Interaction of plasmas with lithium and tungsten fusion plasma facing components

Fiflis, Peter Robert

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https://hdl.handle.net/2142/90576 Copy

Description

Title

Interaction of plasmas with lithium and tungsten fusion plasma facing components

Author(s)

Fiflis, Peter Robert

Issue Date

2016-04-20

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

Ruzic, David N.

Doctoral Committee Chair(s)

Ruzic, David N.

Committee Member(s)

• Allain, Jean P.
• Brooks, Caleb
• Curreli, Davide
• Thomas, Brian

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)

• Fusion
• Plasma facing components (PFC)
• Lithium
• Stability
• Tungsten
• Nanostructuring

Abstract

One of the largest outstanding issues in magnetic confinement fusion is the interaction of the fusion plasma with the first wall of the device; an interaction which is strongest in the divertor region. Erosion, melting, sputtering, and deformation are all concerns which inform choices of divertor material. Of the many materials proposed for use in the divertor, only a few remain as promising choices. Tungsten has been chosen as the material for the ITER divertor, and liquid lithium stands poised as its replacement in higher heat flux devices. As a refractory metal, tungsten’s large melting point and thermal conductivity as well as its low sputtering yield have led to its selection as the material of choice of the ITER divertor. Experiments have reinforced this choice demonstrating tungsten’s ability to withstand large heat fluxes when adequately cooled. However, tungsten has shown a propensity to nanostructure under exposure within a certain temperature range to large fluxes of helium ions. These nanostructures if disrupted into the plasma as dust by an off-normal event would cause quenching of the plasma from the generated dust. Liquid lithium, meanwhile, has gathered growing interest within the fusion community in recent years as a divertor, limiter, and alternative first wall material. Liquid lithium is attractive as a low-Z material replacement for refractory metals due to its ability to getter impurities, while also being self-healing in nature. However, concerns exist about the stability of a liquid metal surface at the edge of a fusion device. Liquid metal pools, such as the Li-DiMes probe, have shown evidence of macroscopic lithium displacement as well as droplet formation and ejection into the plasma. These issues must be mitigated in future implementations of liquid lithium divertor concepts. Rayleigh-Taylor-like (RT) and Kelvin-Helmholtz-like (KH) instabilities have been claimed as the initiators of droplet ejection, yet not enough data exists to delineate a stability boundary. The influences of plasma pressure and current driven instabilities on lithium surfaces that lead to droplet ejection are investigated to determine which of the two effects is dominant for a given set of plasma conditions. This work studies the influence of large plasma fluxes on these two materials to better inform the selection and design of plasma facing components (PFCs). The nanostructuring of tungsten was investigated to determine the mechanisms by which tungsten nanostructures so that its formation may be mitigated. Experiments investigated the dependence of nanostructuring on temperature, looked at the morphological evolution, and grew nanostructures on a variety of metals to examine their similarity to tungsten. Additionally, a computational model is presented for the initial stages of fuzz formation showing good quantitative and qualitative agreement with experimental observations. The influences of RT and KH instabilities on the surface of liquid lithium were experimentally observed and quantified on the ThermoElectric-driven Liquid-metal plasma-facing Structures (TELS) chamber at the University of Illinois at Urbana-Champaign and the stabilizing effect of surface tension, an effect employed by the LiMIT concept as well as other liquid lithium concepts, was studied, and the stability boundary afforded by surface tension was compared between experiment, computational simulation, and theory.

Graduation Semester

2016-05

Type of Resource

text

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http://hdl.handle.net/2142/90576

Copyright and License Information

Copyright 2016 Peter Robert Fiflis. All rights reserved.

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