Ionization enhanced ion collection by a small floating grain in plasmas

It is demonstrated that the ionization events in the vicinity of a small floating grain can increase the ion flux to its surface. In this respect the effect of electron impact ionization is fully analogous to that of the ion-neutral resonant charge exchange collisions. Both processes create slow ion which cannot overcome grain' electrical attraction and eventually fall onto its surface. The relative importance of ionization and ion-neutral collisions is roughly given by the ratio of the corresponding frequencies. We have evaluated this ratio for neon and argon plasmas to demonstrate that ionization enhanced ion collection can indeed be an important factor affecting grain charging in realistic experimental conditions.