Small-amplitude positron-acoustic double layers

Small-amplitude positron-acoustic double layers (PA-DLs) in a four-component plasma model consisting of two-temperature positron, isothermal electrons, and immobile ions are investigated. Due to the relative particle fractions and the temperature ratio, our plasma model can admit compressive as well as rarefactive PA-DLs. As the relative number of hot positrons increases, the negative PA-DL shrinks and, beyond a certain critical value, develops into a positive structure allowing therefore the existence of compressive PA-DLs. This critical value is lowered as the relative number of hot electrons decreases. The critical Mach M-c number, above which only rarefactive PA-DL is observed, is shifted towards higher values as the relative amount of hot positrons becomes important. A decrease of the temperature ratio sigma=T-e/T-ph provides qualitatively the same results but with a net shift of M-c towards lower values.