Power handling of liquid metal infused trenches in the EAST Tokamak
Flowing liquid metal PFCs offer an attractive solution to the problems currently facing conventional solid high-Z PFCs such as: the sputtering of high-Z impurities into the core, fuzz formation from helium implantation and thermal damage (melting, blistering and cracking.) By presenting a constantly refreshing surface to the plasma liquid metal PFCs protect against damage by removing heat via convection from the system and by acting as a particle sink thereby preventing sputtering of high-Z atoms. Lithium is the popular choice for liquid metal PFCs as it is a low-Z element and is a strong chemical getter so can remove impurities from the plasma greatly enhancing confinement time within the plasma. Lithium also reduces the recycling of hydrogenic species at the plasma edge decreasing the collisional cooling in these regions consequently keeping the edge of the plasma hot. Maintaining a hot plasma edge flattens the temperature gradient across the plasma which in turn suppresses the anomalous turbulent transport, stabilising the plasma against ELMs and greatly increasing confinement time. In order to develop lithium PFC technology: three generations of a FLiLi type limiter have been tested in EAST. The tests in EAST have shown the inclusion of FLiLi to increase the confinement time within the tokomak, reduce large scale ELM and produce ELM free periods as well as reducing the recycling within the plasma increasing stored plasma energy. In continuation of this work a LiMIT type limiter was exposed to the EAST plasma and this experimental campaign will form the basis of this work. The LiMIT design consisted of a trenched front face with 2mm wide 1mm trenches, these trenches allow for lithium flow to be driven by TEMHD via the Seebeck effect. Previous work at UIUC has shown the ability for a LiMIT design to produce velocities of ∼ 60cms−1 and withstand heat fluxes of 10MWm−2 under electron beam exposure. Complementary to the experimental data a COMSOL model has been developed to calculate the heat absorbed into the TZM backplate of LiMIT by correlating the simulated temperature profiles over the plasma exposure with the experimentally measured TC. This data can then be combined with calculations of the heat absorbed into the lithium film via convection and the power dissipated by a lithium vapour cloud formed above the plate to produce a measurement of the total heat flux experienced by the plate and begin to benchmark the heat handling capabilities of the LiMIT design. The most extreme conditions faced by the LiMIT plate occurred with 6.7MW of auxiliary heating with the plate positioned 2cm from the separatrix. In this case a conductive heat strike of 8.2MWm−2 was calculated in COMSOL further supplemented with 5MWm−2 dissipated by the vapour cloud. This result displays the ability of the LiMIT design to exceed the maximum heat limit for tungsten PFCs of 10MWm−2 and further still the LiMIT design withstood multiple high powered NBI shots with heat fluxes above 10MWm−2 with no macroscopic damage caused to the plate highlighting LiMIT’s ability to protect against multiple exposure to high temperature plasma. Increase in plasma performance was noted with lithium evaporation and subsequent redepostion on EAST’s divertor regions: during RF heated shots this effect conditioned the device leading to a transition from low to high confinement operation and with NBI heating; shots subsequent to large lithium evaporation from LiMIT produced plasmas with increased density, stored energy and confinement time due to a decrease in the global recycling. Development of the COMSOL model elucidated the interaction between the EAST plasma and LiMIT. The plasma strike points were found to be localised to the ion and electron sides of the plate, with the ion side experiencing a heat flux an order of magnitude higher than the electron side.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign