Plasma-neutral interactions as an energy sink in the edge of the Madison Symmetric Torus

Author / Creator

Norval, Ryan Joseph, author

Available as

Online

Physical

Summary

Reversed field pinch (RFP) plasmas are high density, moderate temperature plasmas, which efficiently utilize magnetic fields for fusion research and astrophysical studies. RFPs are operated with ei...

Reversed field pinch (RFP) plasmas are high density, moderate temperature plasmas, which efficiently utilize magnetic fields for fusion research and astrophysical studies. RFPs are operated with either a multihelicity (MH) or a quasi-single helicity (QSH) magnetic core. Core plasma confinement and heating is the main goal of RFP research. The influence of the plasma edge on the RFP energy balance has not been systematically studied. The Madison Symmetric Torus (MST), a large RFP device with a limiter, is ideal for studying the plasma-wall interaction (PWI) in the RFP edge. The RFP edge is a domain with a high fraction of neutral particles produced by neutralization of impinging plasma ions on the vessel wall and limiter. Neutral particles affect the plasma energy balance through the processes of dissociation, ionization, charge exchange, and radiation. In this work, boundary-viewing cameras are used to image the plasma edge. Absolute calibration of the camera system enables measurement of the D[subscript alpha] photon flux generated by PWI. Langmuir probes measure electron density (n[subscript e]) and electron temperature (T[subscript e]) in the edge. Core n[subscript e] and T[subscript e] are measured by an interferometer and a Thomson scattering diagnostic respectively. Knowledge of n[subscript e] and T[subscript e] is required to convert photon fluxes into particle fluxes by converting D[subscript alpha] atomic line emission intensities into particle fluxes using appropriate atomic data for excitation and radiative decay of the relevant line transitions. A helical bulge in the plasma pressure was discovered in QSH plasmas. The edge pressure maximum is phase-aligned to the magnetic mode in the plasma core domain. By referring to these experimental data from cameras and Langmuir probes, a three-dimensional (3D) plasma edge temperature and density was constructed and used to the fully model the 3D kinetic neutral particle model EIRENE. A method of comparison between modeled EIRENE images of D[subscript alpha] emission with experimental data served as a first detailed benchmark for MST. Synthetic images are compared to experimental images validating the EIRENE model. For the first time, 3D profiles of neutral density in MST are constructed using EIRENE. This fully 3D neutral distribution then enabled an investigation of the role of neutral particles to the RFP energy balance. Neutral particles account for a significant percentage of power loss in QSH plasmas. Neutral particles account for up to 30% of the power losses in the plasma edge domain of MST. The main fraction is established by electron-impact ionization and molecular dissociation events. The remaining fraction is dissipated by charge-exchange. By injecting gas directly into the helical bulge, a QSH plasma may be fueled at 74% efficiency compared to 52% fueling efficiency at other locations. The localization of the PWI in the QSH mode may be exploitable in future RFPs by designing divertor-like edges where particles can be effectively pumped out. In addition control of the PWI could reduce energy losses in QSH plasma by up to 20% of total input power. Understanding the magnitude of these losses and what drives them can lead to improved optimization of the RFP as a fusion device.

Show More