference patterns are compared with theory.