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Erosion and Thermomechanical Behavior of Metallic Foams in Plasma Environments

  • Author(s): Nadvornick, Warren Isamu
  • Advisor(s): Ghoniem, Nasr M
  • et al.
Abstract

Because of its high melting-point and geometric flexibility, reticulated tungsten foam has been proposed for a variety of applications in extreme heat environments, particularly associated with exposure to photon and energetic particle fluxes in plasma environments. Photons, ions and electrons deposit their energy deep into the foam, thus reducing steep temperature gradients associated with extreme transient thermal loading. Smaller temperature gradients result in reduced thermal stresses and thermal shock on the one hand, and the deep penetration of particles can lead to re-trapping of sputtered material and a degree of self-healing on the other. To understand the relationship between foam geometry (i.e. porosity, ligament size, and cell dimensions) and resistance to thermal shock and erosion phenomena, a hierarchy of models for the mechanical, thermal and erosion properties of the foam are developed and presented. A method for geometric construction of as-fabricated foam is developed, where 2-D X-ray tomography data is assembled into 3-D volume elements that can be computationally manipulated for either particle transport models or continuum physics modeling. Finite Element models of reconstructed foam are then developed to determine the dependence of its effective thermal and mechanical properties on the main geometric descriptors; namely the solid volume fraction and the number of Pores Per Inch (PPI) that is typically used to describe its average cell size. While scaling relationships of linear properties that depend on the volume fraction alone (e.g. elastic modulus) are consistent with simple rules-of-mixture, detailed computational models are shown to be necessary for scaling of non-linear properties, such as thermal conductivity and the plastic modulus. A particle model based on transport of volume elements is developed to study plasma interaction with the complex topology of the reticulated foam. We show that the foam possesses a self-healing property in the plasma environment that is a result of self-trapping and re-deposition of sputtered material. These findings have been corroborated with dedicated experiments on tungsten foam exposed to helium and xenon plasmas in the PISCES facility at UCSD. The explicit geometric model of the foam is utilized in a transport study of secondary electron emission from metallic foams. Applications of the model to the calculation of the secondary electron emission shows that foam structures can effectively suppress SEE, consistent with experimental findings on copper. Finally, a continuum multiscale model is developed, where the effective thermal and mechanical properties are used in large scale applications. The model captures the thermomechanical response of the foam in steady high-heat flux emanating from the low-pressure plasma of fusion energy devices and the transient response associated with thermal shock in intense high-enthalpy plasma in arc-jets. The thermomechanical model is finally used for experimental planning and testing of a prototypical “finger” module, designed as an element of a divertor component in a fusion device.

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