Magnetization of bulk superconductors using thermally actuated magnetic waves

A novel technique is proposed to magnetize bulk superconductors, which has the potential to build up strong superconducting magnets. Instead of conventionally using strong magnetic pulses, periodical magnetic waves with strength as low as that of rare-earth magnets are applied. These magnetic waves travel from the periphery to the center of a bulk superconductor and become trapped little by little. In this way, bulk superconductors can gradually be magnetized. To generate these magnetic waves, a thermally actuated magnet was developed, which is constructed by a heating/cooling switch system, a rare-earth bulk magnet, and a Gadolinium (Gd) bulk. The heating/cooling switch system controls the temperature of the Gd bulk, which, along with the rare-earth magnet underneath, can transform thermal signals into magnetic waves. The modeling results of the thermally actuated magnet show that periodical magnetic waves can effectively be generated by applying heating and cooling pulses in turn. A YBCO bulk was tested in liquid nitrogen under the magnetic waves, and a notable accumulation of magnetic flux density was observed. © 2006 IEEE.

High-precision digital lock-in measurements of critical current and AC loss in HTS 2G-Tapes

Application of High Temperature Superconducting (HTS) has been increasingly popular since the new superconducting materials were discovered. This paper presents a new high-precision digital lock-in measurement technique which is used for measuring critical current and AC loss of the 2nd Generation HTS tape. Using a lock-in amplifier and nano-voltage meter, we can resolve signals at nano-volt level, while using a specially designed compensation coil we can cancel out inductive by adjusting the coil inductance. Furthermore, a finer correction for the inductive component can be achieved by adjusting the reference phase of the lock-in amplifier. The critical current and AC loss measurement algorithms and hardware layout are described and analyzed, and results for both numerical and experimental data under varieties of frequencies are presented. © 2008 SICE.

Superconducting magnetic bearings for energy storage flywheels

We are investigating the use of flywheels for energy storage. Flywheel devices need to be of high efficiency and an important source of losses is the bearings. In addition, the requirement is for the devices to have long lifetimes with minimal or no maintenance. Conventional rolling element bearings can and have been used, but a non-contact bearing, such as a superconducting magnetic bearing, is expected to have a longer lifetime and lower losses. At Cambridge we have constructed a flywheel system. Designed to run in vacuum this incorporates a 40kg flywheel supported on superconducting magnetic bearings. The production device will be a 5kW device storing 5 kWh of retrievable energy at 50,000 rpm. The Cambridge system is being developed in parallel with a similar device supported on a conventional bearing. This will allow direct performance comparisons. Although superconducting bearings are increasingly well understood, of major importance are the cryogenics and special attention is being paid to methods of packaging and insulating the superconductors to cut down radiation losses. The work reported here is part of a three-year program of work supported by the EPSRC. © 1999 IEEE.

Design of brushless doubly-fed machines based: On magnetic circuit modeling

This paper proposes a magnetic circuit model (MCM) for the design of a brushless doubly-fed machine (BDFM). The BDFM possesses advantages in terms of high reliability and reduced gearbox stages, and it requires a fractionally-rated power converter. This makes it suitable for utilization in offshore wind turbines. It is difficult for conventional design methods to calculate the flux in the stator because the two sets of stator windings, which have different pole number, form a complex flux pattern which is not easily determined using common analytical approaches. However, it is advantageous to predict the flux density in the teeth and air-gap at the initial design stage for sizing purposes without recourse finite element analysis. Therefore a magnetic circuit model is developed in this paper to calculate the flux density. A BDFM is used as a case study with FEA validation. © 1965-2012 IEEE.

Soft ferrite used as thermal-magnetic conversion intermedium in the flux pumping technology

Recent progress in material science has proved that high-temperature superconductors, such as bulk melt-processed yttrium barium copper oxide (YBCO) single domains, have a great potential to trap significant magnetic fields. In this paper, we will describe a novel method of YBCO magnetization that only requires the applied field to be at the level of a permanent magnet. Instead of applying a pulsed high magnetic field on the YBCO, a thermally actuated material (TAM), such as Mg0.15}hbox{Cu}0.15} hbox{Zn0.7 Ti0.04}Fe1.96boxO4, has been used as an intermedium to create a travelling magnetic field by changing the local temperature so that the local permeability is changed to build up the magnetization of the YBCO gradually after multiple pumping cycles. It is well known that the relative permeability of ferrite is a function of temperature and its electromagnetic properties can be greatly changed by adding dopants such as Mg or Ti; therefore, it is considered to be the most promising TAM for future flux pumping technology. Ferrite samples were fabricated by means of the conventional ceramic method with different dopants. Zinc and iron oxides were used as raw materials. The samples were sintered at 1100 C, 1200 C} , and 1300 C. The relative permeability of the samples was measured at temperatures ranging from 77 to 300 K. This work investigates the variation of the magnetic properties of ferrites with different heat treatments and doping elements and gives a smart insight into finding better ferrites suitable for flux pumping technology. © 2002-2011 IEEE.

