Plasma Boundary Interactions - I 1

A study has been made of additional factors describing plasma-boundary interactions. The theoretical analysis of the sheath has been extended to include a transverse plasma flow outside the sheath region. The sheath model has been made more general, and also reveals the importance of the accommodation coefficients at the boundary surface. The presence of ionization and collisions in the sheath has also been studied theoretically. The interpretation of the experimentally observed threshold in the tangential electrode drag as a function of transverse magnetic field and of arc current has suggested the possibility of a threshold in the tangential accommodation coefficient. The threshold tangential energy appears to be proportional to the sputtering threshold and can be related to the threshold for displacement of surface atoms. The relationship between plasma phase velocity and particle velocity was studied and it was shown that there is not necessarily a simple connection: The related problem of arc retrograde motion (where plasma phase and particle motion have opposite signs) was studied and a simple model was proposed. It appears that the plasma propagates in the retrograde direction because of an increased net ion generation rate due to the inhibited diffusion loss of ions and electrons in the region of higher magnetic field. The importance of direct measurement of ion and atom velocity by means such as the Doppler shift is discussed.