A primary goal of F-TRIDYN is to make the code available for use in code- coupling situations. F-TRIDYN has been coupled to both plasma and mate-
rial codes. On the plasma side, F-TRIDYN allows as input incident energy distributions and mixed composition of incident ion species. F-TRIDYN tracks particles that sputter, reflect, implant, or transmit in the surface as well. On the materials side, these tracked particles can be used to model fast ion processes so that thermal and other, slower processes can be handled by a different code.
As part of the PSI-SciDAC project, F-TRIDYN has been coupled in a preliminary fashion to the codes Xolotl[37] and GITR[53]. Xolotl is an ADR material code that tracks the motion and reactions of impurities and defects in a Tungsten lattice. Implanted ions, whose distribution profiles are created with an F-TRIDYN simulation, are allowed to evolve in the lattice. Damage and sputtered particles are also tracked using F-TRIDYN simulations. GITR is an impurity transport code that uses F-TRIDYN to handle the response of the surface under plasma contact. Sputtered target atoms are tracked as impurities in the plasma.
Coupling to both plasma and surface codes allows F-TRIDYN to bridge the gap between processes in the plasma and processes in the surface. These processes interact in a complex way that is not easily handled by analytic methods. Bridging this gap brings about the possibility of whole-device modeling, allowing for the operation of a plasma device to be simulated at all relevant time and length scales simultaneously. Performing this task is a milestone for the PSI-SciDAC project. F-TRIDYN has been upgraded to include output files for file-based code coupling that TRIDYN does not have, including implanted ion tracking, PKA and SKA tracking, and more. Code- coupling improvements, including additional input and output options, will be a focus of future development of F-TRIDYN.
Figure 5.16: Dynamic composition over fluence of an evolving Tungsten target with 100 eV, normal incidence Beryllium ions bombarding it.
6
Conclusions
Understanding the complex interplay between plasma and surfaces is an important step to advancing the field of plasma physics. Plasma-Material Interactions comprise a rich and intricate set of processes that will play a significant role in the engineering of future plasma devices. Approaching PMI with simulations allows for the calculation of physical quantities unap- proachable by analytical or theoretical approaches. The inherent mathemat- ical complexity of the whole problem can be reduced to discrete processes whose relationships can be simulated explicitly. Using simulations to un- derstand PMI is the next step forward to understanding the role that the surface will play in future plasma devices such as fusion power plants. Code- coupling provides a set of strategies for compartmentalizing the problem of PMI into separate domains with their own time and length scales, allowing for multiple, independent simulation methods to be linked. This paves the way toward simulating whole devices, from the material bulk to the surface, from the surface to the plasma sheath, and from the plasma sheath inward into the bulk of the plasma.
F-TRIDYN has been developed with code-coupling in mind. F-TRIDYN includes models of surface roughness, a physical parameter whose influence on PMI is not fully understood. As the interface between the plasma and the material bulk, F-TRIDYN captures the complex behavior of ion-material interactions with rough and variously composed surfaces. F-TRIDYN’s li- braries and upgrades allow for simple use as an independent code and as part of a suite of codes coupled using file-based code-coupling procedures.
F-TRIDYN reproduces theoretical and experimental results on the ef- fect of surface roughness on sputtering, among the most important ion- material interactions. These results show the power and practicality the fractal surface model offers. Work on an alternative, statistical model of surface roughness is promising and offers significant speed increases over the fractal surface model.
7
Future Work
Future work on F-TRIDYN will focus first on improving code-coupling for the PSI-SciDAC project funded by the U.S. Department of Energy. Addi- tionally, the statistical model of surface roughness and its results and prop- erties will be investigated. F-TRIDYN will also continue to be developed as an independent platform, with additional libraries to simplify ease-of-use of the code. Additional physics, such as improvements to the scattering model and inclusion of more accurate electronic stopping are also being considered, promising to potentially lower the minimum energy of validity of the BCA model. All of these efforts will continue to expand understanding of ion- material interactions, especially in a plasma environment such as in a fusion reactor. Engineering of fusion power plants and operation of experimen- tal reactors such as ITER will rely on the results of whole-device models, and the inclusion of F-TRIDYN in those codes allows for more accurate and faster modeling of relevant surface interactions than alternatives such as MD codes or sputtering yield formulae.
Statistical surface models of surface roughness have already proved to be a fast, efficient alternative to explicit, morphological models. For this work, a normally distributed surface was chosen. Other distributions may yield more realistic results. Comparison of both fractal, non-fractal explicit, and various statistical models will provide crucial insight into which models are most useful for simulating PMI.
Additional possible areas of investigation include the reformulation of electronic stopping from first principles to attempt to capture low tempera- ture behavior as well as high temperature behavior. A significant gap exists in atomic simulations of PMI between MD and BCA codes. There is no framework that captures behavior between the fully-detailed, atomic mo- tions of MD codes and the binary, approximate collisions of BCA codes. Modification of the electronic stopping theory included in BCA codes to a more detailed model of electronic interaction may be the key in bridg- ing the gap between BCA and MD codes. Lindhard-Shcarff theory is well founded and is useful for a wide range of incident ion energies, but when implemented in a BCA code it fails to capture the detailed electronic inter-
actions that happen at the surface for low energy incident ions. Low energy interactions are important to a number of physical processes, such as adsorp- tion, desorption, and chemical reactions. Currently, the only framework for simulating these interactions is MD, which is computationally expensive, especially for complex systems that display long-range effects such as the Tungsten-Hydrogen-Helium system that drives the formation of Tungsten fuzz.
F-TRIDYN is currently not written to be fully parallelized, despite the readiness of the BCA framework to be fully parallelized. This is another potential area of work and would require the redevelopment of the base F-TRIDYN code in order to achieve full parallelization. If this can be ac- complished, however, it promises orders of magnitude faster BCA codes in the future. Since BCA codes are Monte Carlo codes, each thread, repre- senting the motion of one particles at a time, is independent of every other thread. Graphical Processor Units (GPUs) have thousands of cores, each of which could run an independent thread. Expansion of BCA codes to GPUs will be an important step in securing their place in the multi-code framework of HPC plasma codes.
An investigation of the experimental validity of the fractal surface model as compared to other models of surface roughness is also warranted. Using experiments, it will be possible to determine which model of surface rough- ness is most appropriate for a given system. Exploration of the effect of fractal dimension irrespective of the relevant length scale may also provide insight into the development of atomic scale structures via ion bombard- ment. Fractal theory remains a promising area of development for modeling many physical processes.
Modeling of surface evolution in a BCA framework has been achieved in a preliminary fashion in F-TRIDYN. This method has lead to interesting results, including the appearance of multiple equilibria of fractal dimension of a surface after ion bombardment. Surface modification via plasma bom- bardment remains an active area of research. If the fractal-BCA surface evolution model compares well to experiment, it may prove useful in ap- proximating the broad effect of surface modification post ion-bombardment. In situations where PMI is used for large-scale effects, such as increasing surface roughness to improve the efficacy of a subsequent chemical reaction, the BCA-fractal surface evolution model may be used to choose ion energy and angle of incidence required to achieve the desired surface morphology.
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