An examination of the effects of neutral particles on the edge plasma in Tokamaks

The recycling of neutral atoms in the plasma edge is an area of increasing interest in Tokamak plasma physics. Strong evidence has been presented recently that suggests there is a strong link between neutral particles and the overall confinement of the plasma. Experimental studies of neutral particle dynamics [1] and penetration into the core plasma [2] have stimulated investigations of the effects of neutrals upon edge phenomena such as MARFEs [3, 4] (Multifaceted Axisymmetric Radiation from the Edge), the L-H (Low-to-High Mode) transition [5--7], the structure of the edge pedestal [8, 9] in H-mode plasmas, and the interpretation of thermal transport coefficients from measurements of edge density and temperature gradients [10]. One such piece of evidence (and major motivation for our research) comes from a series of experiments carried out on the Tokamak TEXTOR that qualitatively demonstrates there is a direct link between density limits and neutral particle density in the plasma edge. The details of this experiment are found in Chapter 2 of this thesis.;Many studies have focused on the investigation of edge pedestal fueling by neutrals produced from ions recycling from edge plasma material surfaces. At issue is whether the edge pedestal is fueled primarily by neutrals recycling from ion fluxes incident on the divertor target plates, as has long been assumed, or by neutrals recycling from the main plasma chamber wall produced by radial convective ion fluxes in the SOL (scrape-off layer). Evidence for both divertor recycling [11, 12] and main chamber recycling [13--15] stimulated an extensive data collection, analysis and modeling effort for a series of DIII-D discharges [16], which concluded that divertor recycling was the primary fueling mechanism in DIII-D.;One purpose of this thesis is to make use of this extensive data compilation/analysis and background plasma calculation database that has been established for this series of DIII-D discharges for a detailed analysis of neutral particle recycling and edge pedestal fueling in an H-mode Tokamak plasma with the 2D neutral particle transport code GTNEUT [17, 18]. We have chosen the deterministic GTNEUT code instead of the more familiar Monte Carlo codes such as DEGAS2 [19] and EIRENE [20] which are frequently used with plasma fluid codes such as SOLPS [21] and UEDGE [22] for neutral recycling calculations in Tokamak plasmas because of the difficulty of obtaining sufficiently good statistics to calculate accurate neutral profiles with Monte Carlo (see e.g. Fig. 17 in Ref. [21]). The GTNEUT code is discussed in detail in Chapters 3 and 4. The ion flux measurements used in this analysis are discussed in Chapter 5. The neutral particle recycling and pedestal fueling calculations are discussed in Chapter 6. Analyses of the relative importance of neutrals recycled at different spatial locations and discussion of the calculational uncertainties are also presented in Chapter 6. Suggestions for improvements to the GTNEUT code are briefly discussed in Chapter 7. A brief summary and conclusions are presented in Chapter 8.