A Comparative Study Of The Effects Of Liquid Lithium And Tin As Demo Divertor Targets On The Heat Loads And Sol Properties

The behaviour of the scrape-off plasma of the European tokamak DEMO (DEMOnstration tokamak of the economical feasibility of the fusion power) is analysed by means of the 2D edge code TECXY when either lithium or tin are used as liquid target materials in the single null configuration. The targets are modelled as a thin liquid metal layer superimposed on a tungsten substrate that faces the plasma, while its bottom is kept at a fixed temperature. The top target temperature is calculated from the heat transport equation. The evaporation rate so derived and sputtering give the total impurity source strength. The impurity concentration and the involved radiative losses are calculated self-consistently by solving the multifluid plasma transport equations. A scan in the upstream separatrix density range n(e,sep) = 2.5-4.5 x 10(19) m(-3) is carried out for the nominal power input into the scrape off layer, P-SOL = 150MW and in the range P-SOL = 150-250MW for n(e,sep) approximate to 3.5 x 10(19) m(-3). Steady state solutions are found only if the maximum plate temperature is kept below the threshold for a very steep increase in evaporation. The radiative capability and peak load mitigation are higher for Sn than for Li but only slightly at high density. If a ceiling to the Sn concentration is set for high core fusion performance, Sn mitigation at low density may drop below Li. Nonetheless, concerns on the Li option are derived from the very large material consumption and on the fact that the main mechanism of impurity release is sputtering, which is outside a full control of the operator. For both Sn and Li, detachment starts to appear only close to the upper limit of the spanned density range. A regime where evaporation overcomes sputtering is more effective in dissipating the input power, especially for Sn. In this case, a sort of vapour shielding seems to develop attached to the impurity source.