Formation Of Mixed Materials Research Articles

Simulating the nitrogen migration in Be/W tokamaks with WallDYN

The migration of wall material or seeding impurities plays an important role in the formation of mixed materials, the impurity contamination of the plasma and tritium retention. First, this work presents an improved model for the sputtering from mixed material surfaces in WallDYN. Second, we present dynamic SDTrimSP and WallDYN simulations of the nitrogen implantation in Be and the migration of nitrogen in tokamaks with a Be main wall. The simulations with the binary collision code SDTrimSP predict that N accumulates directly at the surface and that the Be erosion decreases with increasing N surface content. A first application of WallDYN to the nitrogen migration with an ITER-like wall indicates that the Be main wall may cause wall pumping of N by co-deposition with Be.

Multiscale modelling of plasma–wall interactions in fusion reactor conditions

The interaction of fusion reactor plasma with the material of the first wall involves a complex multitude of interlinked physical and chemical effects. Hence, modern theoretical treatment of it relies to a large extent on multiscale modelling, i.e. using different kinds of simulation approaches suitable for different length and time scales in connection with each other. In this review article, we overview briefly the physics and chemistry of plasma–wall interactions in tokamak-like fusion reactors, and present some of the most commonly used material simulation approaches relevant for the topic. We also give summaries of recent multiscale modelling studies of the effects of fusion plasma on the modification of the materials of the first wall, especially on swift chemical sputtering, mixed material formation and hydrogen isotope retention in tungsten.

Modeling of Surface Composition Dynamics in the ITER Divertor Region

The formation of mixed materials in the ITER divertor region is a critical materials issue that is expected to have a profound effect on the operation of the machine. This is particularly true for the divertor, the component where most of the energy is deposited during ITER operation. The divertor is made up of three tungsten domes and two vertical plates (inboard and outboard) which will be manufactured with composite carbon fiber according to the latest design. In the case of the first wall, the plasma-exposed surface is made out of beryllium; some of this beryllium is eroded from the first wall and finds its way into the divertor region, where it may strike the carbon vertical plates and get implanted or deposited. Having beryllium in the divertor enhances tritium trapping via codeposition, hence reducing the net available fuel for the reactor; on the other hand, there is evidence of carbon chemical erosion suppression. In this paper, the dynamic composition of a carbon surface under simultaneous bombardment by deuterium, tritium, and beryllium ions is studied. Both carbon and beryllium can redeposit on the target once eroded, so the source of ions receives feedback from the eroded material. Simulation results show that both the partial sputtering of Be and C atoms as well as the bombarding energy play an important role in Be accumulation on the vertical target plate, which can reach up to 50% near the surface. The results also show that the contribution of redeposited species may become so large that the DT bombarding flux may drop to 70% of the incident species.

Implications of the use of Carbon-Based Plasma Facing Components in Next Step Fusion Devices

The implications for the use of carbon based PFCs in next step devices are reviewed, with ITER as the reference example device. Issues with respect to steady-state operation concerning the carbon source at the divertor target in realistic conditions (including impurity seeding and mixed material formation caused by interaction of the plasma with the main chamber PFCs) and their implications for tritium retention are discussed taking into account other restrictions for next step plasma operation, such as the achievement of detached divertor operation with peak power loads <10 MW/m2. The effect of transient power loads on the divertor CFC target caused by ELMs are discussed and the implications for the use of CFCs at the divertor target for regimes with repetitive ELMs, acceptable from the divertor lifetime point of view, are reviewed.

Mixed material formation and erosion

The formation of mixed phases on materials relevant for first wall components of fusion devices is studied under well-defined conditions in ultra-high vacuum (UHV). This is necessary in order to determine fundamental parameters governing the basic processes of chemical reaction, material mixing and erosion. We examined the binary systems comprising of the wall materials beryllium, silicon, tungsten and titanium and carbon, the latter being both a wall material and a plasma impurity. Experiments were carried out to study the interaction of carbon in the form of a vapor-deposited component on clean, well-defined elemental surfaces. The chemical composition and the binding state are measured by X-ray photoelectron spectroscopy (XPS) after annealing treatments. For all materials, a limited carbide formation is found at room temperature. Annealing carbon films on elemental substrate leads to a complete carbidization of the carbon layer. The carbide layers on Be and Si are stable even at very high temperatures, whereas the carbides of Ti and W dissolve. The erosion of these two metals by sputtering is then identical to the pure metals, whereas for Be and Si a protective carbide layer can reduce the sputtering yields.