Speaker
Description
A new divertor concept, referred to as the Diffusion Pump Divertor [McComas et al. US Patent Application #63/919,661], is presented. This concept adapts vacuum-pump technology to enable controlled delivery and removal of vapor within fusion devices. This approach addresses longstanding limitations in present lithium vapor delivery schemes, which rely on evaporation and provide only limited control over both the quantity and spatial distribution of vapor. Poor control can adversely affect plasma performance, leading to undesirable core impurity accumulation or insufficient radiation for heat-flux mitigation. By contrast, the Diffusion Pump Divertor provides a means to inject low-Z metal vapor, such as lithium, into the divertor plasma with high precision while simultaneously removing excess vapor and contaminants during operation. In addition to improving impurity management, the introduced vapor absorbs significant plasma thermal power, offering an additional pathway for divertor heat exhaust. In this paper we present a preliminary design of the Diffusion Pump Divertor. We have evaluated performance through analytical modeling, computational fluid dynamics (CFD), and advanced SOLPS-ITER simulations. The assessment aims to determine the feasibility, operational benefits, and potential performance improvements associated with this concept in fusion-relevant conditions.
The SOLPS-ITER simulations of the boundary plasma-surface interactions employed a National Spherical Torus Experiment Upgrade (NSTX-U) magnetic equilibrium created specifically to examine high-heat-flux scenarios [Emdee and Goldston 2023 Nuclear Materials and Energy 34 101335]. Plasma-facing component envelopes as used in previous lithium vapor box modeling were used for a direct comparison with the vapor box calculations [Emdee and Goldston 2023 Nuclear Fusion 63 096003]. In these simulations, supersonic jets integral to the divertor design were represented by “transparent” surfaces emitting atomic lithium 1.62 eV kinetic energy, corresponding to a Mach 5 jet for a lithium vapor temperature of 900 K. The nozzle fluence was scanned at constant particle energy to study the impact of jet strength on plasma and neutral transport. Activation of the jet produced a measurable increase in neutral deuterium flux toward the divertor target, demonstrating diffusion-pump-like action; by one measure, the increase in pumping speed was about 20x. The ionized deuterium flux, however, decreased due to divertor detachment rather than the pumping mechanism itself. Such reduction is expected in detached divertors, where strong upstream recombination naturally limits ionized flux reaching the target. Despite the reduction in deuterium flux, the pumped flux of deuterium was still calculated to far exceed standard pumping techniques, indicating unique benefits to this divertor design.