Investigation of ion collision effect on electrostatic sheath formation in weakly ionized and weakly collisional plasma

The effects of ion collisions on plasma-sheath formation are investigated experimentally for a low-density and low-pressure discharge. The space potential and ion velocity distribution measurements at high spatial resolution show that the ion collision properties observed in the presheath are maintained in the plasma-sheath transition region. The potential drop in the transition region indicates the existence of ionization as an effect of ion collisions in the transition region owing to the non-negligible density of the electrons penetrating the sheath. Based on comparisons between the space potential measurements and Riemann's presheath-sheath transition solution, the ion collision length lambda (i) was determined as a key parameter in the presheath and transition region. And it represents that the thermal properties of ions and neutral gases affects space potential by the charge exchange and ionization collisions. The existence of the ion collision effect in the transition region suggests possible influence on the incident conditions of ions and electrons near the sheath edge. Consequently, the energy distributions of ions and electrons incident on the material surface facing the sheath are sensitive to the collisionality and operating conditions.