Plasma Wall Transition and Effects of Geometry in Presheath

Abstract

When plasma interacts with the wall of a conductor, electrons due to high mobility reach the wall first and develop negative potential on the wall and very near to the wall plasma is divided into sheath and presheath regions. The quasi-neutral plasma is shielded from the wall by a space charge sheath of the positive ions of the order of few electrons Debye’s lengths (λD) . At the sheath edge quasi neutrality breaks down from presheath side. In asymptotic limit ε=λD/L → 0 varying area of geometry affects the structure of the presheath scale. In addition to geometry, collisions and ionization also affects the presheath structure. But the sheath region is universal and is independent of either of geometry, ionization rate and collision frequency. The region which play the role of a link between these two regions has characteristics of both regions and is known as intermediate region. Even in the absence of ionization source and collision expanding area of geometry can accelerates the ions towards the wall. The characteristic length of the geometric presheath depends on radius of curvature c R = A/A′ , where “A” is the area of geometry and “A′= dA/dz”. If either of ionization or collisions is present along with the expanding area of geometry then dominant factor for the acceleration of ions in the presheath region is not the expanding area of geometry.