On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)

Measuring thermal conductivity in freezing and thawing soil using the soil temperature response to heating

The thermal conductivity of the thin seasonally freezing and thawing soil layer in permafrost landscapes exerts considerable control over the sensitivity of the permafrost to energy and mass exchanges at the surface. At the same time, the thermal conductivity is sensitive to the state of the soil, varying, for example, by up to two orders of magnitude with varying water contents. In situ measurement techniques perturb the soil thermally and are affected by changes in soil composition, for example through variations in thermal contact resistance between sensor and soil. The design of a sensor for measuring the temperature of the soil rather than the axial heating wire temperature has consequences for the modeling of heat flow. We introduce an approximation of heat flow from a heated cylinder with thermal contact resistance between the cylinder and the surrounding medium. This approximation is compared to the standard line source approximation, and both are applied to data measured over a one-year period in northern Alaska. Comparisons of thermal conductivity values determined numerically using the line source solution, line source approximation and the analytical form of the heated cylinder model fall within 10% of accepted values, except for measurements made in pure ice, for which all methods of calculation under-predicted the thermal conductivity. Field data collected from a complete freeze¿thaw cycle in silty clay show a seasonally bimodal apparent thermal conductivity, with a sharp transition between frozen and thawed values during thaw, but a three-month transition period during freezing. The use of soil composition data to account for changes in heat flow due to the effect of latent heat during phase change results in a relationship between soil thermal conductivity and temperature

Lower limb access.

Lower limb vascular access is used as an access site in patients in whom all upper limb possibilities for arteriovenous access creation are exhausted or with bilateral upper limb central vein occlusions. Autologous arteriovenous fistulae (AVF) using the greater saphenous vein have disappointing results apart from the isolated success. Autologous AVF using the femoral vein transposition have good results both in terms of long-term patency and are associated with a 10-fold reduction in infection risk compared with arteriovenous grafts (AVGs). However, a femoral vein transposition is a major undertaking and is associated with an increased risk of ischaemic complications. It is not a good option for patients with established peripheral arterial disease, but may be a good alternative for the younger patient with a high infection risk. The type of lower-extremity vascular access should be carefully tailored to the individual patient