Stopbands in fast ion-acoustic soliton propagation revisited

It was recently discovered that, in a multispecies plasma with two positive ion species (one cold and one warm adiabatic) and Boltzmann-distributed electrons, a “stopband” could occur, i.e., there was a range of velocities where no fast ion-acoustic solitons could propagate between two ranges where propagation was possible. Several extensions were subsequently investigated, including the effects of the cool ions having finite temperature and of nonthermal electron distributions. Efforts were made to estimate existence domains in plasma parameter space, often by ad hoc arguments, but the illustrations were invariably restricted to the same specific set of parameter values or their neighborhoods. In contrast, here, a systematic and structured study is given: physical arguments determine a range of compositional parameters so that a Sagdeev pseudopotential analysis establishes in a consistent way the various curves limiting the existence domains in parameter space. This is done for four models, namely, Boltzmann, nonthermal Cairns, superthermal kappa, and nonextensive Tsallis electron distributions, and for each, existence domains are plotted in the space of ion charge-to-mass ratio and inverse electron temperature, and detailed examples are presented. Contrary to reports in the literature, stopbands are shown to exist for large deviations from isothermality. However, their range in parameter space is shifted from that obtained for Boltzmann electrons. This establishes that the stopband phenomenon is robust and governed primarily by the cold and warm ion properties and the electron temperature, the form of the electron distribution having only a quantitative effect.