D Experimental Study in Tore-Supra Ergodic Divertor Plasmas: Molecular Deuterium Density Measurements

Abstract

The deuterium Dα line emitted in front of an equatorial neutraliser plate in Ergodic Divertor Tore Supra plasmas is a common diagnostic of recycling particles in present Tokamaks. First, the possible perturbation of the Dα spectral lineshape by the nearby HeII line Brβ has been studied. Although its effect on the Dα line profile shape is evaluated to be non negligible in plasmas with large Helium contents, it is concluded that it should be of no influence in usual fusion plasmas. Second, a detailed study of the Dα spectral lineshape has been performed to obtain temperatures and densities of the different neutral deuterium species. A synthetic profile method taking into account Zeeman and Doppler effects has been used. This shows that two deuterium populations with specific behaviours have to be considered: the first one (‘cold atoms’) is composed of atomic deuterium coming from D2 desorbed from the neutraliser plate, while the second one (‘warm atoms’) is composed of charge exchange (CX) and reflected deuterium. The Doppler temperature of cold atoms increases with the edge electron density, nedgee (1.4 eV < TcD < 2.5 eV). The possible heating of cold atoms by D+ ions is evaluated to explain this effect. Molecular and atomic deuterium densities behaviour versus edge electron temperature Te are analysed with the use of the warm and cold Dα brightnesses coupled to a collisional radiative deuterium model. The results are compared to the numerical code EDCOLL. An important molecular density (> 1018 m-3) is found for the whole range of edge electron temperature (10 eV < Te < 40 eV) while the ratio of the atomic to the molecular densities is around 1.8. The density of atoms coming from molecular dissociation is ∼ 0.8 times the density of atoms created by charge exchange or re.ection of deuterons on the neutraliser.