UC San Diego

In this dissertation, we investigate two proposed features aimed at improving the performance of the divertor exhaust system in tokamak nuclear fusion devices. Specifically, we assess the influence of the "inverse" plasma sheath and long-leg divertor configurations on the divertor plasma using numerical simulations. First, we investigate the "inverse" plasma sheath, which has been suggested to prevent the flow of ions to the wall and promote divertor detachment. We use the UEDGE code to simulate the physics of both the inverse and standard (Bohm) sheath regime at the divertor targets. Our results show little difference in the overall plasma state, but an increase in electron heat flux to the divertor targets. Additionally, we observe a bifurcation behavior related to plasma recombination effects, and present an analytical model of this behavior. Second, we evaluate the long outer divertor leg, which is designed to increase volumetric dissipation, enhance turbulence spreading, and extend the distance between the material surface and the fragile core plasma. Using the SOLPS4.3 code, we assess the transition to the detached divertor regime, with scans on plasma density, transport coefficients, and multiple impurity species. Our results show a significant contribution to the energy balance from cross-field transport to the side walls and considerable recycling of impurities along the long leg, enabling delocalization of the radiated heat flux. Overall, this dissertation provides important insights into the physics associated with these proposed divertor features. The results suggest that the inverse plasma sheath may not have a significant impact on divertor plasma detachment, while the long-leg divertor configuration has potential to improve divertor performance by reducing impurity radiation localization and enhancing energy dissipation through cross-field transport. These findings contribute to the ongoing efforts to develop more efficient and effective divertor exhaust systems for tokamak fusion devices.