In 1997 the new `LYRA' divertor went into operation at ASDEX Upgradeand, in parallel, the neutral beam heating power was increased to 20 MW by installation of asecond injector leading to a P/R value of 12 MW/m.Experiments have shown that the ASDEX Upgrade LYRA divertor iscapable of handling such high heating powers. There is an overallreduction of the maximum heat flux in the LYRA divertor byabout a factor of 2 compared with the previous open divertor Div I. This reduction ismainly due to increased radiative losses inside the divertor region,which are caused by an effective reflection of hydrogen neutrals into thehot separatrix region. The main channel of radiative loss is carbon radiation,which cools the divertor plasma down to a few electronvolts, where hydrogen radiation losses become significant.The radiative losses preferentially reduce the power flux atthe separatrix, leading to early detachment around the strike pointposition. With increasing density, the detached regionextends upwards on the vertical target. The power fraction radiated in the LYRA divertor is around 45%and nearly independent of the heating power. This value isa factor of 2 higher than the typical radiation fraction in Div I.B2-EIRENE modelling of the performed experiments supports the experimental finding and refines the understanding of loss processes in the divertor region.
Physics design of new lower tungsten divertor for long-pulse high-power operations in EAST