Collector Probe Measurements of SOL Impurity Accumulation and the Implications of SOL Flows on the Accumulation Amount

A collector probe in its simplest form is a rod inserted into a plasma so that impurities are deposited onto it. These probes are then removed and analyzed to determine the deposition profile both along the length of probe and across the width of it. This dissertation covers a series of collector probes experiments and accompanying interpretive modelling all with the main goal of providing evidence for long-hypothesized near scrape-off layer (SOL) accumulation of impurities that can lead to efficient core contamination. The structure of this dissertation is as follows. A brief outline of fusion energy and why we need it is given in Chapter 1. Chapter 2 goes over the basics of the SOL region in tokamaks, as well as the basics of impurity transport and collector probes. A brief history of collector probes is also presented. Chapter 3 presents collector probe results and trends from the DIII-D Metal Rings Campaign. This includes a description of the hardware and software used, a scaling law to determine what led to the most tungsten deposition on the probes, explaining asymmetries between the two probes faces by how far they were from the separatrix, interpretive 3DLIM simulations of deposition patterns that suggest W radially transports via convection in the far-SOL, and that a \simple" SOL prescription in the far-SOL is most appropriate. Chapter 4 presents a deep-dive analysis into two collector probes that were inserted for similar shots differing primarily in the toroidal magnetic field (BT) direction. It is proposed that the differences in the deposition profiles can be explained in the context of a near-SOL impurity accumulation only occurring in a particular BT direction. In the opposite BT direction, fast SOL flows may flush out impurities that would otherwise accumulate. This hypothesis is studied via DIVIMP and 3DLIM simulations. Chapter 5 introduces a set of methane injection experiments meant to build upon the results of Chapters 3 and 4, and to provide more convincing evidence of near-SOL accumulation. Preparation that went into the experiments and preliminary analysis is presented. Possible areas for dedicated analysis are described. Finally, Chapter 6 puts in context the contributions of this dissertation to the wider field of impurity transport in tokamaks before concluding.

Final version.