Effect of particle-induced electron emission (PIEE) on the plasma sheath voltage

This paper investigates the effect of ion- and electron-induced electron emission from a material wall on the voltage drop across the adjacent plasma sheath ('plasma sheath voltage' (PSV)). For this purpose, a new model involving a collisionless kinetic sheath consistently coupled to a fluid presheath is developed. The underlying analysis is valid for plasmas (both magnetized and unmagnetized) in which the Debye length is much smaller than the relevant characteristic presheath length ('asymptotic two-scale limit'). Material boundaries of particular interest are first walls and divertor target plates bounding magnetically confined fusion plasmas. Majority and impurity ions accelerated from the bulk plasma towards the material boundary release electrons flowing back into the plasma, thus giving rise to a lower PSV than without electron emission. In addition, sufficiently fast electrons from the plasma impinging on the bounding wall produce secondary electrons and are also partially reflected. The present work represents a first step in which the unmagnetized case is considered and electron reflection at the wall is still neglected. Considering typical boundary–plasma conditions and characteristic particle-induced electron emission (PIEE) data (i.e. electron yields and energy distributions), the PSV is self-consistently calculated by means of the new sheath model, showing appreciable effects of PIEE.