Modeling Of Divertor Plasma Sheath And Hydrogen Isotope Invenroty In Mixed Materials

Nuclear fusion is a promising long-term solution to the energy crisis human beings are facing. Tokamak is the most important in all the fusion devices. In a Tokamak, the plasma is powered by either external heating or self-heating by the energy released by fusion reactions in the core plasma. Fraction of the energy stored in the plasma unavoidably escapes and deposits on the plasma-facing components (PFC), either on the first wall or on the divertor. The divertors has been designed as a means of isolating the point of direct plasma-wall contact and allowing the efficient removel of the helium ash and heat flux. Plasma-wall interaction can shorten wall lifetime, increase hydogen isotope inventory, aggravate plasma contamination, and detereriate plasma operation. Therefore, it is necessary and important to study the characteristics of divertor plasmas, plasma-surface interaction, and resulting hydrogen isotope retention. This thesis, with focus on modeling of plasma-wall interation in the divertor region, contains three parts:simulation of divertor plasma with self-developed one-dimensional PIC code (EPPIC1D); modeling of plasma characteristics in the divertor gap region with self-developed two-dimensional PIC code (EPPIC2D); and modeling of hydrogen isotope inventory in different materials.In chapter2, the characteristics of divertor plasmas are investigated dynamically using a one dimension-in-space and three-dimension-in-velocity Particle-In-Cell Monte Carlo collision simulation technique (PIC-MCC), and it is found that a strong magnetic field with a small grazing angle on the divertor plate gives rise to a set of hybrid electrostatic waves distinctive frequencies. These waves are then identified as being related, respectively, to the lower hybrid wave mode and electron Bernstein wave modes. The wave associated with the lower hybrid wave mode dominates the potential fluctuation, modulates the incident energy flux onto the divertor plate in a large magnitude and causes the wave-like variation of impurity production from the carbon based divertor plates. By varying the incident angle and strength of the magnetic field, the effects of magnetic field on the low frequency wave and the erosion rate are also studied.In chapter3, the plasma characteristics in the divertor gap region are studied with a two dimension-in-space and three-dimension-in-velocity (2d3v) PIC-MCC model. The gaps between divertor tiles are ideal hideouts of impurities. To investigate the impurity deposition in the gaps, the thesis employs a2d3v PIC-MCC model to study the plasma characteristics inside and outside the tile gap. With the energy fluxes to the tile surfaces obtained from the simulations, the erosion rates of carbon-based tile plates are further evaluated. In addition, effects of magnetic field strength on the energy flux are also discussed.In chapter4, a fexible standalone model (a function module for other powerful fluid packages like SOLPS) is developed to simulate the hydrogen isotope inventory (HII) in the PFCs. Coupled with a heating model, the particle-balance-based model for reaction-difusion and HII in metal and porous media (carbon and co-deposited layers) is presented. Sample results, for illustrating the capability of this model, show good qualitative agreement with experimental measurements. To study the bubble growth processes during HII, a rivised model is developed based on a rate equation, in which hygrogen iosotope (HI) molecules can be produced via recombination processes inside the bubble, and HI molecules can also be dissoved into the bulk wall due to very high pressure inside the bubble. Details of this model and sample results are shown in detail.