M. Warrier

Subroutines for some plasma surface interaction processes: physical sputtering, chemical erosion, radiation enhanced sublimation, backscattering and thermal evaporation ☆

A suite of FORTRAN subroutines/functions to generate data using empirical formulas for physical sputtering of mono-atomic targets for any elemental incident ion (atom), chemical erosion of graphite, Radiation Enhanced Sublimation (RES) of graphite, the number and energy backscattering coefficients for any elemental incident ion (atom) on a compound target and thermal evaporation of graphite is presented. Since chemical erosion, RES and thermal evaporation depend on the surface temperature of graphite, a subroutine implementing the 1-D heat diffusion equation to determine the temperature of any plasma-facing graphite surface is implemented. As an example to illustrate the use of these subroutines/functions, a simple model for the erosion of a plasma-facing surface, consisting of a simple collisionless sheath model, a 1-dimensional steady state heat diffusion model and 0-dimensional steady state particle balance at the target is developed and a sample listing of the program is presented.

Modelling of hydrogen isotope inventory in mixed materials including porous deposited layers in fusion devices

Hydrogen isotope inventory (HII) is a key issue for fusion devices such as ITER. Simultaneous use of Be, W and C as the wall material for different parts of plasma-facing components (PFCs) will bring in material mixing issues, which compound that of hydrogen isotope retention. To simulate the hydrogen inventory in the PFCs, we have developed a flexible standalone model called HIIPC (Hydrogen Isotope Inventory Processes Code). The particlebalance-based model for reaction–diffusion and HII in metal and porous media (mainly carbon and co-deposited layers) is presented, coupled with a heating model which can calculate the temperature distribution. Some sample results are given to illustrate the model’s capabilities and show good qualitative agreement with the experiment. (Some figures may appear in colour only in the online journal)

Effect of the porous structure of graphite on atomic hydrogen diffusion and inventory

A multi-scale model for particle diffusion in porous structures is used to study the effect of the internal structure of graphite on atomic hydrogen transport and inventory in graphite. The diffusion of trace amounts of atomic hydrogen are modelled as a trapping–detrapping mechanism within the porous network typical in graphites. Activation energies for the different traps are taken from experiments and from molecular dynamics simulations. Different diffusion mechanisms dominate at different graphite temperatures. It is seen that the trace diffusion coefficient of hydrogen scales as the square of the jump lengths of the dominant mechanism. Depending on the detrapping mechanism and on the internal structure of graphite, the jump lengths after each detrapping event can vary over a few orders of magnitude. This gives rise to the possibility of anomalous diffusion in graphite. The effect of closing the pores of graphite on atomic hydrogen diffusion is also presented.

Extensions to the SOLPS edge plasma simulation code to include additional surface interaction posibilities

The SOLPS suite of codes is used extensively to model the edge plasma of existing tokamaks, and for design studies of new tokamaks. At its core is the combination of a fluid plasma code, B2, and a Monte Carlo neutrals code, Eirene. Usually, the interaction with the wall is handled by Eirene, but B2 also has the capability of treating neutrals with a fluid model, and so both codes need to deal with the interaction of the plasma and neutrals with material surfaces. A number of recent enhancements to the plasma–wall models, particularly on the B2 side, have been made, and are described. In particular, the plasma–wall interaction model has been extended to include the modelling of mixed materials (such as is proposed for ITER).

Multi-scale modeling of hydrogen isotope transport in porous graphite

We describe a general multi-scale method to model hydrogen isotope transport, over length scales from angstroms to centimeters and time scales from picoseconds to several seconds, in a complex three-dimensional porous geometry using molecular dynamics, kinetic Monte Carlo and Monte Carlo diffusion simulations. we present comparisons with experimental results for hydrogen diffusion and hydrogen atom desorption from graphite. Finally, we demonstrate the flexibility of the computational tools used to tackle problems at different scales.

Comprehensive suite of codes for plasma-edge modelling

The various aspects of plasma-edge physics are included in a comprehensive suite of codes having applications from industrial plasmas to fusion devices. Here the basic ideas, status, and relationship of the codes are summarized: Plasma-wall interaction effects on a microscopic length-scale (e.g., chemical sputtering effects) are studied with molecular dynamics. Mesoscale effects (e.g., sputtering and diffusion in amorphous materials) are analysed with Monte Carlo methods (kinetic Monte Carlo with input from molecular dynamics or experiment or binary collision approximation). A full kinetic description (including ions, electrons, neutrals and their collisions) is possible for some low-temperature plasmas (e.g., electron cyclotron resonance heated methane plasmas) and for qualitative studies of edge plasma effects in fusion edge plasmas. Fluid transport codes for the edge of magnetically confined plasmas (2D tokamaks, tokamaks with ergodic perturbations, 3D stellarators) are necessary for understanding better the complex physics in such devices. The different code levels provide physics which is embodied in simplified models for those above and below it in the hierarchy or for those linked across various boundary regions.