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Dynamic resistance can be observed in a superconducting tape carrying a DC current which is exposed to an oscillating magnetic field. This effect is attributed to the interaction between the transport current and moving fluxons, and can occur in various superconducting components including high temperature superconducting (HTS) flux pumps, fast-ramping magnets and HTS rotating machines. Although conventionally expressed in terms of a DC ‘resistance,’ the phenomenon is inherently transient in nature, and the voltage drop across the superconductor follows a time-dependent periodic waveform. Here we present experimental measurements of the dynamic resistance of different REBCO tapes carrying a DC current and exposed to an oscillating perpendicular field. Measurements of both the transient voltage waveforms and the time-averaged DC resistances are compared with numerical finite element simulations obtained using the H-formulation. We observe clear variations between the voltage response from different tapes, which can be understood in terms of their differing J c(B, θ) dependence. In particular, a key feature of the experimentally measured waveforms is the emergence of a split ‘double peak’ at higher applied fields. Graphical visualisations of the finite element data show that this coincides with a periodic increase in J c(B, θ) throughout the tape. This occurs during each cycle at those times when the applied field falls below the shielding threshold of the tape (as the penetrating field within the tape then approaches zero). Our findings show that models which assume a constant J c irrespective of local field strength cannot capture the full range of behaviour observed by experiment. This emphasises the importance of employing experimentally measured J c(B, θ) data when simulating transient effects in HTS materials.

Dynamic resistance is observed in type-II superconductors carrying a DC transport current while simultaneously exposed to an alternating magnetic field. The appearance of a non-zero resistance is attributed to the interaction between the transport current and moving fluxons. This effect is relevant to many superconductor applications such as high-temperature-superconductor (HTS) flux pumps, DC/AC magnets, synchronous machines, and persistent current switches. Here, we present a finite element method (FEM) analysis of both the time averaged dynamic resistance and the instantaneous current sharing behaviour in a cable comprised of a stack of four YBCO thin films connected in parallel. Numerical modelling was performed using the H -formulation method implemented in the commercial software COMSOL. The model employs experimentally measured values of the angular dependence of the critical current I c( B, θ ) and the flux creep exponent n ( B, θ ). A single threshold field is observed, above which a finite dynamic resistance is observed in all tapes simultaneously. The time-averaged dynamic resistance of individual tapes tends to be larger for the exterior tapes than the interior tapes, but this difference decreases as the total transport current in the cable increases. We attribute this to shielding currents flowing in the exterior tapes during the majority of the cycle, which displace net DC current into the interior tapes. However, the relative proportion of DC transport current flowing in the exterior and interior tapes is also observed to vary periodically once per half cycle of the applied field. This is due to the periodic trapping of return screening currents in the interior tapes.

The emergence of a potential drop across a current-transporting type-II superconducting loop under a perpendicular oscillating magnetic field is revealed. We have derived analytical formulae to describe the effect under DC transport current in 1D, based on Bean’s critical state model. The analytical formulae are verified by a finite element model. To exploit this effect, we have developed a transformer-like ‘resistive switch’, and experimentally observed a switching effect. This work demonstrates a physically important general insight of the interaction between DC transport currents and time-varying magnetic fields in type-II superconducting loops, which extends beyond the well-known ‘dynamic resistance’ effect. It also provides a useful view on the interaction between a ‘transport-current’ and a ‘screening-current’ in the superconductor. The resulting demonstrated switch has the potential to be used in a variety of applications including superconducting rectifiers, fault current limiters, and superconducting magnetic energy storages.

Mounting evidence has demonstrated that a specialized extracellular matrix exists in the mammalian brain and that this glycoprotein-rich matrix contributes to many aspects of brain development and function. The most prominent supramolecular assemblies of these extracellular matrix glycoproteins are perineuronal nets, specialized lattice-like structures that surround the cell bodies and proximal neurites of select classes of interneurons. Perineuronal nets are composed of lecticans, a family of chondroitin sulfate proteoglycans that includes aggrecan, brevican, neurocan, and versican. These lattice-like structures emerge late in postnatal brain development, coinciding with the ending of critical periods of brain development. Despite our knowledge of the presence of lecticans in perineuronal nets and their importance in regulating synaptic plasticity, we know little about the development or distribution of the extracellular proteases that are responsible for their cleavage and turnover. A subset of a large family of extracellular proteases (called a disintegrin and metalloproteinase with thrombospondin motifs [ADAMTS]) is responsible for endogenously cleaving lecticans. We therefore explored the expression pattern of two aggrecan-degrading ADAMTS family members, ADAMTS15 and ADAMTS4, in the hippocampus and neocortex. Here, we show that both lectican-degrading metalloproteases are present in these brain regions and that each exhibits a distinct temporal and spatial expression pattern. Adamts15 mRNA is expressed exclusively by parvalbumin-expressing interneurons during synaptogenesis, whereas Adamts4 mRNA is exclusively generated by telencephalic oligodendrocytes during myelination. Thus, ADAMTS15 and ADAMTS4 not only exhibit unique cellular expression patterns but their developmental upregulation by these cell types coincides with critical aspects of neural development.

Critical current and AC loss in coil windings are two important factors for various HTS applications. Many coated conductors exhibit asymmetry in the variation of the critical current with magnetic field angle. This asymmetry results in different coil critical current values depending on the orientation of the conductors in the coil windings. We report critical current and AC loss results at 77 K and 65 K for three hybrid coil assemblies which have a common central winding and different arrangements of the end windings. We found a difference greater than 13% in both the critical current and the AC loss results for the different arrangements. The results imply that if we wind coil assemblies smartly even using the same materials and the same design, we can not only improve critical current but also reduce AC loss significantly.

