Algirdas Baskys

Pulsed-Field Magnetization of Superconducting Tape Stacks for Motor Applications

The potential of (RE)BCO superconducting bulks in rotating machine designs has been explored through numerous experimental prototypes, with the bulks being magnetized to act as held poles. However, stacks of superconducting tapes have emerged as a promising alternative for trapped held magnets partly because of their suitability for the pulsed-field method of magnetization, which is considered the most practical method of trapping flux. The benefits of using a stack of tapes as rotor held poles suitable for motors are reported. The ability to have a long rectangular stack allows for motor designs with more efficient held poles in terms of the flux produced per unit area of the pole and easy scalability. Such a rectangular stack was experimentally magnetized for the first time using a race-track-shaped pulsed-held coil giving a highly uniform and well-defined trapped held. The unique self-supporting 120 mm by 12 mm stack was produced by compressing a high-temperature-superconducting tape coated with a thin layer of solder. Shorter rectangular stacks were pulse magnetized over a temperature range of 10-77 K using a fully automated pulsed magnetization system.

The effect of stabilizer on the trapped field of stacks of superconducting tape magnetized by a pulsed field

Stacks of high temperature superconducting tape, magnetized using pulsed fields, provide a new type of permanent magnet using superconductors. To optimize the trapped field in such stacks, the role of stabilization layers was investigated by pulse magnetizing a 12 mm square stack of 15 tape layers over a temperature range of 15-77 K. The stacks consisted of commercial tape with a silver stabilizer of 1-3 mu m or tape with an additional 20 mu m layer of copper on top of 1 mu m of silver. It was found that the trapped field and flux are relatively insensitive to the stabilizer thickness, and 1 mu m of silver only, led to the highest trapped field. An FEM model was also developed for a stack that considered for the first time both the actual thickness of metallic and superconducting layers, to investigate the effect of heating and heat transfer when a stack of tapes is magnetized.

Magnetic levitation using high temperature superconducting pancake coils as composite bulk cylinders

Stacks of superconducting tape can be used as composite bulk superconductors for both trapped field magnets and for magnetic levitation. Little previous work has been done on quantifying the levitation force behavior between stacks of tape and permanent magnets. This paper reports the axial levitation force properties of superconducting tape wound into pancake coils to act as a composite bulk cylinder, showing that similar stable forces to those expected from a uniform bulk cylinder are possible. Force creep was also measured and simulated for the system. The geometry tested is a possible candidate for a rotary superconducting bearing. Detailed finite element modeling in COMSOL Multiphysics was also performed including a full critical state model for induced currents, with temperature and field dependent properties and 3D levitation force models. This work represents one of the most complete levitation force modeling frameworks yet reported using the H-formulation and helps explain why the coil-like stacks of tape are able to sustain levitation forces. The flexibility of geometry and consistency of superconducting properties offered by stacks of tapes, make them attractive for superconducting levitation applications.

Self-Supporting Stacks of Commercial Superconducting Tape Trapping Fields up to 1.6 T Using Pulsed Field Magnetization

(RE)Ba2Cu3O7-δ bulks are well known for their ability to trap high magnetic fields; however, it has been recently shown by the current authors that stacks of commercial YBCOcoated conductor tape can outperform YBCO bulks of similar size at temperatures below 60 K due to their superior thermal stability during multipulse magnetization. The latest progress in a comprehensive study being undertaken to optimize and tailor the stacks for applications will be discussed. In this paper, a method of binding layers of superconducting tape is reported, namely, soldering of Pb-Sn solder-coated high-temperature superconducting tape developed by SuperOx. The performance of tape before and after the soldering procedure is discussed, and a 100-layer stack, trapping a field of up to 1.6 T above the surface after pulse magnetization at 10 K, is reported.

Simulation and experiments of Stacks of High Temperature Superconducting Coated Conductors Magnetized by Pulsed Field Magnetization with Multi-Pulse Technique

