Ion-Induced Surface Modification of Refractory Metals Under Fusion-Relevant Conditions

Theodore J. Novakowski, Purdue University

Abstract

Magnetic confinement fusion reactors aim to achieve clean, large-scale power production through sustained fusion of hydrogen fuel. Utilizing strong magnetic fields to compress ionized hydrogen plasma, reactors harness fusion energy by directly absorbing heat and energetic fusion byproducts in the inner reactor walls. While major advances in fusion reactor technology have been made over the past several decades, finding materials to withstand these harsh radiation environments remains a preeminent issue for reliable reactor operation. Tungsten (W), due to its high heat tolerance and erosion resistance, has made it to the forefront of candidate plasma-facing materials (PFMs) for fusion applications and is currently the material of choice for the International Thermonuclear Experimental Reactor (ITER). However, under fusion-relevant plasma irradiation conditions, W has been shown to undergo significant surface morphology changes including the formation of fine surface nanostructures. There are serious concerns that the resulting surface structures may negatively impact reactor performance. Alternative (non-W) refractory metals have also been considered as PFMs due to their similarly high thermal conductivity, low erosion rates, and reasonably low hydrogen isotope retention. However, there are relatively few experimental studies investigating how surface morphologies of these alternative refractory metals might evolve under fusion-like irradiation conditions. Fundamentally, these alternative refractory metals all exhibit similar chemical affinities for hydrogen and helium isotopes and have similar crystal structures. One would then expect these materials to exhibit similar surface structuring mechanisms to W under helium and hydrogen ion irradiation. Yet differences in material properties imply that surface structuring in different materials may occur under somewhat different irradiation conditions. Since operating conditions (wall temperatures, ion and neutron fluxes) are only vaguely defined for future fusion reactors, it is possible that one of these alternative refractory metals could behave more ideally than W for a specific regime of operating conditions. Without systematically studying the response of these refractory metal surfaces to fusion-relevant irradiation conditions, however, the optimal use of these alternative materials may never be fully realized. Furthermore, by studying a wide range similar materials, one may gain unique insights into the nature of fusion radiation damage that would be difficult to ascertain through studying a single material. The following studies presented in this dissertation are then intended to assess the response of refractory metal surfaces under fusion-relevant ion irradiation conditions through systematic experimental investigations. First, temperature- and fluence-dependent surface structure evolutions of refractory metals (Ta, Nb, Mo, and V) under low-energy, high-flux He + ion irradiation were studied. It was found that temperature ranges for structure formation, normalized for each material’s respective melting temperature, are relatively consistent with trends observed in W: structure formation occurs in the temperature range ~0.25 < T/T m < ~0.55, where T _m is the melting temperature. If nanostructure formation must be strictly avoided for reliable reactor operation, different refractory metals could be used for different reactor temperature regimes based on their relative melting temperatures. Furthermore, fluence-dependent studies reveal that Ta has an impeded nanostructure growth rate compared to W. While W is generally perceived to have more appropriate bulk thermal and mechanical properties, Ta’s improved resistance to surface structure formation suggests that its properties would be less likely to degrade over the lifetime of the reactor; this could be a redeeming quality for the use of Ta as a PFM. Because of this intriguing result, the response of Ta surface morphology to D + ion irradiation was then studied to gain a more complete idea of how the material would behave in a fusion reactor irradiation environment. Future investigations will focus on how these ion irradiations will affect hydrogen isotope retention to address fuel conservation and safety issues.

Degree

Ph.D.

Advisors

Sizyuk, Purdue University.

Subject Area

Nuclear engineering|Materials science

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