Fundamental mechanisms of deuterium retention in lithiated graphite plasma facing surfaces

Chase N Taylor, Purdue University

Abstract

Plasma impurities and undesirable deuterium recycling degrade plasma confinement and impede a sustainable fusion reaction. This occurs by inducing plasma instabilities and reducing plasma temperature. Lithium wall conditioning has been used in fusion devices including TFTR, CDX-U, FTU, TJ-II, MAST and NSTX as a means to reduce plasma impurities and improve deuterium retention, resulting in significant enhancements in plasma performance. These improvements have come via a reduction in deuterium recycling in addition to a reduction in oxygen and carbon impurities. NSTX, with ATJ graphite walls, is the leading fusion device in lithium research. Many previous studies have investigated deuterated lithium, deuterated graphite, and lithiated graphite in order to understand fundamental properties and particular applications. Deuterium irradiation of lithiated graphite studies are few in number and no systematic research has been conducted to determine the fundamental mechanisms by which deuterium is retained in lithiated graphite. This work presents controlled laboratory studies that use X-ray photoelectron spectroscopy (XPS) to identify the fundamental chemical interactions in lithiated graphite. Li-O chemical interactions are observed in the photoelectron energy spectrum at 529.5 eV after thermally depositing lithium onto ATJ graphite. Deuterium retention induces Li-O-D and Li-C-D interactions which are observed at 529.9 eV and 291.2 eV, respectively. Examination of NSTX post-mortem tiles confirms the formation of Li-O-D and Li-C-D chemical interactions and validates the procedures in these experiments. Prior to these findings, deuterium was assumed to bind exclusively with lithium to form stoichiometric LiD. Instead, we find that in a graphite matrix, lithium will always bind with oxygen and carbon (when present) prior to the introduction of deuterium. The deuterium saturation of lithiated graphite is also assessed using XPS and results indicate that saturation occurs at a deuterium fluence of ∼ 2.9×10 17 cm−2. This implies that the NSTX deuterium flux of 1017 – 1018 cm−2 s−1 saturates the typical 10-100 nm lithium evaporations after a single plasma discharge. Atomistic simulations synergistically corroborate the above experimental findings. Experiments show significant influence of oxygen in retaining deuterium. Density functional theory simulations were updated to include oxygen and lithium in a carbon matrix at concentrations observed in experiments (∼20%). Results show that deuterium preferentially chooses to be near and bind with oxygen. Later experiments demonstrate the role of oxygen in retaining deuterium, but also show that lithium is required to attract sucient quantities of oxygen to the surface and to retain the oxygen. This dissertation conclusively demonstrates that the mechanism by which deuterium is retained in lithiated graphite is through a lithium-catalyzed oxygen-deuterium bond.

Degree

Ph.D.

Advisors

Allain, Purdue University.

Subject Area

Nuclear engineering

COinS