Flow control and associated technologies to advance the application of TEMHD-driven liquid lithium in fusion devices

As the fusion research community trends toward building larger and hotter devices, evidence points to the fact that solid plasma facing components will not be able to endure the conditions without extensive damage. Plasma-materials interactions with these surfaces lead to material defects, impurity formation, and cooling of the edge plasma. In order to alleviate these serious issues, liquid metal concepts are being heavily researched as alternative plasma facing components. Liquid lithium has shown the most promise, as its use in fusion devices has led to increased confinement time, less wall recycling and improved impurity control, and enhanced plasma performance. Many early devices used lithium evaporation or pool melting to introduce lithium, but the high reactivity of liquid lithium quickly led to passivation of the surface. To mend this problem, flowing liquid lithium systems have been developed that provide a constantly refreshing liquid lithium surface to the regions of plasma interaction. The Liquid Metal Infused Trench (LiMIT) concept, developed at the University of Illinois at Urbana-Champaign (UIUC), utilizes the thermoelectric magnetohydrodynamic (TEMHD) effect to passively drive liquid lithium through solid metal trenches. The LiMIT device has been successfully tested at UIUC and in devices around the world, such as the HT-7 tokamak and the Magnum PSI linear plasma device, at heat fluxes of up to 3 MW/m2. While sustained flow has been observed in many cases both horizontally and at an arbitrary angle to horizontal, methods to control and constrain the flow are lacking. This thesis focuses on modeling and experimental techniques meant to aid in lithium flow control in LiMIT devices. A compact flow module was developed that utilizes the nozzle effect to drive high-velocity flow when impacted with high local heat fluxes. The proof of concept testing showed sustained flow between 2 and 10 cm/s in the device, and associated modeling predicts velocities up to 60 cm/s will be attainable once used with large heat fluxes. The dryout phenomenon, where high local acceleration of flow depresses lithium surfaces and exposes the solid trenches, is investigated via multiphysics modeling. The models developed recreate experimental observations, and were used to predict that a step increase of the height of the bottom of the LiMIT trenches can effectively mitigate dryout risk in future devices. For flow of 1 cm/s in a 5 mm deep trench, a step increase of 1.8 mm is most effective, while for 10 cm/s flow, a step increase of 2.7 to 3.0 mm works to diminish dryout. Finally, a method to control the wetting properties of liquid lithium on stainless steel and molybdenum is developed. Pulsed laser interaction with the metal surfaces creates relatively ordered micro and nanostructures that serve to increase the wetting temperature of lithium. On stainless steel, this increase is 83 °C (to 398±4 °C), and on molybdenum, it is 77 °C (to 401±4 °C). Furthermore, it is shown that the change in wetting temperature increase can be used to accurately predict the surface roughness of the structured materials, or that experimental observations of a structured surface can be used to predict the wetting temperature. Overall, the models and technologies presented herein describe various methods of controlling and constraining lithium in a flowing liquid lithium device. The information can be used on future iterations of LiMIT testing on larger devices, like the design for a LiMIT limiter for the Experimental Advanced Superconducting Tokamak (EAST) presented in this work. As flowing liquid lithium concepts continue to be developed, the adaptable models and technologies shown here will be used to inform the design process and inform engineering decisions, in order to further the applicability of liquid lithium in large scale fusion devices