Simulation And Analysis Of The Interactions Between Nano-dust And Plasma-facing Materials In Tokamak

The dust issues are the key difficulties on the way to reality the commercial fusion reactors. Dust particles are not only one of the sources of impurities which rises the difficulty of plasma operation, but also interact with plasma-facing materials (PFM) which leads the modification or even damage on PFM and produce sputtering atoms. A systematically research on the interactions between dust and PFM has been performed in the present thesis. Firstly, the electrostatic force of dust particles in plasma sheath is studied and compared with other forces acting on dust, such as friction force, gravity etc. The motion equation is obtained which offers the initial velocity of dust landing on PFM. Secondly, with the aid of molecular dynamics, the simulation on the interactions between dust and PFM is performed. Results shows that the incident velocity, size as well as the number of dusts effect the interaction results. Finally, a predictive model is developed based on collision theory. This model can offer the results of dust-PFM interactions.With the aid of bispherical and bipolar coordinates, analytic calculations are performed on the dust particles touching or untouching PFM surface in the researching on dust. The analytic calculations offer the electrostatic field in plasma sheath with the existence of dust particle. The charge density on dust and PFM surface as well as the relations between dust potential and total charge are also obtained. Based on these results, the electrostatic force is obtained and compared with other possible forces acting on dust in order to calculate the motion equation of dust in sheath. The impacting velocity of dust particles to PFM can be computed from the equation which offers the foundation of dust-PFM interaction.A series of factors which are possible to effect the interactions between dust and PFM have been discussed, for example, impacting velocity/energy, size, angle and elements and the number of impacting dusts. Because beryllium (Be) and tungsten (W) are used as first wall and divertor respectively in ITER, Be/W dust impacting to Be/W PFM surface are discussed in present thesis. The results show that three different effects on PFM surface present under different impacting velocity. In low-velocity case, dust particles stick on PFM surface and is not deformed. Dust particle can form loose thin- film on PFM surface. In high-velocity case, dust do not damage PFM but can produce compact film. When dust velocity is high enough, dust particles can damage PFM surface and cause sputtering. The damage depends on dust velocity. Meanwhile, heaver dust, W dust for example, are easier to damage PFM surface while lighter ones are difficult to damage PFM. Because the studying methods are quite similar to the cluster/solid interactions so after the discussion of dust/PFM interaction, we further make some statements on the effect of energetic cluster on solid and the sputtering and cratering process in order to extend our research field.Based on conservation laws and collision theory, an analytic model is developed on the interactions of dust/PFM. By comparing with molecular dynamics simulation and experimental results from present thesis and other previous papers, the model can precisely predict the dust-deforming energy, PFM damage energy and the dimensions of the crater. The behaviors of other possible dusts, such as Fe dust, Mo dust and C dust are predicted based this model.The results presented in this thesis are helpful for predicting the lifetime of plasma-facing materials and its surface modifications during long-time discharge. These results deepen the understanding of dust issues which offer valuable experiences and foundations for plasma-facing materials as well as the design of ITER and future Tokamak devices.