The role of self-consistency in double layer calculations

In a number of calculations involving nonlinear structures such as double layers and solitons, the Korteweg-de Vries theory is applied beyond its range of validity or approximations are made which are not self-consistent. Some situations arising in the theory of double layers in fluid multispecies plasmas are pointed out and attention is drawn to arbitrary-amplitude numerical calculations which yield results that are physically very different from those of the approximate theories. In particular, severe restrictions on the occurrence of ion-acoustic double layers in negative-ion plasmas and of electron-acoustic double layers in two-electron-component plasmas are found