In pulsed field magnetization (PFM), the phenomenon of flux jump is capable of driving magnetic flux vortexes into GdBCO superconducting bulk center to aid full magnetization. Varieties of homogeneous critical current density (Jc) models have been implemented to reproduce flux jumps, but simulated multi-physical responses differ from experimental observations. This paper proposes a modified Jc model to consider r-z plane Jc inhomogeneity and simulates flux jumps under experimental conditions by solving a 2D axisymmetric electromagnetic-thermal coupled model. A numerical treatment is developed to reflect the break of shielding current during flux jumps. The accuracy of our model is verified by comparisons of the calculation results of trapped magnetic fields (BT) and the PFM and field cooling (FC) experimental results. On this basis, we investigate the improvement of inhomogeneous Jc model and obtain the multi-physical responses which have better agreement with the experimental results compared to homogeneous Jc model. Moreover, to further test the ability of the inhomogeneous Jc model to predict anisotropy of spatially magnetic field distribution, the simulated BT profiles in top and bottom surfaces of HTS bulk at 77 K are compared to the experiments. This study may provide a new approach for modelling the inhomogeneity of Jc characteristics and a useful analysis tool for industrial devices using high-temperature superconductor (HTS) bulk magnets.
A high temperature superconducting (HTS) magnetically levitated testbed has been developed for the steady thrust measurement of miniature ion electrospray thruster. The structure of the testbed mainly consists of an HTS composite bearing, a magnetic shielding plate, an active electromagnetic brake and a laser displacement sensor. The steady thrust is described as a function of the equilibrium angle displacement of the floating frame. Furthermore, the mechanical behaviors of HTS composite bearing were studied via finite element simulation and experiments, which include the load capacity, levitation stiffness and background noise. The results show that the thrust testbed can keep in low noise and have a load capacity up to 4 kg. According to the ignition testing of the electrospray thruster, the thrust force of 25.2 mN was measured by the testbed, which is close to the design value of miniature ion electrospray thruster.
Characterized by very low rotational drag, applications of high temperature superconductor (HTS) bearings have been expanded in some high precision instruments. We developed a sensitive magnetic suspension stand based on an Evershed-type hybrid HTS bearing to measure the micro newton level thrust. The hybrid HTS bearing is to use the strong attractive force of a permanent magnet (PM) biased bearing to support main loading in the bearing, while the instability in the PM biased bearing was compensated by the magnetic stability from an HTS bearing. Compared to the single PM/HTS bearing, the hybrid HTS bearing is intended not only to support larger load and but also to suppress the rotation loss and levitation drift. Loading capacity, loss and damping torque were explored at different design parameters such as bearing gaps L1, L2, field cooling height (FCH) of bulk HTSs. Spin down testing suggested that the loss of the hybrid HTS bearing can be reduced by raising the HTS bearing gap L2 or reducing the field cooling height. The hybrid HTS bearing showed a typical oscillation behavior in the extreme low frequency, and torsional pendulum testing suggested that the spring constant k of the hybrid HTS bearing can be as low as 36.39 μN·m/°. A micro-thrust stand based on the hybrid HTS bearing was established to test an electrospray thruster in the vacuum chamber, and the measurement result of 26.61 μN presents the ability of micro-thrust stand to achieve μN level testing.
The diamagnetic levitation technique can be applied in non-destructive testing for identifying cracks and defects in magnetic materials. Pyrolytic graphite is a material that can be leveraged in micromachines due to its no-power diamagnetic levitation on a permanent magnet (PM) array. However, the damping force applied to pyrolytic graphite prevents it from maintaining continuous motion along the PM array. This study investigated the diamagnetic levitation process of pyrolytic graphite on a permanent magnet array from various aspects and drew several important conclusions. Firstly, the intersection points on the permanent magnet array had the lowest potential energy and validated the stable levitation of pyrolytic graphite on these points. Secondly, the force exerted on the pyrolytic graphite during in-plane motion was at the micronewton level. The magnitude of the in-plane force and the stable time of the pyrolytic graphite were related to the size ratio between it and the PM. During the fixed-axis rotation process, the friction coefficient and friction force decreased as the rotational speed decreased. Smaller-sized pyrolytic graphite can be used for magnetic detection, precise positioning and other microdevices. The diamagnetic levitation of pyrolytic graphite can also be used for detecting cracks and defects in magnetic materials. We hope this technique will be used in crack detection, magnetic detection and other micromachines.
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