Dynamic model of a plasma structure with an intermediate double-layer, formed outside an anodic plasma contactor

The plasma structure around an electron-collecting, plasma contactor consists of two quasineutral regions separated by an intermediate, free double layer(DL). A dynamic model of this three-region structure is here established, based in a linear perturbation of the complete steady solution. Time-dependent conditions at the mobile DL are obtained; these consist of Bohm and Langmuir laws, jump conditions on the accelerated species and barrier conditions on the confined ones. Boundary conditions at the contactor surface and at the undisturbed, ambient plasma are analyzed in detail. The dynamic response to perturbations in plasma emission is the combination of a quasisteady electron mode and several ion–electron modes, with different characteristics in the core and presheath. Mode interaction takes place through the DL and the electric potential. At large frequencies, the DL displacement is restricted by ambient ions, which tend to be rigid. The stability of the steady solution is studied through a nonlocal dispersion relation. It is found that this plasma structure does not develop the radial, ion–electron (macro)instability, and, in particular, the Buneman instability.