Bohm sheath criteria and double layers are investigated in general
plasmas containing a number of lighter, thermal species, described by Boltzmann
distributions, in the presence of a number of heavier species described
fluid theory. A nonlinear description for one-dimensional stationary self-similar
electrostatic modes is given using the Sagdeev potential. We generalize
Bohm sheath criteria by giving appropriate multispecies definitions of
ion-acoustic velocities, leading to a global supersonic condition. We also
plasmas with only one Boltzmann species, to show that double layers cannot
supported at all, no matter how many cold fluid species are present, and
do so by
analytical rather than numerical means, based on the full Sagdeev potential.
expansions for weak double layers occurring in the literature can only
claim to find
double layers, erroneously as we indicate, by not confirming whether the
consistency relations are obeyed, and also by not checking that
‘neglected’ terms in the expansion overwhelm those retained.