Deuterium atomic cross section uncertainties and molecular recycling effects in the divertor region of tokamaks

Abstract

How predictions of deuterium atomic reaction rates in tokamak divertors are altered by changes in cross section data which are comparable with the uncertainty in published data is investigated. An extended neutral diffusion theory calculation, which includes D atoms and ground and excited state D2 molecules, is performed on a fixed background plasma. For the atoms, the principal relevant mechanisms are elastic scattering, charge exchange, ionization, and recombination. It is evaluated that the charge exchange and electron processes are known to an accuracy better than +/−25%, and it is concluded that this level of accuracy is sufficient for the present state of divertor calculations. Of significant concern is the fact that classical elastic cross sections which are widely used in divertor calculations differ substantially from a more accurate set of cross sections based on quantum mechanics, and that the latter produce predictions of neutral density and ionization rate which differ by orders of magnitude from predictions based on the classical cross sections. Opacity of the plasma to Lyman alpha radiation influences transport for high density plasma situations. The explicit representation of incident particles recycling as molecules is shown to significantly affect the neutral atom density and reaction rates near the divertor plate. In particular, the dissociation of even a small fraction of excited molecules significantly reduces the plasma ion population and increases the neutral atom population just in front of the plate in nearly detached plasmas.