Optimization of pumping efficiency and divertor operation in DEMO

In the present work a sensitivity analysis of the pumping performance of a standard divertor design for two extreme dome cases (with and without) and different pumping port locations is performed. Such an investigation re-assesses the role of the divertor dome in the design of a DEMO divertor cassette. The non-linear neutral gas flow in the private flux and sub-divertor region is modeled based on the Direct Simulation Monte Carlo (DSMC) method, which takes into account the intermolecular collisions as well as the interaction of the molecules with the stationary walls. For this specific configuration, three different pumping port locations, namely in the low and high field bottom sides of the sub-divertor and directly under the dome haven been chosen. It is shown that the optimum pumping port location is found to be directly under the dome, since the pumped particle flux is increased by a factor 2–3 compared to the one, where the port is located inside the low and high field side divertor “shoulders”, respectively. In addition, the divertor dome physically restricts the conductance between the private flux region and the main chamber, enabling the compression of the neutral gas. However, the dome has no direct influence on the macroscopic parameters as the number density and the temperature at the pumping port. Furthermore, it is shown that without the dome, a strong outflux of neutrals towards the plasma core and through the x-point and its vicinity can be expected. This outflux can be reduced by a factor of 2 by positioning the pumping port directly under the dome. Finally it is noted that in all the obtained calculations, the flow field remains homogeneous without the presence of vortices. This can be explained by the fact that the studied geometry does not include any high curvature surfaces, which promote the formation of such flow structures

Metamorphosis of plasma turbulence-shear flow dynamics through a transcritical bifurcation

The structural properties of an economical model for a confined plasma turbulence governor are investigated through bifurcation and stability analyses. A close relationship is demonstrated between the underlying bifurcation framework of the model and typical behavior associated with low- to high-confinement transitions such as shear flow stabilization of turbulence and oscillatory collective action. In particular, the analysis evinces two types of discontinuous transition that are qualitatively distinct. One involves classical hysteresis, governed by viscous dissipation. The other is intrinsically oscillatory and non-hysteretic, and thus provides a model for the so-called dithering transitions that are frequently observed. This metamorphosis, or transformation, of the system dynamics is an important late side-effect of symmetry-breaking, which manifests as an unusual non-symmetric transcritical bifurcation induced by a significant shear flow drive.Comment: 17 pages, revtex text, 9 figures comprised of 16 postscript files. Submitted to Phys. Rev.

Observation of an impurity hole in a plasma with an ion internal transport barrier in the Large Helical Device

Extremely hollow profiles of impurities (denoted as “impurity hole”) are observed in the plasma with a steep gradient of the ion temperature after the formation of an internal transport barrier (ITB) in the ion temperature transport in the Large Helical Device [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. The radial profile of carbon becomes hollow during the ITB phase and the central carbon density keeps dropping and reaches 0.1%?0.3% of plasma density at the end of the ion ITB phase. The diffusion coefficient and the convective velocity of impurities are evaluated from the time evolution of carbon profiles assuming the diffusion and the convection velocity are constant in time after the formation of the ITB. The transport analysis gives a low diffusion of 0.1?0.2 m2/s and the outward convection velocity of ~1 m/s at half of the minor radius, which is in contrast to the tendency in tokamak plasmas for the impurity density to increase due to an inward convection and low diffusion in the ITB region. The outward convection is considered to be driven by turbulence because the sign of the convection velocity contradicts the neoclassical theory where a negative electric field and an inward convection are predicted