Ionization induced instability in an electron collecting sheath

A numerical investigation of the electron collecting sheath about a positively biased probe in the ionosphere has been conducted to determine the effect of ionization within the sheath. This report is directed at the space charge limited regime appropriate to objects with characteristic size much larger than the ambient Debye length, and uniform surface potential much greater than the ambient plasma temperature. A fluid approximation is employed to model the dynamics of the secondary plasma produced within the sheath. The results indicate that when the secondary ion production exceeds a critical level, the sheath edge propagates outward in an explosive fashion, in a manner similar to the propagation of double layers. This critical level is shown to be equivalent to a mean free path for ionization by electrons lambda sub ie < (sq. rt. (m(sub i)/m(sub e))) (D(sub S)/alpha) where D sub S is the sheath thickness and alpha is a geometry dependent parameter. Usual ionosphere-probe interactions appear to be sub critical, but an enhanced neutral background might be produced by shuttle contaminants, could reach the critical level, and trigger the instability.