In the last campaign, the TJ-II heliac has been operated under lithium-coated walls, representing the first stellarator ever working under these boundary conditions. Enhanced density control and discharge reproducibility, leading to the drastic enlargement of the operational window, have been obtained. A strong decrease in recycling together with changes in the shot by shot fuelling characteristics and in the wall particle inventory have been recorded. These changes, associated with the new wall scenario, had led to a long-lasting good density control. The new conditions were also mirrored in the plasma profiles under NBI heating scenarios with increased peaking of the electron density profiles. Fuelling rates corresponding just to the nominal beam current were obtained for the first time, and transitions from bell to dome-type plasma profiles, with different collapsing limits, were observed and tentatively ascribed to changes in the local edge power balance. ELM-type activity was observed in concomitance to reduced fluctuation levels and confinement improvement. Record values of plasma energy content were measured at central densities up to 8 × 10 19 m −3 under Li-coated walls.

This paper presents the latest results on confinement studies in the TJ-II stellarator. The inherently strong plasma–wall interaction of TJ-II has been successfully reduced after lithium coating by vacuum evaporation. Besides H retention and low Z, Li was chosen because there exists a reactor-oriented interest in this element, thus giving special relevance to the investigation of its properties. The Li-coating has led to important changes in plasma performance. Particularly, the effective density limit in NBI plasmas has been extended reaching central values of 8 × 1019 m−3 and T e ≈ 250–300 eV, with peaked density, rather flat T e profiles and higher ion temperatures. Due to the achieved density control, a second type of transition has been added to the low density ones previously observed in ECRH plasmas: higher density transitions characterized by the fall in Hα emission, the onset of steep density gradient and the reduction in the turbulence; which are characteristics of transition to the H mode. Confinement studies in ECH plasmas indicate that lowest order magnetic resonances, even in a low shear environment, locally reduce the effective electron heat diffusivities, while Alfven eigenmodes destabilized in NBI plasmas can influence fast ion confinement.

In this paper a non linear mathematical model to describe absorption phenomena in polymers\ud is proposed. The model is established assuming that the diffusing penetrant causes a\ud deformation which induces a viscoelastic stress responsible for a convective field. This convective\ud field is defined to represent an opposition of the polymer to the Fickian diffusion.\ud Several numerical examples show the effectiveness of the model

In this paper we study the convergence of a centred finite difference scheme on a non-uniform mesh for a 1D elliptic problem subject to general boundary conditions. On a non-uniform mesh, the scheme is, in general, only first-order consistent. Nevertheless, we prove for s ∈ (1/2, 2] order O(h s)-convergence of solution and gradient if the exact solution is in the Sobolev space H 1+s (0, L), i.e. the so-called supraconvergence of the method. It is shown that the scheme is equivalent to a fully discrete linear finite-element method and the obtained convergence order is then a superconvergence result for the gradient. Numerical examples illustrate the performance of the method and support the convergence result.

This paper deals with the supraconvergence of elliptic finite difference schemes on variable grids for second order elliptic boundary value problems subject to Dirichlet boundary conditions in two-dimensional domains. The assumptions in this paper are less restrictive than those considered so far in the literature allowing also variable coefficients, mixed derivatives and polygonal domains. The nonequidistant grids we consider are more flexible than merely rectangular ones such that, e.g., local grid refinements are covered.The results also develop a close relation between supraconvergent finite difference schemes and piecewise linear finite element methods. It turns out that the finite difference equation is a certain nonstandard finite element scheme on triangular grids combined with a special form of quadrature. In extension to what is known for the standard finite element scheme, here also the gradient is shown to be convergent of second order, and so our result is also a superconvergence result for the underlying finite element method. o