Weak double layers

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Abstract
The characteristics of weak double layers with potential steps e Delta phi /Te approximately=5 are studied in a triple plasma device and compared with a kinetic model. The double layers studied differ from those previously investigated in two significant ways; first, the double layers are much weaker and second, trapped ions play a much less important role. In this study two plasmas at different potentials but with similar characteristics are separated by grids from a target chamber so that ions enter only at the high potential side and electron beams enter at the low potential side. In addition, thermal electrons enter at the high potential side and are trapped by the double layer. The authors were able to produce very one-dimensional laboratory double layers, and they explain many of the features of the laboratory double layers using a one-dimensional kinetic model. The model calculations illustrate the role of the trapped ions in determining the position of the double layer, and they show the dependence of the length of the double layer on the entering ion and electron fluxes. Ion beams were found to have velocities much greater than the Bohm velocity. In general, the authors find that the characteristics of weak double layers are more sensitive to the boundary conditions.

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