Multifractal Analysis of Plasma Irradiated Tungsten Alloy Samples

If the world's energy demand grows at the same rate as in previous decades, sooner or later we face an energy crisis. One solution is fusion energy. The main problem with fusion energy is the choice of proper plasma facing material for the fusion chamber and the most effective evaluation of the durability of this material. The interior wall materials must withstand constant heat and particle flows that affect and damage the material. At present, tungsten is going to be used as the internal wall material in future plasma fusion plants. To characterize the behavior of plasma facing materials under intense plasma flows, the materials are currently analyzed using SEM (scanning electron microscope) images of the surfaces and cross-sections, estimating the role of different kinds of defects (e.g., droplets, cracks, blisters, etc.). However, there is still no reliable method to compare the abundance and the role of different defects on the surface of the irradiated materials. In this research, we propose the multifractal formalism for characterizing the distribution and properties of the defects. In this study, we investigate the surface multifractality of two tungsten alloys (95% W, 1.66% Fe, 3.33% Ni and 97% W, 1% Fe, 2% Ni) and pure (double forged) tungsten specimens, using the box counting method. The test specimens are irradiated with one and two series of deuterium plasma pulses, prior to the analysis in an irradiation mode similar to fusion devices. Then the SEM images are taken to perform further analysis. The box counting method allows defining material defects from SEM pictures and predicting material behavior during irradiation and multifractal spectra enable us to distinguish the samples irradiated with different heat and particle loads.