Surface Response of Lithium Coatings on High Z Refractory Metal under Deuterium And Helium Ion Bombardment

Anton L Neff, Purdue University

Abstract

Lithium has proven to be a very interesting plasma facing component (PFC) material. It has been used as a wall coating in multiple research tokamaks, and at many of these facilities, lithium has improved confinement time by reducing hydrogen recycling from the walls of the reactor. Tungsten is also being considered as a durable PFC for fusion environments. The International Tokamak Experimental Reactor (ITER) will use a full tungsten divertor. Many operating tokamaks are using ITER like walls for testing ITER designs. From these tests and from controlled experiments, tungsten has shown some detrimental material changes under low energy deuterium and helium ion irradiation. Some of these material defects are holes, bubbles, voids, and fuzz. Because of the use of lithium in a number of tokamaks and the use of tungsten in many test reactors as well, studies have begun to study the behavior of lithium on the refractory metal tungsten. These studies deposited 100 and 500 nm of lithium on commercial tungsten discs and irradiated them with 100 eV deuterium, 1 keV helium, and deuterium and helium from two separate ion guns. The thicknesses of the lithium layers were determined by the temperature of the evaporator, over time, entered into a model using the Knudsen equation combined with an effusion model. This model was verified using AFM scans and SEM cross section micrographs of lithium layers on silicon substrates. Irradiations were carried out to a medium deuterium fluence of (1-3)×1017 D+/cm2. The surface chemistry of the samples before, after, and during irradiation was characterized using x-ray electron spectroscopy (XPS) in situ. These studies show that lithium on tungsten retains deuterium in a similar manner as does lithium on carbon; however, when helium is a secondary ion during irradiation, it can inhibit the deuterium retention in the lithium. In addition, the lithium is not eroded from the surface of the tungsten at the fluences that were achieved is this study, but exhibits a self healing behavior after irradiation by deuterium. In addition to these studies, textured refractory metals have been irradiated with argon at 150 eV to determine the effect of surface morphology on the sputter yield. These samples were provided by Ultrament Inc. The samples were irradiated up to a fluence of (1-3)×1017 Ar+/ cm2 at both normal and 45° ion impact incidence. The relative sputter yields were measured using a quartz crystal microbalance (QCM). These relative sputter yields, when compared to a polycrystalline tungsten disc, showed that mass deposited on the QCM decreased from the textured surfaces. The studies also observed that the relative sputter yield decreased when the ions were impacting at 45° compared to normal incidence.

Degree

M.S.

Advisors

Allain, Purdue University.

Subject Area

Nuclear engineering|Materials science

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