The investigation of sweeping speed on the magnetization of a 2 inches diameter YBa2Cu3O7-δ thin film with the circular-type magnetic flux pump

A circular-type magnetic flux pump (CTMFP) device was built to study the flux dynamics on a 2-inch-diameter YBCO thin film. This CTMFP is composed of two CTMFP coils, with each CTMFP coil containing concentric three-phase windings and a dc winding. We connected the three-phase windings to the output of a commercial inverter. By changing the output frequency of the inverter, the sweeping speed of the circular-shaped travelling magnetic wave can be changed. The connection of the phase coils follows the forward consequence, so that the circular-shaped travelling magnetic wave travels inward to the center. The output frequency f was changed from f = 0.01 to 1000.0 Hz. The YBCO sample was sandwiched between the two CTMFP coils to experience the circular-shaped travelling magnetic wave. It was found that the increase of the flux density in the center of the film is independent of the sweeping frequency. In high frequency f = 1000.0Hz, even if the waveform had changed a lot, the increment is still the same as in low frequencies. © 2012 IEEE.

Theoretical and experimental magnetization loss comparison between IBAD coils and RABiTS coils

This paper presents a comparative study of ac magnetization losses in two types of 2 G HTS racetrack coils. The magnetic substrate made by RABiTS is the main difference between the two types, because ferromagnetic loss caused by magnetic substrate is accounted into the total ac losses. IBAD and RABiTS tapes were successfully wound into racetrack shape with identical geometry. The measurements were carried out by using electromagnetic method with pick-up coils under a sinusoidally varying external magnetic field, with amplitudes up to 27 mT, ranging from 10 Hz to 100 Hz at a temperature of 77 K. The field was oriented perpendicularly to the surface of the tapes. Experimental measurements were validated by applying theoretical models and the results showed that the magnetization loss in the MAG RABiTS coil is always higher than that in the NON MAG coil due to the presence of the magnetic substrate, which increases the magnetic field penetration into the coil and causes higher magnetic flux density within the penetrated region. © 2002-2011 IEEE.

The use of an MgB2 hollow cylinder and pulse magnetized (RE)BCO bulk for magnetic levitation applications

Both MgB2 and (RE)BCO bulk materials can provide a highly compact source of magnetic field when magnetized. The properties of these materials when magnetized by a pulsed field are potentially useful for a number of applications, including magnetic levitation. This paper reports on pulsed field magnetization of single 25 mm diameter (RE)BCO bulks using a recently constructed pulse magnetization facility, which allows an automated sequence of pulses to be delivered. The facility allows measurement of force between a magnetized (RE)BCO bulk and a bulk MgB2 hollow cylinder, which is field cooled in the field of the magnetized (RE)BCO bulk. Hysteresis cycling behavior for small displacement is also measured to extract the stiffness value. The levitation forces up to 500 N were obtained, the highest ever measured between two bulks and proves the concept of a bulk-bulk superconducting bearing design. © 2002-2011 IEEE.

Shielding efficiency and E(J) characteristics measured on large melt cast Bi-2212 hollow cylinders in axial magnetic fields

We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets can also act as efficient magnetic shields. The magnetic screening properties under an axial DC magnetic field are characterized at several temperatures below the liquid nitrogen temperature (77 K). Two main shielding properties are studied and compared with those of Bi-2223, a material that has been considered in the past for bulk magnetic shields. The first property is related to the maximum magnetic flux density that can be screened, Blim; it is defined as the applied magnetic flux density below which the field attenuation measured at the centre of the shield exceeds 1000. For a cylinder of Bi-2212 with a wall thickness of 5 mm and a large ratio of length over radius, Blim is evaluated to 1 T at T = 10 K. This value largely exceeds the Blim value measured at the same temperature on similar tubes of Bi-2223. The second shielding property that is characterized is the dependence of Blim with respect to variations of the sweep rate of the applied field, dBapp/dt. This dependence is interpreted in terms of the power law E = Ec(J/Jc)^n and allows us to determine the exponent n of this E(J) characteristics for Bi-2212. The characterization of the magnetic field relaxation involves very small values of the electric field. This gives us the opportunity to experimentally determine the E(J) law in an unexplored region of small electric fields. Combining these results with transport and AC shielding measurements, we construct a piecewise E(J) law that spans over 8 orders of magnitude of the electric field.

Use of 2G coated conductors for efficient shielding of DC magnetic fields

This paper reports the results of an experimental investigation of the performance of two types of magnetic screens assembled from YBa2Cu3O7-d (YBCO) coated conductors. Since effective screening of the axial DC magnetic field requires the unimpeded flow of an azimuthal persistent current, we demonstrate a configuration of a screening shell made out of standard YBCO coated conductor capable to accomplish that. The screen allows the persistent current to flow in the predominantly azimuthal direction at a temperature of 77 K. The persistent screen, incorporating a single layer of superconducting film, can attenuate an external magnetic field of up to 5 mT by more than an order of magnitude. For comparison purposes, another type of screen which incorporates low critical temperature quasi-persistent joints was also built. The shielding technique we describe here appears to be especially promising for the realization of large scale high-Tc superconducting screens.