Particle transport in density gradient driven TE mode turbulence

The turbulent transport of main ion and trace impurities in a tokamak device in the presence of steep electron density gradients has been studied. The parameters are chosen for trapped electron (TE) mode turbulence, driven primarily by steep electron density gradients relevant to H-mode physics, but with a transition to temperature gradient driven turbulence as the density gradient flattens. Results obtained through non-linear (NL) and quasilinear (QL) gyrokinetic simulations using the GENE code are compared with results obtained from a fluid model. Main ion and impurity transport is studied by examining the balance of convective and diffusive transport, as quantified by the density gradient corresponding to zero particle flux (peaking factor). Scalings are obtained for the impurity peaking with the background electron density gradient and the impurity charge number. It is shown that the impurity peaking factor is weakly dependent on impurity charge and significantly smaller than the driving electron density gradient.Comment: 11 pages, 6 figures. Submitted to Nuclear Fusion SP

Effect of Lateral Constraint on the Mechanical Properties of a Closed-Cell Al Foam: Part II. Strain-Hardening Models

Experimental results, presented in the companion article, show that the compressive deformation of a closed-cell Al foam under lateral constraint is characterized by significant strain hardening. This enhanced hardening is due to the change in stress state from uniaxial to triaxial, which additionally contributes to friction between the deforming foam and the walls of the constraining sleeve. Detailed analysis, employing two different types of deformation models, is presented in this article in order to rationalize the experimental observations. In the heterogeneous model, it is assumed that plastic deformation is similar with and without constraint and that it occurs via collective plastic collapse of cells. The bands, thus formed, elastically bear the lateral stresses and give rise to friction. In the homogeneous deformation model, it is assumed that the deformation mode is different under constraint and involves uniform densification, which leads to inherent hardening as well as additional friction. By comparing the model predictions with experimental observations, it is suggested that the plastic strain hardening of the metallic foam under constraint is due, in equal measure, to the triaxial state of stress and friction. Mechanistically, the material deforms principally by collective cell collapse, though there is some evidence of concurrent homogeneous deformation