EDGE2D-EIRENE Study on Tungsten Fluid Versus Tungsten Kinetic

New materials for plasma facing components in the next generation tokamak ITER are beryllium for the main chamber, CFC and a tungsten divertor. This combination will be tested at JET during the ITER like wall project, the current status of which is summarised in [1]. Recently the edge modeling code package EDGE2D-EIRENE [2] at JET has been extended to handle the new wall materials. First tungsten transport studies with EDGE2D-EIRENE, where the impact of bundling charge states on impurity screening has been analysed, have been presented [3]. Bundling schemes are important for W transport modeling with EDGE2D, because simulating all 74 charge states is computationally extremely expensive and numerical artefacts can occur. Thus bundled charge state schemes are introduced to reduce the number of charge states in the computation. The bulk plasma as well as the impurity ions are described within EDGE2D as fluids, which requires the assumption of full thermalisation of the particular ions. From ionisation rates ofWtaken from the ADAS data base [4] and assuming ne ~ 1013/cm3 and Te = 10eV (near the divertor) the lifetimes of the lower charge states of tungsten W1-4+ is below 10-4s and can be even shorter further upstream in the JET SOL where temperatures reach more than 200eV. In contrast equilibration of heavy W ions released into a 10eV plasma at wall temperature takes approximately 150ms, thus only W5+ and higher ionised W charge states can be fully thermalised, hence the criteria of full thermalisation, which is required for a fluid description, might be violated for W1-4+. Only recently the kinetic Monte Carlo code EIRENE has been expanded with a new trace ion option, referred to as Trace Ion Module (TIM). The new module comprises the numerical algorithms required to solve the drift kinetic equation in the tokamak geometry with Monte Carlo approach. The EIRENE code has become an integrated code package which unifies kinetic neutral particle dynamics with kinetic trace ions in one single code. Beside other kinetic impurity transport codes, e.g. DIVIMP or DORIS, the trace ion module utilises finite element methods, which is new and allows calculating drifts on unstructured grids. Moreover the Coulomb collision operator has been further extended compared to e.g. DORIS. The details of the trace ion module are summarised in [5]. Currently EIRENE is used by many plasma transport codes, e.g. EDGE2D, SOLPS and EMC3, for describing the dynamics of neutrals in the divertor region.With the trace ion module transport ofWions is described kinetically and can be benchmarked against the fluid transport model of EDGE2D and other fluid codes. This approach allows to solely compare the nature of the transport models, because in both cases (fluid or kinetic) the same EDGE2D plasma solution for the bulk plasma is used, the boundary conditions are the same as well as the atomic physics and the grid.
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