High temperature superconducting (HTS) bulks or stacks of coated conductors (CCs) can be magnetized to become trapped field magnets (TFMs). The magnetic fields of such TFMs can break the limitation of conventional magnets (<2 T), so they show potential for improving the performance of many electrical applications that use permanent magnets like rotating machines. Towards practical or commercial use of TFMs, effective in situ magnetization is one of the key issues. The pulsed field magnetization (PFM) is among the most promising magnetization methods in virtue of its compactness, mobility and low cost. However, due to the heat generation during the magnetization, the trapped field and flux acquired by PFM usually cannot achieve the full potential of a sample (acquired by the field cooling or zero field cooling method). The multi-pulse technique was found to effectively improve the trapped field by PFM in practice. In this work, a systematic study on the PFM with successive pulses is presented. A 2D electromagnetic-thermal coupled model with comprehensive temperature dependent parameters is used to simulate a stack of CCs magnetized by successive magnetic pulses. An overall picture is built to show how the trapped field and flux evolve with different pulse sequences and the evolution patterns are analyzed. Based on the discussion, an operable magnetization strategy of PFM with successive pulses is suggested to provide more trapped field and flux. Finally, experimental results of a stack of CCs magnetized by typical pulse sequences are presented for demonstration.

Modeling of Trapped Fields by Stacked (RE)BCO Tape Using Angular Transversal Field Dependence

Stacks of superconducting (RE)BCO tape are gaining popularity as a potential alternative for superconducting bulks for trapped field applications. This is partly due to versatility and uniformity of the starting material, allowing for more deterministic prediction of field profile and magnitude. However, most FEM models of trapped field magnets do not incorporate parameters such as critical current and n-value dependence on the angle of applied magnetic field, leading to only qualitative modeling results. More quantitative results can be obtained from incorporating more data for superconductivity and thermal properties of the material. Such models can be used as a starting point for most geometries and both trapped field and current transport modeling problems. An FEM model of a stack of tapes was constructed using the H formulation, incorporating goniometric critical current and n-value measurements. The modeling results were compared to field cooling experiments for stacks of different heights. The experiment and modeling show good agreement.

Magnetic Levitation Between a Slab of Soldered HTS Tape and a Cylindrical Permanent Magnet

Stacks of commercial high-temperature superconducting tape can be cut and soldered together to form slabs of a large range of shapes and sizes. They are most interesting for magnetic levitation applications due to the flexibility of geometry, allowing them to be created in large thin slabs suitable for planar rotary magnetic bearings and linear maglev bearings. In this paper, the axial levitation force was measured between a field cooled slab of 30 × 30 mm and a 25-mm-diameter rare-earth permanent magnet (PM), which produced a cylindrically symmetric field necessary in the context of rotary bearings. The force results were compared with that achieved between the same PM and a larger 43-mm-diameter bulk MgB2 disk, as well as to FEM modeling using the Perfectly Trapped Flux approximation.

Composite superconducting bulks for efficient heat dissipation during pulse magnetization

Pulsed field magnetization is the most practical method of magnetizing a (RE)BCO bulk, however large heat generation limits the trapped field to significantly less than possible using field cooling. Modelling has been used to show that effective heat removal from the bulk interior, using embedded metallic structures, can enhance trapped field by increasing thermal stability. The reported results are for experimental pulsed magnetization of a thin walled YBCO sample with 55 vertical holes embedded with high thermal conductivity wires. A specially designed copper coldhead was used to increase the trapped field and flux of the perforated YBCO by about 12% at 35 K using a multi-pulse magnetization. Moreover, by filling the perforations with copper, the central trapped field was enhanced by 15% after a single-pulse at 35 K. 3D FEM computer model of a perforated YBCO bulk was also developed showing localised heating effects around the perforations during pulse magnetisation.

Spark-Discharge Plasma as a Method to Produce Low AC Loss Multifilamentary (RE)Ba2Cu3 O7 Coated Conductors

If coated conductors are to be used in large-scale ac applications such as motors and generators, energy losses must be minimised. Hysteretic ac losses can be reduced by dividing the coated conductor into filaments. In this study, a new method for producing filamentary coated conductors is presented. An electrical spark discharge was used to selectively degrade regions of superconducting tape. The robust, noncontact and scalable method was used to striate tapes into four filaments. The filamentary samples had lower ac losses than nonstriated tapes with less than a 7% reduction in current carrying capacity.

Toward Uniform Trapped Field Magnets Using a Stack of Roebel Cable Offcuts

Stacks of high-temperature superconducting tape can be magnetized to produce a variety of different trapped field profiles in addition to the most common conical or pyramidal profiles. Stacks of tape using discarded Roebel cable offcuts were created to investigate various stacking arrangements with the aim of creating a stack that can be magnetized to produce a uniform trapped field for potential applications such as NMR. A new angled stacking arrangement proved to produce the flattest, most uniform field of all the overlapping stacking arrangements and has the potential to be scaled up. FEM modeling in COMSOL was also performed to complement the measurements and explain the limitations and advantages of the stacking arrangements tested.