The Massachusetts Institute of Technology, the University of Cambridge in the United Kingdom, and Tsinghua University in Beijing, China, are collaborating to design, construct, and test a 20 m, direct current, superconducting MgB2 and YBCO power cable. The cable will be installed in the State Key Laboratory of Power Systems at Tsinghua University in Beijing beginning in 2013. In a previous paper [1], the cryogenic system was briefly discussed, focusing on the cryogenic issues for the superconducting cable. The current paper provides a detailed discussion of the design, construction, and assembly of the cryogenic system and its components. The two-stage system operates at nominally 80 K and 20 K with the primary cryogen being helium gas. The secondary cryogen, liquid nitrogen, is used to cool the warm stage of binary current leads. The helium gas provides cooling to both warm and cold stages of the rigid cryostat housing the MgB2 and YBCO conductors, as well as the terminations of the superconductors at the end of the current leads. A single cryofan drives the helium gas in both stages, which are thermally isolated with a high effectiveness recuperator. Refrigeration for the helium circuit is provided by a Sumitomo RDK415 cryocooler. This paper focuses on the design, construction, and assembly of the cryostat, the recuperator, and the current leads with associated superconducting cable terminations.

The depth extent, strength, and composition of oceanic detachment faults remain poorly understood because the grade of deformation‐related fabrics varies widely among sampled oceanic core complexes (OCCs). We address this issue by analyzing fault rocks collected from the Kane oceanic core complex at 23°30′N on the Mid‐Atlantic Ridge. A portion of the sample suite was collected from a younger fault scarp that cuts the detachment surface and exposes the interior of the most prominent dome. The style of deformation was assessed as a function of proximity to the detachment surface, revealing a ∼450 m thick zone of high‐temperature mylonitization overprinted by a ∼200 m thick zone of brittle deformation. Geothermometry of deformed gabbros demonstrates that crystal‐plastic deformation occurred at temperatures >700°C. Analysis of the morphology of the complex in conjunction with recent thermochronology suggests that deformation initiated at depths of ∼7 km. Thus we suggest the detachment system extended into or below the brittle‐plastic transition (BPT). Microstructural evidence suggests that gabbros and peridotites with high‐temperature fabrics were dominantly deforming by dislocation‐accommodated processes and diffusion creep. Recrystallized grain size piezometry yields differential stresses consistent with those predicted by dry‐plagioclase flow laws. The temperature and stress at the BPT determined from laboratory‐derived constitutive models agree well with the lowest temperatures and highest stresses estimated from gabbro mylonites. We suggest that the variation in abundance of mylonites among oceanic core complexes can be explained by variation in the depth of the BPT, which depends to a first order on the thermal structure and water content of newly forming oceanic lithosphere.

Tsinghua University, University of Cambridge, and MIT are collaborating to design, construct, and test an MgB2 dc cable for microgrid applications. Two-stage current leads with nitrogen vapor cooled copper and gaseous helium cooled hightemperature superconductor will be used. The two-stage cryostat will be cooled using a single helium fan with a recuperator. The helium gas used to cool the MgB2 to...

Tsinghua University, University of Cambridge, and MIT are collaborating to design, construct, and test an MgB2 dc cable for microgrid applications. Two-stage current leads with nitrogen vapor cooled copper and gaseous helium cooled high-temperature superconductor will be used. The two-stage cryostat will be cooled using a single helium fan with a recuperator. The helium gas used to cool the MgB2 to 20-25 K will be cooled with a two-stage GM cryocooler. The cryogenic stability of the cable will be presented. Designs of cryostats are being investigated. Semi-rigid cryostats, with low heat leak, are being designed and will be tested as part of this program. The system will operate up to 1 kV and 1 kA, limited by the present power equipment. However, the MgB2 conductor is being designed for 5 kA, and may be tested at low voltage with upgraded current leads. A 30-m cable will be tested using facilities at Tsinghua University in Beijing.

Losses within a pulse tube cryocooler (PTC) are dominated by regenerator losses that scale directly with the magnitude of the mass flow rate within the regenerator. Therefore, in order to maximize PTC performance it is necessary to minimize the ratio of the mass flow rate in the regenerator to the acoustic power. This is accomplished by controlling the phase between the mass flow and the pressure with a phase shifting device installed at the warm end of the pulse tube. The most common device, the inertance tube, has significant disadvantages including limited achievable phase angles and large mass and volume. Also, once installed the inertance tube is not tunable. It has been proposed that the inertance tube be replaced with a hybrid mechanical/electrical phase shifting system. The damping for this system is provided by an eddy current damper and can be controlled via an applied external magnetic field that provides active real-time phase control. This paper presents an analytical model of a bellows phase shifting mechanism for a PTC. The model is used to determine properties of the phase shifting mechanism (volume, mass, spring constant, damping force, etc.) based on typical PTC operating conditions. Initial experimental results are also presented.

Regenerator models used by designers are macro‐scale models that do not explicitly consider interactions between the fluid and the solid matrix. Rather, the heat transfer coefficient and pressure drop are calculated using correlations for Nusselt number and friction factor. These correlations are typically based on steady flow data. The error associated with using steady flow correlations to characterize the oscillatory flow that is actually present in the regenerator is not well understood. Oscillating flow correlations based on experimental data do exist in the literature; however, these results are often conflicting. This paper uses a micro‐scale computational fluid dynamic (CFD) model of a unit‐cell of a regenerator matrix to determine the conditions for which oscillating flow affects friction factor. These conditions are compared to those found in typical pulse tube regenerators to determine whether oscillatory flow is of practical importance. CFD results clearly show a transition Valensi number beyo...

Hybrid solar lighting (HSL) systems distribute natural sunlight to luminaires located in office or retail buildings in order to reduce energy consumption associated with conventional lighting systems. HSL systems reduce energy consumption directly by reducing the lighting energy and indirectly by reducing the associated cooling loads. A key component of the HSL system is the fiber optic bundle (FOB) that transmits the light from the collector to the luminaire. The observed thermal failure of the FOB when exposed to concentrated sunlight has motivated the development of a thermal model of this component. This paper describes the development of a predictive thermal model of the heat transfer in an FOB for an HSL system. The model is verified experimentally against temperature measurements obtained in the lab under controlled conditions and provides a powerful design tool that can be used to evaluate alternative thermal management strategies.