Characterization of thermomechanical damage on tungsten surfaces during long-duration plasma transients

Abstract

A new experimental facility constructed at UCLA for the simulation of high heat flux effects on plasma-facing materials is described. The High Energy Flux Test Facility (HEFTY) is equipped with a Praxair model SG-100 plasma gun, which is nominally rated at 80 kW of continuous operation, of which approximately 30 kW reaches the target due to thermal losses. The gun is used to impart high intermittent heat flux to metal samples mounted within a cylindrical chamber. The system is capable of delivering an instantaneous heat flux in the range of 30-300 MW/m 2 , depending on sample proximity to the gun. The duration of the plasma heat flux is in the range of 1-1000 s, making it ideal for studies of mild plasma transients of relatively long duration. Tungsten and tungsten-copper alloy metal samples are tested in these transient heat flux conditions, and the surface is characterized for damage evaluation using optical, SEM, XRD, and micro-fabrication techniques. Results from a Finite Element (FE) thermo-elastoplasticity model indicate that during the heat-up phase of a plasma transient pulse, the majority of the sample surface is under compressive stresses leading to plastic deformation of the surface. Upon sample cooling, the recovered elastic strain of cooler parts of the sample exceeds that from parts that deformed plastically, resulting is a tensile surface self-stress (residual surface stress). The intensity of the residual tensile surface stress is experimentally correlated with the onset of complex surface fracture morphology on the tungsten surface, and extending below the surface region. Micro-compression mechanical tests of W micro-pillars show that the material has significant plasticity, failing by a "barreling"mode before plasma exposure, and by normal dislocation slip and localized shear after plasma exposure. Ongoing modeling of the complex thermo-fracture process, coupled with elasto-plasticity is based on a phase field approach for distributed fracture, and a discrete cracking approach, with cracks represented by Volterra dislocations.

Experimental measurements of surface damage and residual stresses in micro-engineered plasma facing materials

“…The differing stress response can be further attributed to the small scale of the geometry. Designing structures at the micron and nano scales has been shown to induce the well known "size effect"in which traditionally brittle materials can sustain large amounts of plastic strain without fracture [11,12,13]. Microarchitected coatings harness the superior mechanical properties afforded by the size effect into the design scheme, this results in a heat shield which benefits from both the freedom of expansion provided by independently moving microfeatures as well as a betterment of inherent mechanical strength common to micron and submicron scale structures.…”

“…In order to test the response of microarchitected surfaces under a thermal load, two W substrates were coated with a dendritic surface consisting of Re pillars covered with a thin layer of W. SEM images of these coatings, fabricated by Ultramet Inc., are shown in figure 2. The samples were then exposed to a cyclic thermal loading, using a high heat flux testing apparatus, designed at UCLA and described in reference [13]. The specimens, given internal identifiers of PW60 4 and PW60 5, are shown in their pristine (unexposed) condition in figure 4.…”

“…Experiments on planar tungsten samples under high cooling and long duration plasma transients (t on =6 s, t off =4 s ) have shown that the samples begin to recrystallize after roughly 1100 cycles of Γ=14.5±1.5 MW/m 2 . The appearance of recrystallization leads to the generation of microcracks on the specimen surface [13]. X-ray diffraction (XRD) measurements of pre-exposure and post-exposure planer tungsten can be seen in figure 10.…”