Some investigations of the ion acoustic wave problem in collisionless plasmas. (Thesis)

A survey is presented of the Risø studies of linear ion acoustic waves in a single-ended Q-machine during the period 1969-7H. Some of the main results are:
1) The construction of an electrostatic ion energy analyzer for determination of both unperturbed and perturbed ion velocity distribution functions.
2) Propagation properties of grid-excited pulses and waves were calculated analytically by means of Green's functions. It was concluded that a single Landau-mode does not constitute a satisfactory description of the wave propagation close to the grid. This was confirmed experimentally.
3) It was shown that any density wave pattern obtainable as a solution to the Vlasov equation with the collective term included can also be obtained with purely freely streaming ions alone.
4) Clear-cit evidence of collective interaction in connection with density pulses was found by measuring the perturbed ion velocity distribution function.
5) It was concluded that the damping of ion acoustic waves can best be described by a phase mixing of almost freely streaming ions. The wave potential energy in the electron fluid is absorbed by the bulk of the ions in the background plasma; the absorption spectrum is centered around the phase velocity but has a rather wide shape.