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Damage Studies of Tungsten Samples Using Dense Plasma Focus Devices



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Design and studies of plasma facing material for a fusion reactor is an engineering challenge. The focus of this M.Sc. thesis research project studies the interaction between the helium (He) ion beam produced in a dense plasma focus (DPF) device and the tungsten samples (poly-W and nano-W). Re-commissioning and optimization of a 1 kJ Mather-type UofS-I DPF was completed at the University of Saskatchewan. The United Nations University/ International Centre for Theoretical (UNU/ICPT) DPF device operated with helium working gas at the Nanyang Technological University (NTU), Singapore was used to study damage effects of tungsten sample with pulsed plasma irradiation under a simulated damage condition similar to that in a fusion reactor. Effect of irradiation of 2.3×10^28 m^-2 s^-1 helium ion flux on the PLANSEE double forged tungsten samples of size 1×10×10 mm^3 was investigated. Poly-W samples were irradiated for 5, 10, 15 and 20 shots from a distance 7 cm from the central anode of the DPF device. Surface defects due to He exposure were studied with SEM micrograph. High heat loads resulted in blisters and micro-cracks on the sample surface. With increase in the number of shots the density of the blisters increased and the crater-like He bubbles on the W surface were observed. Re-solidification of the melted and sputtered surface have been noticed as well. Nano-structuration of W was realized with UNU/ICPT DPF in argon working gas at 50 Pa pressure. Successive plasma pulses increased size of nanoparticles and led to particle agglomeration. At 10 shots, uniformly distributed highly dense nanoparticles of the size 20-50 nm have been synthesized. Nanostructured samples were then exposed to He plasma under the same conditions used for the poly-W samples. Instead of blisters and holes, micro-cracks and nanopores have been found on the synthesized Nano-W. BSE imaging of the poly-W and nano-W gives an evidence of trapped He bubbles on the poly-W sample surface and He desorption around grain boundaries in nano-W samples. EDX spectra showed the presence of Cu impurities due to sputtering from the anode in the DPF device. The XRD analysis of the exposed sample shows peak shifting toward higher diffraction angles and peak broadening of the prominent peak in the poly-W as well as nano-W samples. The comparative studies between poly-W and nano-W samples under irradiation of He ions seem to support the previous suggestions that nano-W is more favorable to be used as an alternative for plasma facing materials.



Plasma, Plasma Physics, Plasma Facing Materials, Tungsten, Helium irradiation, Damage Studies



Master of Science (M.Sc.)


Physics and Engineering Physics




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