A Feasibility Assessment of Liquid Metal Divertors

Abstract

The divertor technology has become the focus of concern for prospective steady state tokamak reactors. The imposed heat flux and particle flux conditions cast doubt on the feasibility of any solid surface divertor. The aim of this work is to evaluate the feasibility of the existing concepts of liquid metal divertors from both the physics and engineering points of view. It is found that lithium is not a favorable liquid metal due to the large tritium inventory that may develop in the form of solid hydride LiH. Gallium, on the other hand, does not form hydride within the temperature range of interest, and hence is considered a favorable material. Slowly flowing thin film and pool type divertors are found to be undesirable owing to the possible blistering erosion and resulting plasma contamination. The popular concept of self-cooled liquid metal film divertor suffers mainly from the linear MHD instability, in addition to other complicating factors such as the variation of the liquid metal electric property, dynamics of halo current, and the compatibility problem of insulator coating with the liquid metal. The liquid gallium droplet curtain divertor is evaluated to be the most feasible. However, unless an effective helium pumping scheme can be developed, the goal of controling the neutral recycling coefficient by liquid metal divertors can not be accomplished.