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.