Abstract-BEST PATHS (acronym for "BEyond State-of-theart Technologies for rePowering Ac corridors and multiTerminal HVDC Systems") is a collaborative project within the FP7 framework of the European Commission that includes an MgB 2 -based power transmission line among its five constituent demonstrators. Led by Nexans and bringing together transmission operators, industry and research organizations, this demonstrator aims at validating the novel MgB 2 technology for very high power transfer (gigawatt range). The project foresees the development of a monopole cable system operating in helium gas in the range 5-10 kA/200-320 kV, corresponding to a transmitted power from 1 to 3.2 GW. The main research and demonstration activities that will be pursued over the four-year project duration are: 1) development and manufacturing of MgB 2 wires and of the cable conductor; 2) design and manufacturing of the HVDC electrical insulation of the cable; 3) optimization of the required cryogenic system; 4) electromagnetic field analysis; 5) design and construction of a prototype electrical feeding system including terminations and connectors; 6) testing of the demonstrator; 7) study of grid connection procedures and integration of a superconducting link into a transmission grid; and finally, 8) a socio-economic analysis of the MgB 2 power transmission system. CIGRÉ recommendations will be used to take into account the established international practices, and guidance will be given on newly addressed technical aspects. An overview of the project is presented in the paper, including the main tasks and challenges ahead, as well as the partners and their roles.

The blocking of fine-capillary tubes used as flow impedances in 4 He evaporation cryostats to achieve temperatures below 4.2 K is generally attributed to nitrogen or air impurities entering these tubes from the main bath. The failure of even the most rigorous low-temperature laboratory best practices aimed at eliminating the problem by maintaining the cleanliness of the helium bath and preventing impurities from entering the capillary tubes suggests that a different cause is responsible for the inexplicable reduction of impedance flow. Many low-temperature research laboratories around the world have suffered this nuisance at a considerable financial cost due to the fact that the affected systems have to be warmed to room temperature in order to recover their normal low-temperature operation performance. Here, we propose an underlying physical mechanism responsible for the blockages based upon the freezing of molecular H 2 traces present in the liquid-helium bath. Solid H 2 accumulates at the impedance low-pressure side, and, after some time, it produces a total impedance blockage. The presence of H 2 traces is unavoidable due its occurrence in the natural gas wells where helium is harvested, forcing gas suppliers to specify a lower bound for impurity levels at about 100 ppb even in high-grade helium. In this paper, we present a simple apparatus to detect hydrogen traces present in liquid helium and easily check the quality of the liquid. Finally, we propose a solution to eliminate the hydrogen impurities in small-and large-scale helium recovery plants. The solution has been implemented in several laboratories that previously experienced a chronic occurrence of blocking, eliminating similar occurrences for more than one year.

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Abstract. The transfer of liquid helium (LHe) at decant stations with flexible transfer lines is characterized by high evaporation losses. These losses are mainly caused by the static heat leak and the specific pressure drop of the transfer line. An extensive series of tests allows the characterization of a flexible LHe transfer line regarding its specific transfer performance, outlet quality, static heat leak, and overall pressure drop at Reynolds numbers from 340,000 to 740,000. Implemented pressure and temperature sensors allow studies of the fluid behavior during the transfer line operation. The achieved measurement results provide the basis for a new low loss flexible transfer line.