Effect of asymmetric secondary emission in bounded low-collisional E × B plasma on sheath and plasma properties

Abstract

In a weakly-collisional bounded plasma, secondary electrons emitted from the walls can transit the plasma gap and reach the opposite wall. To accurately predict wall potentials and energy fluxes, one must account for this effect in the flux balance. We present 1D3V particle-in-cell simulations of a plasma slab in an E × B field where the emission yield is different at each wall and varied in a wide parameter range. Analytical formulas are derived to explain the dependence of wall potentials on the emission from both walls. When emission yields are asymmetric, the wall with a weaker emission charges more negatively to compensate the emission imbalance. We show that a small imbalance of emission can lead to relatively large wall potential changes. Moreover, details of secondary electron emission energy distribution are important to take into account for correct calculation of this effect; for example, a small fraction of energetic elastically backscattered secondary electrons can further enhance the wall potential difference. Both walls are usually negatively charged by plasma. But for certain magnitudes of the E × B field, a distinct state appears with a positive surface charge at one wall and a negative surface charge at the other, causing a drastic change in the plasma properties.