The use of beryllium, carbon and tungsten as plasma-facing materials (PFMs) in ITER calls for dedicated investigations on their behaviour under the expected particle and power loads and neutron irradiation. Their simultaneous use implies the formation of mixed materials during plasma operation, which can have significantly different properties compared with the initial materials concerning thermo-mechanical behaviour and T retention. This contribution presents the latest results on these issues mainly achieved under the umbrella of the European taskforce on plasma wall interaction. While laboratory results provide the basis for the understanding of the basic properties and the behaviour of PFMs, experiments in fusion devices are indispensable in order to test their integral performance, incorporating also the internal feedback on the plasma properties. For this reason ASDEX Upgrade has been converted to a full W device, while JET is beginning operation with its ITER-like wall consisting of Be main chamber plasma-facing components and a W divertor. In parallel, dedicated W experiments are performed in several devices in order to investigate specific uncertainties such as the characteristics of melt-layer movement in the presence of a strong magnetic field. Based on the results on ASDEX Upgrade, which reveal a narrower operational space as compared with the operation with graphite plasma-facing components but also provide tools for a successful operation with tungsten, the experiments at JET will provide a unique opportunity to address specific issues related to the parallel use of Be and W as PFMs with plasma parameters closest to those of ITER. Amongst these are the anticipated but still to be demonstrated reduction in hydrogen retention compared with a carbon device, the test of conditioning procedures, mixed materials effects, erosion and transport, the effect of edge localized modes (ELMs) and ELM mitigation methods as well as the behaviour of melt layers and their influence on plasma operation under steady state and transient heat loads.
Chapter 8: Plasma operation and control