Investigating the power dissipation mechanics in a lithium vapor cloud during vapor shielding regimes

Rizkallah, Rabel

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https://hdl.handle.net/2142/121314 Copy

Description

Title

Investigating the power dissipation mechanics in a lithium vapor cloud during vapor shielding regimes

Author(s)

Rizkallah, Rabel

Issue Date

2023-06-27

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

• Andruczyk, Daniel
• Curreli, Davide

Doctoral Committee Chair(s)

• Andruczyk, Daniel
• Curreli, Davide

Committee Member(s)

• Ruzic, David N
• Morgan, Thomas W

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Nuclear fusion
• liquid lithium
• lithium vapor cloud
• plasma chemistry
• plasma facing components
• plasma heat flux mitigation
• plasma-material interactions
• plasma physics
• vapor shielding.

Abstract

The quest to resolve the plasma material challenge in the development of fusion devices have led the fusion community to consider liquid metals as alternatives to the traditional solid high-Z solutions. Lithium in particular is gathering much deserved attention, with its low atomic number and low melting temperature making it an ideal candidate for liquid metals based plasma facing component concepts. Over numerous experiments and studies, lithium was proven to enhance plasma performance, reducing the impurity content and allowing to operate in low recycling regimes. Lithium has also been shown more recently to help in mitigating the high incident plasma fluxes by radiating considerable portions of the power away from the plasma facing component. This behavior known as \textit{vapor shielding} is the result of the formation of lithium vapor cloud in between the lithium surface and upstream plasma. Once the surface temperature of the lithium becomes high enough, the lithium cloud can effectively shield the beneath surface. A surface locking temperature around 800 degrees C has been observed in past experiments, with lithium vapor shielding successfully protecting tungsten targets for heat fluxes up to 29 MW/m2 and transient pulses of 0.5 - 0.75 MJ/m2 at 100 Hz. The locking of the surface temperature indicates that the vapor cloud is able to effectively keep up with the increase in upstream heat flux, radiating more power away as needed, and delivering reduced powers to the targets. Lithium however is not without its inconveniences. The low recycling regimes are achieved due to the lithium high fuel retention capabilities which is particularly problematic for the tritium because of its limited supply. Lithium vapor shielding is also possible because of the high evaporation rate of lithium. This can lead to considerable amounts of lithium contaminating and diluting the fusion plasma. Nonetheless, solutions to both these problems have been proposed and are being actively researched. The huge benefits that lithium can bring to the fusion research make it important to continue investigating it as a working plasma facing component solution. The focus of the presented work is on the lithium vapor shielding phenomenon. The exact mechanics behind it are unknown as well as is the limitation of the vapor cloud. Two candidates come to mind to explain the power radiation: the heavy lithium evaporation at the surface and the plasma chemistry happening inside the cloud. Experiments have been conducted on Magnum-PSI, a linear plasma device operated at the Dutch Institute for Fusion Energy Research, to gather experimental data to further understand and characterize the lithium cloud. A surface locking temperature around 790 degrees C was observed and held up to steady-state heat fluxes of 20 MW/m2. Calorimetry data have shown steady increase in the dissipated power between a reference target and a lithium target under increasing heat flux conditions. Vapor shielding was also insensitive to the addition of helium and neon species with the same locking temperature achieved regardless of their addition or not to the Magnum-PSI plasma. Attempts at estimating the electron temperature and density inside the cloud found mixed success. The temperature at or very near the lithium surface was measured via the Boltzmann plot method around 0.25 - 0.3 eV for the plasma conditions of Magnum-PSI. Unfortunately, axial scans from the lithium surface to the upstream plasma did not return temperatures of the core plasma but rather seemed to get stuck at the edge. Fitting of the hydrogen-beta emission profiles from a high accuracy 2.25 meters spectrometer was also unsuccessful at resolving the atom temperature because of heavy Zeeman splitting. Therefore, the two-temperature population of the Magnum-PSI plasma could not be correlated to the electron temperature inside the cloud, and determining the electron density was accompanied by strong uncertainties. Nonetheless, the data suggest a slight drop in the electron density inside the cloud, without clearly forming a region of heavy recombination as past literature suggested for the Magnum-PSI plasma conditions. Another experimental campaign was run on the HIDRA classical stellarator at the University of Illinois at Urbana-Champaign with the specific goal of measuring lithium redeposition rates and recreating vapor shielding regimes inside a toroidal device. The measured redeposition rates suggest a value around 0.95, which is lower than the expected >0.999 from the literature, in agreement with past measurements on Magnum-PSI. The lithium surface temperature locking was also observed on HIDRA for a few shots but in general, running lithium experiments at high temperature was shown to be problematic for the device plasma conditions. The heavy lithium evaporation quickly overtakes the plasma fuel flow and a transition to a lithium plasma happens usually very quickly. While HIDRA operates at relatively low densities compared to larger devices, this does illustrate the need to capture or control the lithium vapor before it reaches the fusion plasma to avoid excessive contamination. From the obtained experimental data, calculations have shown that mass loss from the lithium evaporation and sputtering cannot account for the power dissipated by the vapor cloud unless extremely low redeposition rates are assumed. Such low rates are in contradiction with how long the lithium targets are able to carry on without running out of lithium. A plasma chemistry model was then developed to compute the power loss per lithium particle due to electron impact excitation, ionization and recombination. Reactions driven by heavy particle interactions were disregarded because of the low temperature of the vapor clouds studied experimentally. A global plasma reaction solver named CRANE was used to solve for the steady-state content of a zero dimensional vapor cloud for different lithium excited states and ion stages which was used to obtain a bulk power loss per lithium particle. The analysis has shown that lithium is very efficient at radiating power away for temperatures below or near 1.0 eV. Past that point, there is a steep drop in the power dissipation potential, reaching a minimum at the first ionization energy of lithium before going back up but never reaching back the capacities attained under 1.0 eV. This behavior is more pronounced for longer lithium residence times inside the cloud. Calculations also shown that redeposition rates below 0.99 are needed to justify lithium vapor shielding from a plasma chemistry description, in agreement with the experimentally measured rates on HIDRA. CRANE was then coupled to Zapdos, a plasma transport solver to expand to a one dimensional model. The Zapdos-CRANE implementation was tuned to reproduce similar lithium clouds to the observed ones during the Magnum-PSI experiments and successfully demonstrated the ability of the vapor cloud to radiate enough power away once enough lithium is fed to the cloud with vapor shielding able to dissipate much higher powers than those experimentally tested. The model also showed the limitation of the vapor cloud at higher electron temperatures. Once the electron temperature inside the cloud reaches values around the ionization energy of lithium, between temperatures of 3.0 eV and 7.0 eV, the massive drop in power dissipation capacity of the lithium results in a very strong reduction in the shielding efficiency. This reduction is so strong that vapor clouds in such temperature conditions will not be able to protect the plasma facing component from the upstream plasma. Hence, lithium vapor shielding appears to only really be efficient for electron temperatures around or below 1.0 eV, where the power dissipation capacity of the cloud can be a very useful tool to mitigate the plasma fluxes and protect the targets.

Graduation Semester

2023-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/121314

Copyright and License Information

Copyright 2023 Rabel Rizkallah

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