Formation of stable inverse sheath in ion-ion plasma by strong negative ion emission

The effect of strong charged particle emission on plasma-wall interactions is a classical, yet unresolved question in plasma physics. Previous studies on secondary electron emission have shown that with different emission coefficients, there are classical, space-charge-limited, and inverse sheaths. In this letter, we demonstrate that a stable ion-ion inverse sheath and ion-ion plasma are formed with strong surface emission of negative ions. The continuous space-charge-limited to inverse ion-ion sheath transition is observed, and the plasma near the surface consequently transforms into pure ion-ion plasma. The results may explain the long-puzzled experimental observation that the density of negative ions depends on only charge not mass in negative ion sources.