Plasma-Wall Interaction with Strong Electron Emission Revisited

Half a century ago, Hobbs and Wesson derived a solution for the plasma sheath at a planar surface with emission coefficient $\gamma $ [1]. They predicted that the floating sheath potential remains negative when $\gamma $ \textgreater 1. Variations of their ``space-charge limited'' (SCL) sheath model have long been used to estimate the particle and energy fluxes at strongly emitting surfaces [2]. Recent theory, simulation and experimental studies show that another plasma-wall equilibrium is possible when $\gamma $ \textgreater 1. In the ``inverse regime'' [3], the sheath potential is positive, repelling ions from the wall. The quasineutral density gradient and force balance in the ``inverted presheath'' are much different from the Bohm presheaths contained in the SCL models. It turns out that a SCL plasma-wall equilibrium is only stable under the assumption of zero ionization inside the sheath. Otherwise, the cumulative trapping of new ions in the SCL's potential ``dip'' will force a transition to the inverse regime [4]. It follows that only an inverse equilibrium should be possible in practice at floating surfaces with strong secondary, thermionic or photoelectron emissions. Applications will be discussed. [1] G. D. Hobbs and J. A. Wesson, Plasma Phys. \textbf{9}, 85 (1967) [2] (review) S. Robertson, Plasma Phys. Control. Fusion \textbf{55}, 093001 (2013) [3] M. D. Campanell, Phys. Plasmas \textbf{22}, 040702 (2015) [4] M. D. Campanell and M. V. Umansky, Phys. Rev. Lett. \textbf{116}, 085003 (2016)