Neural circuit formation demands precise timing of innervation by different classes of axons. However, the mechanisms underlying such activity remain largely unknown. In the dorsal lateral geniculate nucleus (dLGN), axons from the retina and visual cortex innervate thalamic relay neurons in a highly coordinated manner, with those from the cortex arriving well after those from retina. The differential timing of retino- and corticogeniculate innervation is not a coincidence but is orchestrated by retinal inputs. Here, we identified a chondroitin sulfate proteoglycan (CSPG) that regulates the timing of corticogeniculate innervation. Aggrecan, a repulsive CSPG, is enriched in neonatal dLGN and inhibits cortical axons from prematurely entering the dLGN. Postnatal loss of aggrecan from dLGN coincides with upregulation of aggrecanase expression in the dLGN and corticogeniculate innervation and, it is important to note, is regulated by retinal inputs. Taken together, these studies reveal a molecular mechanism through which one class of axons coordinates the temporal targeting of another class of axons. National Institutes of Health NIH: EY012716 NIH: EY021222

Development of visual system circuitry requires the formation of precise synaptic connections between neurons in the retina and brain. For example, axons from retinal ganglion cells (RGCs) form synapses onto neurons within subnuclei of the lateral geniculate nucleus (LGN) [i.e., the dorsal LGN (dLGN), ventral LGN (vLGN), and intergeniculate leaflet (IGL)]. Distinct classes of RGCs project to these subnuclei: the dLGN is innervated by image-forming RGCs, whereas the vLGN and IGL are innervated by non-image-forming RGCs. To explore potential mechanisms regulating class-specific LGN targeting, we sought to identify differentially expressed targeting molecules in these LGN subnuclei. One candidate targeting molecule enriched in the vLGN and IGL during retinogeniculate circuit formation was the extracellular matrix molecule reelin. Anterograde labeling of RGC axons in mutant mice lacking functional reelin (relnrl/rl) revealed reduced patterns of vLGN and IGL innervation and misrouted RGC axons in adjacent non-retino-recipient thalamic nuclei. Using genetic reporter mice, we further demonstrated that mistargeted axons were from non-image-forming, intrinsically photosensitive RGCs (ipRGCs). In contrast to mistargeted ipRGC axons, axons arising from image-forming RGCs and layer VI cortical neurons correctly targeted the dLGN inrelnrl/rlmutants. Together, these data reveal that reelin is essential for the targeting of LGN subnuclei by functionally distinct classes of RGCs.

Most widely used practical superconductors (i.e., NbTi, Nb3Sn, Bi2Sr2Ca2Cu3O10+x ) are manufactured in a form of multigranular superconducting filaments embedded in a metallic matrix. The performance of these multifilamentary conduits at different physical conditions has been a topic of extended research over the last decades. In this paper we targeted to reveal the precise onset of the electric power dissipation in one of these composite superconductors, Bi2Sr2Ca2Cu3O10+x (so-called 1G HTS wire), at the conditions when external magnetic field, B appl, is applied in the maximum Lorentz force geometry. As we showed earlier (Talantsev et al 2017 AIP Advances 7 125230), at self-field conditions the transition to the dissipative state in 1G wire upon increasing the transport current, I, despite a multifilamentary wire design sharply steepens at a threshold current, I c,surfB, at which a simultaneous and abrupt crossover from a non-linear to a linear dependence of the perpendicular component of the local magnetic flux density, B surf(I), measured at the conduit surface occurs. We found that the same transition takes place in 1G HTS wire with an applied magnetic field, B appl, and thus, the definition of critical current in multifilamentary superconductors based on I c,surfB can be extended to in-field conditions for 1G HTS wire. We also studied effects of the flux trap and magnetic hysteresis upon increasing/cycling the transport current, I, in this conduit superconducting wire.

At least three forms of signaling between pre- and postsynaptic partners are necessary during synapse formation. First, “targeting” signals instruct presynaptic axons to recognize and adhere to the correct portion of a postsynaptic target cell. Second, trans-synaptic “organizing” signals induce differentiation in their synaptic partner so that each side of the synapse is specialized for synaptic transmission. Finally, in many regions of the nervous system an excess of synapses are initially formed, therefore “refinement” signals must either stabilize or destabilize the synapse to reinforce or eliminate connections, respectively. Because of both their importance in processing visual information and their accessibility, retinogeniculate synapses have served as a model for studying synaptic development. Molecular signals that drive retinogeniculate “targeting” and “refinement” have been identified, however, little is known about what “organizing” cues are necessary for the differentiation of retinal axons into presynaptic terminals. To identify such “organizing” cues, we used microarray analysis to assess whether any target-derived “synaptic organizers” were enriched in the mouse dorsal lateral geniculate nucleus (dLGN) during retinogeniculate synapse formation. One candidate “organizing” molecule enriched in perinatal dLGN was FGF22, a secreted cue that induces the formation of excitatory nerve terminals in muscle, hippocampus, and cerebellum. In FGF22 knockout mice, the development of retinal terminals in dLGN was impaired. Thus, FGF22 is an important “organizing” cue for the timely development of retinogeniculate synapses. This work was supported by The Thomas F. Jeffress and Kate Miller Jeffress Memorial Trust (Michael A. Fox), the VCU Presidential Research Initiative Program (PRIP) Award (Michael A. Fox), and the National Institutes of Health [NIH; EY021222 (Michael A. Fox); NS070005 (Hisashi Umemori)]. Microscopy was performed at the Virginia Commonwealth University Department of Anatomy and Neurobiology Microscopy Facility supported, in part, with funding from NIH–National Institute of Neurological Disorders and Stroke Center Core Grant 5P30 NS047463-02. We thank Dr. C. K. Chen for providing anti-melanopsin antibodies.