The nature of an electron sheath in steady-state, low-pressure plasma

Abstract

Summary form only given. Electron sheaths are electric fields that form near plasma boundaries to preserve quasineutrality in the bulk plasma by reducing the positive ion current lost to a boundary. Previous conjectures hypothesize that electron sheath formation near a positive anode is a solution to the current balance when the ratio of the anode area, Aa, to the chamber wall area, Aw, is less than the square root of the ion to electron mass ratio, Aa/A w<(me/mi)1/2. However, we show that the general nature of an electron sheath exhibits a potential dip which allows the electron sheath solution for a larger anode than is predicted by this relationship. Potential dips have been observed in electron sheaths, but were attributed to an ion pumping mechanism to an insulator present near the probe. We show that the dip is necessary even when no insulator is present and that the ion pumping needed for a steady-state potential well to exist is provided naturally by the plasma potential adjusting to decrease from the well bottom to the chamber wall. Data were taken for low-pressure argon plasma generated by hot filaments and confined in a multidipole chamber. Bulk plasma density, electron temperature and potential were measured with a planar Langmuir probe. Plasma potential profiles in the sheath regions were measured with an emissive probe operated in the limit of zero emission