Boyang Shen

Voltage-ampere characteristics of YBCO coated conductor under inhomogeneous oscillating magnetic field

Direct current carrying type II superconductors present a dynamic resistance when subjected to an oscillating magnetic field perpendicular to the current direction. If a superconductor is under a homogeneous field with high magnitude, the dynamic resistance value is nearly independent of transport current. Hoffmann and coworkers [Hoffmann et al., IEEE Trans. Appl. Supercond. 21, 1628 (2011)] discovered, however, flux pumping effect when a superconducting tape is under an inhomogeneous field orthogonal to the tape surface generated by rotating magnets. Following their work, we report the whole Voltage-Ampere (V-I) curves of an YBCO coated conductor under permanent magnets rotating with different frequencies and directions. We discovered that the two curves under opposite rotating directions differ from each other constantly when the transport current is less than the critical current, whereas the difference gradually reduces after the transport current exceeds the critical value. We also find that for diff...

Origin of dc voltage in type II superconducting flux pumps: field, field rate of change, and current density dependence of resistivity

Superconducting flux pumps are the kind of devices which can generate direct current into superconducting circuit using external magnetic field. The key point is how to induce a dc voltage across the superconducting load by ac fields. Giaever (1966 IEEE Spectr. 3 117) pointed out flux motion in superconductors will induce a dc voltage, and demonstrated a rectifier model which depended on breaking superconductivity. van de Klundert et al (1981 Cryogenics 21 195, 267) in their review(s) described various configurations for flux pumps all of which relied on inducing the normal state in at least part of the superconductor. In this letter, following their work, we reveal that a variation in the resistivity of type II superconductors is sufficient to induce a dc voltage in flux pumps and it is not necessary to break superconductivity. This variation in resistivity is due to the fact that flux flow is influenced by current density, field intensity, and field rate of change. We propose a general circuit analogy for travelling wave flux pumps, and provide a mathematical analysis to explain the dc voltage. Several existing superconducting flux pumps which rely on the use of a travelling magnetic wave can be explained using the analysis enclosed. This work can also throw light on the design and optimization of flux pumps.

HTS Persistent Current Switch Controlled by AC Magnetic Field

High-temperature superconducting (HTS) magnets operating in persistent current mode can achieve better field stability as well as reduce loss compared with being powered by an external current source. Persistent current switch (PCS) is essential for HTS magnets to operate in persistent current mode. In this paper, we proposed, fabricated, and tested an ac-field-controlled HTS PCS. In the PCS, we used the dynamic resistance of a type-II superconductor as the source of switching-off resistance. An ac magnetic field was applied perpendicular to the surface of a piece of YBCO tape, thus forming the PCS. The performance of the PCS was tested by charging a small superconducting coil under an ac field of different magnitudes and frequencies. The switching-off resistance of the PCS in the experiment is less than 0.2 mΩ, which may be promising for various usages in HTS magnets.

Study of the Pulsed Field Magnetization Strategy for the Superconducting Rotor

High magnetic field can be trapped in a bulk type-II superconductor through different magnetization methods. In our fully high-temperature superconducting (HTS) synchronous motor, the pulsed field magnetization (PFM) method is adopted due to its reduced cost, feasibility, and adaptability on the assembling of our HTS motor system. The HTS rotor was designed with 75 YBCO HTS bulks mounted on the surface of the rotor to make two pairs of poles after the PFM. In this paper, we give a PFM circuit used to generate a pulsed magnetic field about 1.5 T at the center of the magnetizing coil lasting less than 1 s by discharging a capacitor bank. We also investigate the homogeneity of the pulsed magnetic field produced by the coils. The main strategy for magnetizing the superconducting rotor is designed and investigated, and the rotor field profile after the whole magnetization procedure is measured and discussed as well.

Magnetization of Coated Conductor Stacks Using Flux Pumping

Stacked-coated conductor tapes have demonstrated superior properties over bulk superconductors, including the flexible stacking configuration, appealing mechanical properties, and the good cross field performance. This makes stacked high-temperature superconducting (HTS) tapes a promising alternative to HTS bulks in many applications despite of its cost, such as rotary motors and levitation. The conventional magnetization methods, such as pulse field magnetization and field cooling, require large magnetic field to be applied on an area to cover the whole HTS stacks, which seriously affects its applicability. In this paper, HTS flux pumping, a novel technique developed for HTS coil energization, is proposed to be used to magnetize stacked HTS tapes. HTS tapes used in this paper have slits in the middle between two tape ends, called partially slit tape (PST). The PSTs operate as flat HTS coils in persistent current mode once magnetized. The experimental results show flux pumping, with the traveling-wave based mechanical moving magnet design, is able to magnetize the stacked PSTs at a weak level compared to its potential.

Feedback Control of a Rectifier Type HTS Flux Pump: Stabilizing Load Current With Minimized Losses

Flux pumps are able to compensate slow current decay in high-TC superconducting (HTS) magnets through noncontact approaches. It is a promising alternative for power sources and thick current leads in operating HTS coils. Following the previous work of a rectifier flux pump, we developed a feedback control system to achieve flexible control of load current. Experimental results show that the flux pump can stabilize load current at a preset level with the help of the control system. Power loss of using the flux pump is also compared with that of using current leads.

Uniform Magnetic Field Mapping With HTS Coils: Conceptual Design and Optimization

High critical current densities, high critical magnetic fields, improved mechanical properties, and high operating temperatures have made the application of 2G high-temperature superconductors (HTSs) in high field magnets attractive. However, the magnet technology of HTS conductors has not been well developed, both in its technology problems and applications. In this paper, a novel configuration of mapping uniform magnetic field using HTS coils is conceptually designed. The configuration is a tube with a rectangular cross section using two groups of HTS coils to cancel out and produce uniform field in the middle. This design can be used as an MRI magnet or for an accelerator. The HTS coils are connected in series to two dc sources and are individually scaled to produce magnetic field of various magnitude on the region of interest (ROI). A genetic algorithm (GA) and a proposed field correction algorithm are used to optimize the uniformity by manipulating the field produced by each magnet in this magnet system.