AN ANALYTICAL AND NUMERICAL INVESTIGATION OF ION-ACOUSTIC-WAVES IN A 2-ION PLASMA

The ion acoustic dispersion relation for a plasma containing two disti nct ion species is investigated over a wide range of plasma conditions . An approximate general analytic solution to the dispersion relation has been found, and is shown, by comparison to accurate numerical solu tions of the individual modes, to be remarkably precise. This solution provides for the first time a systematic account of the totality of i on acoustic modes of the two-ion system. It has been found that ion ac oustic modes consist of two types of modes: (a) at least one, and, at most, two weakly damped modes for which \omega(I)/omega(R)\ much-less- than 1, and (b) an infinity of critically damped modes for which omega (I)/omega(R) congruent-to -1. The critically damped modes are organize d into two distinct categories: (a) modes for which \omega\/k > upsilo n(F) (upsilon(F) is the thermal speed of the fast ion species); and (b ) modes for which upsilon(S) < \omega\/k < upsilon(F) (upsilon(S) is t he thermal speed of the slow ion species). The critically damped modes with \omega\/k > upsilon(F) are further organized into three distinct classes: (1) modes with phase speeds characterized by upsilon(F), (2) modes with phase speeds characterized by upsilon(F)upsilon(S)/square- root upsilon(F)2 - upsilon(S)2, and (3) modes with phase speeds charac terized by upsilon(S). The critically damped modes with upsilon(S) < \ omega\/k < upsilon(F) belong to a single class, and are characterized by upsilon(S). Generally, it is found that there are one, or, at most, two modes with relatively small damping, while most of the remaining modes are too strongly damped to be physically realized. It has also b een found possible to maximize the ion acoustic damping in a two-ion p lasma by a judicious choice of the relative ion concentrations. More s pecifically, an admixture of the lighter ion species will maximize the damping coefficient over a wide range of plasma conditions. This is i mportant for certain gas-filled, inertial-fusion targets of current in terest, where it is desirable to minimize the stimulated Brillouin bac kscatter process by maximizing the damping.