Two more classes of monotonic double layers complementing the class of beam-type double layers are investigated analytically, and their range of existence is explored in the small amplitude limit. One class preferentially exists for hot ions and electron drifts of the order of electron thermal velocity. The second one, instead, assumes hot electrons and needs almost current-free conditions. The first class, called SEADL, is based on the slow electron acoustic branch and exhibits a tuning-fork configuration in the electron phase space. Its density decreases with increasing potential. The second one (SIADL) rests on the slow ion acoustic branch and. consequently, has a tuning-fork pattern in the ion phase space. Its density increases with the potential. Both classes are found to be linearly stable with respect to one-dimensional, but unstable with respect to two-dimensional electrostatic perturbations. A comparison with experiments suggests an identification of the second type with the double layers obtained by Hollenstein
Asymmetric one-dimensional slow electron holes
