Numerical Study Of The Erosion Of Tungsten Divertor Targets Caused By Edge Localized Modes

Strong plasma-wall interaction in the tokamak raises extremely high demands for candidate materials. Tungsten (W) has been chosen to be the material for the divertor armour of tokamak ITER due to its high thermal conductivity, high melting point, low tritium inventory, etc. But tungsten will undergo melting and evaporation under the transient events, e.g., the Edge Localized Modes (ELMs). The movement of melting layer leads to surface roughness and even to droplet splashing. Surface roughness will reduce thermal conductivity of tungsten, degrade its mechanic strength, succumb to erosion, and initialize arcing. Heat loads brought by large ELMs may damage the target plates and thus significantly reduce the lifetime of divertor tiles. Therefore, it is very important to study the thermal erosion of tungsten under the ELMs.The thermal erosion of divertor target induced by ELMs for both EAST and ITER has been evaluated in the dissertation. This dissertation employs finite volume methods to treat the varying interface between solid and liquid phases with satisfactory accuracy by tracking the moving boundary adequately with a smooth solid-liquid interface movement. The major works of this dissertation are summarized as follows:In chapter 2, a one-dimensional (Id) heat conductivity model, including evaporation, radiation, and melting processes, has been developed to investigate the thermal erosion of divertor target. The erosion of tungsten divertor targets caused by ELMs for EAST under current and possible future operation parameters is first studied. Based on current experimental data of heat fluxes on the carbon-fibre composites divertor in EAST, the surface temperature of slab-shaped tungsten is evaluated numerically. It is found that current Type I ELMs do not bring any noticeable changes to the tungsten target, the surface temperature being raised only several tens degrees. Simulation results indicate that ELMs will not become a concern for EAST tungsten wall for the time being and the near future, as long as much more severe transient events, e.g., disruption, can be avoided. The erosion of divertor targets resulting from ELMs under a wide range of operation parameters for the future tokamak ITER is also subsequently studied. Furthermore, the relationships between thermal erosion yield of slab-shaped tungsten and ELMs frequency, and ELMs peak heat flux, and inter-ELMs heat flux are also discussed, respectively.In chapter 3, the two-dimensional (2d) model is subsequently extended from the Id model and then used to study the thermal performance of the divertor tile with different edge shapes. To reflect the geometrical effects of castellated divertor tiles on the properties of its adjacent plasma, the energy flux density distribution arriving at the castellated divertor tile surface is evaluated first by a two-dimension-in-space and three-dimension-in-velocity particle-in-cell plus Monte Carlo Collisions code, and the obtained energy flux distribution is then used as input for the heat conduction model. The simulation results show that the divertor tiles with any edge shape of interest (rectangular edge, slanted edge, and rounded edge) would melt, especially, in the edge surface region of facing plasma poloidally under typical heat flux density of a transient event of type-I ELMs for ITER, deposition energy of 1 MJ·m-2 in a duration of 600 ?s. In comparison with uniform energy deposition, the vaporizing erosion is reduced greatly but the melting erosion is aggravated noticeably in the edge area of plasma facing diveror tile. Of three studied edge shapes, the simulation results indicated that the divertor plate with rounded edge is the most resistant to the thermal erosion.In chapter 4, a 2d fluid dynamics model is further developed by solving the liquid hydrodynamic Navier-Stokes equation together with the 2d heat conductivity equation for studying the erosion of slab-shaped tungsten targets in ITER during ELMs. In the model, typical interaction forces responsible for melt layer motion are taken into account. Simulations are carried out for ELMs under a wide range of fusion plasma performance parameters, and corresponding results indicate that the lifetime of W plates is determined mainly by the evolution of the melt layer. As a consequence of this melt layer motion, melted tungsten is flushed to the periphery, a rather deep erosion crater appears, and at the crater edges large humps of tungsten form during ELMs. The humps at both the crater edges under an ELM heat load with a Gaussian power density profile are almost the same at height if the J×B force is neglected, while the hump on the side opposinig to the magnetic field force is noticeably higher, caused by the JxB force.