Development of the Leidenfrost divertor- advancing our understanding of liquid lithium
A seminar by Thomas Morgan
Abstract: Solving the heat exhaust problem is one of the biggest challenges facing the development of fusion energy. Unmitigated heat loads to the diverter exceed all material limits, and therefore power density levels need to be strictly controlled. Active management of this is challenging from a control perspective however, as actuation with e.g. gas injection has slow responses and hard limits. Too little gas and the diverter melts, too much and the plasma disrupts. Using a liquid lithium wall which evaporates, the strong interaction between the plasma and the vapour cloud reduces the heat load via passive stabilization which depends directly on the heat load arriving at the target to set the evaporation rate. In other words a negative feedback loop exists which makes this an inherently stable arrangement. This version of the vapour box diverter put forward by Goldston and colleagues [1,2] can be termed a Leidenfrost diverter- analogous to how the Leidenfrost effect allows water droplets to survive due to a cushion of vapour on a hot surface like a frying pan. This approach presents two key challenges however: to prevent excessive accumulation of the evaporated material in the core plasma, where the impurity ions can degrade the fusion output, and to avoid long term fuel-ion wall retention due to trapping of hydrogen isotopes (HI) inside the liquid lithium. We have carried out experiments using the unique linear plasma devices at DIFFER- Magnum-PSI and Upgraded Pilot-PSI to tackle these challenges. A vapour box module was designed and built to improve our physics understanding of the mechanisms of vapour shielding. Testing in Magnum-PSI demonstrated power reductions of greater than 50%, with strong confinement of lithium towards the target. This is mediated by strong charge-exchange interactions between Li and H, in line with SOLPS-ITER simulations. At the same time experiments show that Li-HI co-deposits, which would be the main cause of tritium loss, do not form at high temperatures such as those expected on the first wall of a reactor, and that HI can be effectively removed by isotope exchange. This talk will outline these results and how this provides an exciting path forward for solving heat exhaust in fusion reactors of the future.
[1] R J Goldston et al Phys. Scr. T167 (2016) 014017
[2] R.J. Goldston et al. Nuclear Materials and Energy 12 (2017) 1118–1121
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