ION-ACOUSTIC SOLITONS IN A WARM PLASMA INCLUDING NEGATIVE-IONS IN THE VICINITY OF CRITICAL DENSITY

Ion-acoustic solitons in a warm plasma with different adiabatic ion constituents (with the same temperature) and isothermal electrons in the vicinity of critical negative ion density are studied by use of the reductive perturbation theory. The basic set of equations is reduced to an evolution equation which includes quadratic and cubic nonlinearities. The effective potential of this equation agrees exactly, for small wave amplitudes, with the Sagdeev potential obtained from the exact stationary solution of the original fluid equations. This implies that the evolution equation holds not only in the vicinity of the critical ion density, but also in the whole range of the negative ions under the condition of small wave amplitude.