A transport study of high density helicon plasma discharges in the Prototype Material Plasma Exposure eXperiment
Date of Award
Doctor of Philosophy
David C. Donovan
Theodore M. Biewer, Maik K. Lang, Zhili Zhang
The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) at Oak Ridge National Laboratory is instrumental in demonstrating the plasma source capability for the steady-state MPEX facility to study plasma-material interactions (PMIs). Proto-MPEX has justified MPEX by examining the efficiency of the helicon source, and auxiliary electron Bernstein wave (EBW) heating and ion cyclotron heating (ICH). This thesis aims to address the particle transport from the source toward the target in Proto-MPEX. The governing transport phenomenon in Proto-MPEX is identified using the electron temperature and density, Mach number, and other derived quantities. Extensive diagnostics coverage along the axial length of the device provided various plasma parameters. The upgraded Thomson scattering system and Mach-Double Langmuir probe were used for the study.A small variation in the axial temperature for both helicon-only and helicon with an auxiliary heating system indicates that Proto-MPEX operates in the sheath-limited regime. The presence of strong parallel convective heat along the axis, except near the source, was experimentally measured and predicted using a data-constrained B2.5 Eirene model. With EBW, the conductive heat flux increased around the launcher but was not found to be significant in comparison. With a small temperature gradient and highly conductive flux, Proto-MPEX plasma transport is suggested to be sheath-limited. Increasing collisionality and density at the target can induce conduction-limited regime in Proto-MPEX. During the efficiency study of the transport with EBW heating, a flat or downhill magnetic field from the heating location to the target was found to be preferential to deposit higher heat flux on to the target.The presence of various plasma sources creates a complex magnetic field geometry in Proto-MPEX. Such field variations create magnetic mirrors, which manifests Gas-dynamic trap for low energy and adiabatic-kinetic trapping for high energy plasmas. Experimentally, a linear increase in trapped density was observed with the mirror ratio for the low temperature population. Temporal profiles from flow measurement with ICH show plasma slow-down and the possibility of flow reversal upstream of the ICH resonance location. Discussions on some of the implication for the MPEX user facility is also presented.
Kafle, Nischal, "A transport study of high density helicon plasma discharges in the Prototype Material Plasma Exposure eXperiment. " PhD diss., University of Tennessee, 2019.