Advancement of liquid lithium plasma-facing components for next-generation fusion devices

O'Dea, Daniel

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https://hdl.handle.net/2142/132568 Copy

Description

Title

Advancement of liquid lithium plasma-facing components for next-generation fusion devices

Author(s)

O'Dea, Daniel

Issue Date

2025-12-04

Director of Research (if dissertation) or Advisor (if thesis)

Ruzic, David

Doctoral Committee Chair(s)

Ruzic, David

Committee Member(s)

• Andruczyk, Daniel
• Curreli, Davide
• Levin, Deborah

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Fusion
• Lithium
• Plasma Facing Components
• Thermoelectric MagnetoHydrodynamics
• Electron Beam
• Plasma

Abstract

Fusion power generation is clearly an attractive solution to the problem of global energy production. It produces no greenhouse gases, is incredibly energy dense and could offer access to limitless energy. Generating and controlling the extreme conditions required to achieve fusion however, poses several large technical challenges. One of these challenges is handling the large heat, plasma and neutron fluxes exhausted from the core plasma. Current tokamak design points assume a steady state heat flux of 10MW/m2 increasing to values >1GW/m2 in transient events and particle fluxes > 1 × 1023m−2s−1. Flowing liquid lithium Plasma Facing Components (PFCs) are a promising candidate for plasma exhaust. A flowing PFC presents a constantly refreshing face to the plasma greatly reducing the erosion and sputtering of the solid substrate below whilst also protecting against catastrophic damage during transient events. These factors are predicted to extend the component lifetime in comparison to solid walls, increasing the duty cycle and thus the economic output of a reactor. Lithium also brings improvements to the plasma performance in a reactor by gettering impurities to reduce the zeff and thus boost confinement time or by offering access to high performance operating modes such as the ’low-recycling regime’. By reducing the recycling of hydrogenic species at the walls of the reactor, the collisionality of the edge will drop and, as such, the temperature will rise. This increased edge temperature can bring a myriad of benefits such as a reduction in anomalous transport, an increase in the burning region of the tokamak and an increase in the scrape off layer power width λq. The integration of liquid lithium into the divertor region of a fusion power plant requires several key issues to be addressed. This work will outline these key issues, the progress made so far to overcome them and the future tasks required to bring the maturity of the liquid lithium technology to a level where it can be installed into a reactor. The experimental test bed used throughout this work is the Actively Pumped Open Surface Lithium LOop (APOLLO) at the University of Illinois at Urbana Champaign (UIUC). Due to lithium’s high chemical reactivity, lithium PFCs must be included in an integrated loop where the fluid can be pumped into a reactor, removed and then diverted to processing chambers for impurity removal and the re-capture of fuel species. APOLLO was commissioned in January 2024 to further develop the technology readiness level (TRL) of lithium components. At the time of writing the key components installed on APOLLO are an open surface lithium divertor module within a magnetic field, an ECR plasma source, an electron beam heat source and a hydrogen distillation column. A new 3D ordered mesh flow plate has been 3D printed from tantalum and installed into the APOLLO PFC. With this upgraded flow plate, 100% lithium coverage has been achieved across the entirety of the flow plate. Complete coverage was observed across the full fluid velocity and magnetic field ranges achievable in APOLLO and was able to be regained after freezing and restarting the flow multiple times. These tests also achieved some impressive engineering feats. Over the 91 days these experiments were run: lithium was molten in the loop for 101 hours, was recirculated for 44 hours and was frozen and restarted 17 times all of which are key steps in showing the reliability of lithium systems. With uniform flow achieved on the plate, experiments studying Thermo-Electric MagnetoHydroDynamics (TEMHD) drive began. This involved the re-commissioning of an electron beam assembly to generate fusion relevant heat fluxes at the lithium PFC. The heat flux produced at the PFC was determined using a calorimeter and a maximum value of 6.1 ± 0.63MW/m2 was achieved. Exposures of flowing lithium to this heat flux revealed that the fluid dynamics were driven by the plate normal field and the truncation of the heat flux across the plate. Throughout the test transient peaks on the order of 0.1g/s were observed with applied heat flux and appear to correlate to the expulsion of fluid from the plate. To interpret the results from the TEMHD experiments, a fluid dynamics simulation was created in COMSOL multiphysics. Once the model had been benchmarked against the experimental observations it was extended up to magnetic fields of 5T to investigate the performance of TEMHD drive in a fusion relevant environment. The final part of this work was the design, construction and characterization of an Electron Cyclotron Resonance (ECR) plasma source. This source was installed so that the retention of hydrogen species into liquid lithium and their subsequent transport around the loop could be elucidated. The source also provides a controlled influx of said species into the lithium flow; allowing for the benchmarking of impurity measurement diagnostics and the determination of the removal efficiency of the distillation column installed into APOLLO. The characterization of the plasma centered around tuning external parameters to maximize the flux of ions and radicals to the lithium surface. An optimized solution was found which generated a fluence of 1.7 × 1018s−1 of hydrogenic species to the plate. Studies of the uptake of helium ash were also studied using the ECR plasma source. Helium fluxes on the order of ×1015s−1 were observed to have been transported from the open surface PFC to the bulk lithium reservoir.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132568

Copyright and License Information

Copyright 2025 Daniel O'Dea

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