A Study On The Behavior Of Deuterium Retention In Carbon, Tungsten And Carbon-tungsten Co-deposition Films

Thermonuclear fusion, as a mankind’s ultimate energy source, has its unique advantages, such as infinite fuel resources no pollution and no high-level radioactive nuclear waste, etc. In future fusion devices (such as ITER), high power pulse and cumulative increase of the operation time and the resulting high heat load, would lead to serious issues from the interaction and effects between plasma and the plasma facing wall materials, fuel retention is one of the key issues. Under the condition of burning plasma, besides heat load, the plasma facing materials (PFM) will sustain the bombardment of energetic hydrogen, helium particles and high-energy neutrons. It will results in the direct fuel atoms (deuterium and tritium) retention from the implantation process and the fuel retention from the co-deposition with eroded PFM impurity atoms. This fuel retention will seriously affect the particle recycling, plasma condition, operating efficiency and security. This thesis is to make an experimental simulation on Helium and Deuterium retention behavior in PFMs, focus on the retention behavior of deuterium and helium atoms during the co-deposition with the eroded species of C and W (PFMs in ITER).Magnetron plasma sputtering was used to simulate the co-deposition process of deuterium and helium with eroded C or W atoms in a D2/He mixed gas atmosphere. Helium and deuterium retention behavior in the deposited films was studied. From simple to complex research process, the study starts with the single-element materials including graphite and polycrystalline tungsten, and finally focuses onto the mixed material of carbon and tungsten.The first part of the paper, in D2+He/Ar mixed gas atmosphere, RF magnetron plasma sputtering was used to investigate the gaseous atoms retention behavior during the co-deposition of deuterium and helium with eroded C atoms and the characteristics of the deposited He charged a-C:D films samples. By controlling the variable method, research emphases were paid to the effects by changing relative helium content, temperature and substrate material on the nature of the co-deposition films. The results indicate that the D retention and film growth rate are different in disparate substrates (Si and W), the substrate temperature can effectively reduce the gas retention in the films, helium introduction make the amorphous deuterated carbon films more disordered but of more graphic carbon atoms, and bubbles, surface cracking and exfoliation were observed at the first time.In the second part, the same methods were used to investigate the gaseous atoms retention behavior during the co-deposition of deuterium and helium with eroded W atoms and the characteristics of the deposited He charged W:D films samples (named He-W:D). By independently changing He partial pressure in the gas mixture and substrate temperature, the effects of helium introduction and temperature on structure and deuterium retention properties of He-W:D films were investigated. The results indicate that deuterium and helium were simultaneously retained in the films and helium introduction greatly affects the deuterium retention, crystal structure properties and surface morphology (peeling, flaking) of the films. Besides, high temperature upon substrate reduced the gaseous atoms retention but made the surface morphology of the films more disordered (severe peeling and flaking). While the different characteristics of the same samples grew on separate substrates (smooth single-crystal silicon and mechanical polishing copper) with different roughness verified that the surface disorder of the film on copper can be suppressed for the larger roughness.In the last part, investigations were concerned on the gaseous atoms retention behavior during the co-deposition of D or D/He atoms with the mixed erosion C and W atoms in the D2/Ar or D2+He/Ar atomospheres and on the characteristics of the deposited He charged C-W:D films samples. Firstly, the investigation was concentrated on the D retention and characteristic of the deuterated C-W mixed films. The results revealed that D concentration is proportional to C/W ratio, which indicates C atoms are the main capture centers of D. Besides, the pressure dependent curves showed an maximum value of C/W and D concentration at 5.0 Pa, while the temperature elevated, C/W and D concentration decreased, the crystal of the C-W films tended to be more graphic with a possible WC1-x phase and the surface became more smooth (the erosion caves disappeared at 723K). Secondly, based on the results of the deuterated C-W mixed films, helium introduction into the mixed atmosphere further made the situation more complicated. The results showed that helium had great effects on D retention, crystal structure and surface morphology of the C-W films. Helium introduction made the C/W and D concentration but growth rate increased and the surface morphology of the C-W films more disorder with larger nano-particles and greater roughness. While the high substrate temperature (>573K) effectively eased the situation, the D and He concentration became fewer and the films surface became more smooth with fewer nano-particles and smaller roughness. The effects of high temperature improved the films crystalline properties and increased the C or WC1-x crystalline phases, thus effectively capture probability between C and D atoms.In summary, when the C and W were used for PFM at the same time, there would be a gaseous atoms charged C、Wor C-W mixed co-deposition films formed on the surface of PFM. During the co-deposition processes, D atoms were mainly trapped by C atoms, besides, the status of the crystal structure of the films had great impacts on D retention, including the defects and the formation of tungsten carbide phase in the films. He introduction makes the co-deposition process more complicated, it brings more point defects in the crystal structure of the films. On the one hand, it increases the D retention, on the other hand, it may also make the surface of the films more disordered with large nano-particles, bubbles and flakes, which would cause the films more easily eroded by the neutral particles and break the stable plasma discharge. Appropriate substrate temperature has significant impacts on gas retention, structure and surface morphology of the films, is an effective method to inhibit deuterium retention and improve the crystal structure of the deposited films.