New insights into sheath modes and current limitation in plasma diodes

Many low-temperature plasma sources and technological devices contain plasma between an electron-emitting cathode and an anode. Such "plasma diodes" can also manifest in tokamak divertors and negative ion sources used for fusion. In all plasma diodes, the plasma properties, electrode heating/erosion, and global current are coupled to the electrode sheaths. Understanding and controlling sheath phenomena remain important for ongoing research in applications of plasma diodes like plasma processing arcs, thermionic converters, emissive probes, Hall thrusters, and hypersonic vehicle transpiration. This tutorial will provide an instructive overview of the possible sheath modes, from historical background to recent advances. For many decades, conventional wisdom was built from reduced models of collisionless cathode sheaths. Such models predict that under increasing emitted flux, the classical Debye sheath transitions to a space charge limited (SCL) sheath with a virtual cathode potential well that blocks some emitted electrons and saturates the transmitted current. A deeper basic understanding of plasma diodes has more recently become possible by kinetically modelling the full potential distribution and current flow through the plasma and both electrode sheaths with collisions, see Ref. [1] and refs. therein. Full plasma diode simulations capture several new fundamental phenomena, e.g.: (a) local ion trapping in virtual cathodes enabling current enhancement far beyond the traditional SCL limit, (b) formation of inverse sheath modes with globally confined ions, (c) "backflow saturation" where the global current is limited below the emission even with a classical cathode sheath, and (d) mode transitions, instabilities and oscillations related to these sheath effects. The presenter will clarify the conditions for phenomena (a-d) to occur, review existing experimental evidence, explore innovative applications, and address open questions. [1] M. D. Campanell, C. Y. Wang, and K. L. Nguyen, Physical Review Letters 134, 145301 (2025).