Effects of Electron Emission on Plasma-Surface Interaction

Most models of sheaths~facing emitting surfaces~invoke assumptions that~the sheath is time-independent,~the~wall potential is negative, ions enter~the~sheath at Bohm velocity, the presheath is weakly affected, and one wall is considered [1]. We present theory and PIC simulations showing that these assumptions can break down in practice. When emission is strong, the sheath potential can become positive, repelling ions from the wall [2,3]. Emitted electrons~entering the plasma~can drastically affect the presheath structure too. If their mean-free-path is large, emitted electrons can transit~the~plasma and~impact~the opposite wall; hence~wall charging becomes a complex global problem [4]. Secondary emission can trigger sheath instabilities~preventing plasma-wall systems from reaching steady state [5,6]. Implications are discussed for tokamaks, Hall thrusters, dusty plasmas, hot cathodes, RF discharges and spacecraft. [1] G.D. Hobbs and J.A. Wesson, Plasma Phys. 9, 85 (1967). [2] M.D. Campanell, A.V. Khrabrov and I. D. Kaganovich, Phys. Rev. Lett. 108, 255001 (2012). [3] M.D. Campanell, submitted to Phys. Rev. E (2013). [4] M.D. Campanell and H. Wang, submitted to Appl. Phys. Lett. (2013). [5] M.D. Campanell, A.V. Khrabrov and I. D. Kaganovich, Phys. Rev. Lett. ~108, 235001 (2012). [6] M.D. Campanell et al., Phys. Plasmas 19, 123513 (2012).