Modeling the transition from a space-charge limited sheath to an inverse sheath*

It was recently discovered that charge-exchange collisions in a space-charge limited (SCL) sheath force a transition to the inverse sheath regime [1]. The collisions cause ions to become trapped in the potential well of the SCL sheath. The accumulating ions neutralize part of the charge in the sheath, collapsing the double layer region and leaving behind an inverse sheath. In experiments looking for a specific regime, it is important to consider the time scale of the transition relative to experimental time scales. Depending on the plasma properties, collision rates and emission parameters, the time required for a transition from the SCL to inverse sheath can span many orders of magnitude. We have developed an analytical model of the SCL-to-inverse transition and tested it against 1D-1V simulations. The model calculations involve additional analytic approximations of useful quantities including the total charge in the layers of a SCL sheath, the length of the virtual cathode, and the length of an inverse sheath. [1] M. D. Campanell and M. V. Umansky, Physics of Plasmas 24, 057101 (2017).