Impurity Profiles in the Diii-D Tokamak.

This thesis analyzes impurity profiles in the DIII-D tokamak. Impurity emissions are studied in magnetically confined fusion research because they are an important component of the overall energy balance. Impurity studies are closely related to issues such as plasma resistivity and energy transport. Two novel, complementary methods were developed to determine the distribution and content of impurities in the DIII-D tokamak. The first method employed a spectrometer designed to record the simultaneous spatial profile history of many spectral lines. An extreme ultraviolet spectrograph diffracts the plasma light from many viewing chords into the plasma so as to record spectra from each chord at the same time. This spectrometer was used to study DIII-D plasmas from October 1986 through July 1988. Applications include the study of rapid redistribution of impurities during discharges exhibiting enhanced energy confinement ("H-mode"). The spectrometer was also used characterize the general impurity characteristics of DIII-D plasma; the dominant impurities in the tokamak are carbon and nickel, which are primary constituents in vacuum vessel wall materials. The second method of impurity profile analysis combines measurements from several (non-spectroscopic) diagnostics to determine impurity concentration profiles. The measured local electron density and temperature, local average ion charge (Z_{eff }), and local radiative emissivity are related by a model to the local carbon and nickel concentrations. The concept used is simple and should be applicable to other tokamaks; however, this research is the first to be reported using this scheme to determine absolute impurity concentration profiles. Application of this method to impurity studies of H-mode discharges of DIII-D has provided some surprising results. As opposed to the dramatic impurity accumulation in the plasma core that has been observed on other tokamaks, in high plasma current (2.0 MA) discharges on DIII-D, central impurity levels remain low (<0.02%) throughout the H-mode, and are lower than those seen in L-mode discharges. An important factor is the reduction in edge impurities by edge localized modes (ELMs). H-mode as obtained with electron cyclotron heating and co-injected neutral beams (NBI) at equivalent power levels have the same global level of impurities, but the distribution is significantly more hollow with co-injected NBI.