Steven Stemmley, Ph.D. Candidate
Dr. David Ruzic, Director of Research
March 26, 2024 | 9:00am - 11:00am CST
This final examination will be held in 101A Talbot Laboratory.
Liquid Lithium Loop Development For Open Surface Plasma Facing Components
ABSTRACT: Fusion energy has the potential to provide abundant energy while being a clean power source, producing little carbon emissions. To produce energy, isotopes of hydrogen, like deuterium and tritium, must be given sufficient energy to overcome their Coulomb barriers and fuse. In order for this reaction to produce net energy, a sufficient amount of them must be held at extremely high temperatures and confined for a long enough time. This often results in the creation of extreme environments within fusion devices which end up interacting with the wall materials, called plasma-facing components (PFCs). These components are expected to handle steady state heat fluxes up to 10MWm−2 during normal operation and up to a 1GWm−2 duringtransient events. Under these heat loads, material damage and erosion is often the result.
However, one way of mitigating this damage is to utilize a liquid metal wall, specifically, liquid lithium. Liquids are inherently a self-healing material, meaning that if they are damaged or experience erosion, more liquid will fill the space of the damaged area. Lithium offers a myriad of plasma benefits, including increased temperatures and confinement times. Liquid PFCs also have a variety of potential concerns including surface stability, wetting control, hydrogen retention, and heat flux handling. Much of the work at the University of Illinois Urbana-Champaign has been done on solving a large number of these issues. The Liquid Metal Infused Trench (LiMIT), developed at Illinois, provides one liquid lithium PFC option for future devices. This concept is able to utilize the heat flux from the plasma and the magnetic fields of the device to provide self-driven flow via thermoelectric magnetohydrodynamics (TEMHD). Versions of this concept have been tested all over the world, including in the HT-7 and EAST tokamaks showing sustained flow and improved plasma performance. Recent development in this technology has led to the construction and testing of 3-D LiMIT structures which can handle significant heat flux without exposing the substructure to the damage.
To date, all of the LiMIT systems and other liquid lithium PFC concepts have been tested in self-contained modules, meaning that the lithium did not leave the system. In future devices, the lithium will need to enter the fusion device, flow across the PFC picking up hydrogen and impurities, and then exit the device to be cleaned. Without this recirculation, the lithium will eventually passivate and the beneficial lithium properties will cease. Thus, this work attempts to take the next step in technological readiness and develop a fully recirculating liquid lithium loop with flow in and out of a magnetic field and across an open surface PFC.
A variety of supporting components had to be designed and developed for use with liquid lithium. These mainly include lithium pumps, flowmeters, and liquid level sensors. It was determined that the pump was required to deliver 1-2 psi of pressure head to overcome the frictional and electromagnetic resistance in the loop. A DC conduction pump, utilizing permanent magnets, was designed to produce this pressure head at 5 g s−1 with 50 A of current. Due to this small flow rate, sensitive DC conduction flowmeters able to produce a measurable signal were developed. Additionally, height sensors were developed utilizing time-domain reflectometry. All three of these components were de-risked and calibrated using a U-shaped experiment called the pump stand.
Another issue surrounding lithium PFCs is the uniform distribution of flow on the surface. Without sufficient coverage, the underlying substructure can be exposed to the plasma leading to damage and erosion. Thus, distributor technologies were investigated using a dehomogenized optimization protocol. Simulations performed in COMSOL optimized the flow to produce a near uniform outlet velocity profile. This information was fed into pattern-generating Turing equations to produce structures that follow the desired vector field. After much iteration, a resulting geometry was produced that had a predicted velocity deviation of 37% with the magnetic field off and 15% with the field at 800 G for a mass flow rate of 5 g s−1. Due to it’s complex structure, it was 3D printed out of 316 stainless steel.
Further experimental systems were developed including the PFC plate to hold the distributor, a reservoir to hold the bulk lithium in the loop, and the Solid/Liquid Lithium Divertor Experiment (SLiDE) chamber. Control electronics and programs were developed along side these systems. Upon testing, 600 g of lithium was required to fill the loop. In the resulting experiments, the distributor was able to fully wet and provide uniform flow at its outlet. At 1 g s−1, the experimental lithium flow deviation in the outlet mass flow rate was found to be 6-8% while the simulated variation in profile was 21-9%, without and with the magnetic field (800 G), respectively. However, even though the distributor performed better than expected, the plate would not fully wet as the surface tension forces associated with a flat plate were too high. Though, evidence strongly suggests the lithium will have no problem wetting the 3-D LiMIT-style system.
The whole loop system was tested extensively for 60 cumulative hours, freezing and reheating three times. One of these experiments was ran continuously for 36 hours, indicating some level of reliability. A peak flow rate of 10 g s−1 was achieved and the distributor, plate, and collector were able to handle this flow without spilling or overfilling. Overall, this new loop system opens a new era of experiments for increasing technological readiness of liquid lithium PFCs.
