Collisional plasma sheath and its effects on the charged particle flux towards the wall

Abstract

Understanding the flux of charged particles toward the wall is crucial as it affects the global properties of the plasma and serves as the basis for diagnostics using electric probes. However, recent theoretical studies suggest that the conventional theories regarding ion and electron flux may not be valid when ions or electrons have sufficient collisionality. In this dissertation, the effects of ion-neutral and electron-electron collisions on ion and electron flux towards the wall, respectively, are experimentally investigated. A versatile plasma device MAXIMUS (Magnetic X-point Simulator System) is utilized for the experiments in this study. We generated plasmas using hot filament cathodes in a cylindrical multidipole vacuum chamber. To advance plasma diagnostics, a new Langmuir probe diagnostic method is developed in this study, which applies power law parametrization of the effective ion collecting area. It is confirmed that our new method exhibits the best fit results among the existing methods. The edge effect area, which refers to the additional surface area of the sheath edge resulting from the finite sheath thickness, is found to be dependent on the neutral gas pressure. Theoretical works related to the modified Bohm criterion and edge-to-center density ratio (h-factor) indicate that ion-neutral collisions reduce the ion flux at the sheath edge. We discover that this ion flux reduction factor can be expressed as a function of the edge effect area, which we systematically scan using our planar Langmuir probe diagnostic method. The experiments are conducted for low to intermediate neutral gas pressures (< 100 mTorr). The experimental results demonstrate that the estimated ion flux at the sheath edge is consistent with predictions in accordance with works using particle-in-cell simulation within the margin of error for He discharges, and the dependence of the reduction rate on species is also discussed. The reduction of ion flux at the sheath edge at intermediate gas pressures highlights the importance of taking into account ion-neutral collisions in plasma applications concerned with plasma-wall interaction. Recent theoretical works on ion-acoustic instability and fluid approach to electrons suggest that the electron-electron collisions can be enhanced to the point where the mean free path of electrons falls within the sheath or presheath length scale, resulting in larger electron flux towards the wall compared to predictions from conventional theories. We employed an edge-effect reduced, guard-ringed Langmuir probe to investigate the unique features in the I-V characteristics that are anticipated to appear with collisional electrons. Additionally, emissive and cutoff probes are utilized to obtain accurate reference parameters for bulk plasma properties. However, the predicted modifications in the electron flux towards the probe, as postulated by the electron fluid theory, are not observed. The absence of these anticipated modifications suggests the absence of enhanced electron-electron collisions resulting from the instability. Therefore, it is suggested that further experimental observations be conducted to test these theories.