Transport of tungsten in the H-mode edge transport barrier of ITER

Abstract

The neoclassical transport of tungsten in the H-mode edge transport barrier (ETB) of ITER is investigated and scaling relations for the transport coefficients are evaluated. The major finding is that the radial convection velocity of tungsten is outward directed for a large proportion of the tested pedestal profiles. This is due to a combination of the high pedestal temperatures and the high separatrix densities making the outward directed temperature screening term the predominant contribution of the collisional convection. The high densities at the separatrix are needed to control the power exhaust and the tungsten sputtering in the divertor and the high pedestal temperatures are expected to be achieved in ITER to meet its fusion performance objectives. The density and temperature profiles, which deliver simultaneously optimum fusion performance, power exhaust and tungsten sputtering control in ITER, also ensure optimum collisional radial transport of tungsten leading to hollow tungsten density profiles in the ETB. In helium H-mode plasmas, which are considered for the initial ITER non-active operation, this very favourable neoclassical transport effect is reduced and only about half of the considered edge plasma profiles yield an outward directed drift.