Description
Tungsten is becoming the leading solution as first wall material of magnetic confinement fusion reactors, as confirmed by the recent decision of ITER to also adopt tungsten in the main chamber.
Tungsten is the metal with the highest melting point and combines low erosion rates with low fuel retention. However, tungsten is also one of the elements which leads to the highest radiation losses in a wide range of fusion relevant temperatures. In this plenary talk, progress in experiments, theory and predictive modelling dedicated to tungsten transport in tokamak plasmas is retraced, highlighting the main milestones.
These in particular include: the development of appropriate strategies to reduce tungsten erosion and limit the tungsten concentration in the plasma; the positive role of central wave heating to prevent central accumulation, through modifications of the tungsten transport and of the main plasma profiles; the complicated impact of plasma rotation on collisional transport, which is understood to be particularly unfavorable in usual plasma conditions in present tokamaks. All transport mechanisms, collisional, turbulent and MHD-driven, are involved and can compete. The understanding of these processes has helped to identify effective actuators to mitigate the risk of excessive core radiation and to increase the predictive capability for present and future tokamaks. Some aspects are still challenging the scientific community, particularly to predict turbulent transport at the plasma edge.
These achievements have progressively allowed the integration of tungsten in tokamak operation, overcoming the challenges provided by the need to successfully combine competing requirements of the plasma edge, where tungsten originates, and the plasma core, where strong accumulation can occur. A successful core-edge coupling is essential for the high plasma performance required in a fusion reactor, but which tungsten makes more difficult to achieve. However, strategies to limit tungsten erosion and edge plasma contamination practically coincide with those which are in any case imposed by the heat exhaust and a long lifetime of the plasma-facing components.
Experimental and theoretical research have advanced together, converging on deeper physics knowledge, controlled plasma operation and more reliable reactor predictions, enabling more future tokamak projects to confidently choose tungsten as plasma-facing material.