Activation of Trapped Field Magnets by Flux Pumping
ABSTRACT High temperature superconducting trapped field magnets (TFM) offer great potential as an alternative to second generation YBCO wire, both in cost and performance. The difficulty of activating this material is the primary problem hindering their incorporation into electromechanical devices. An alternate method called flux pumping is discussed in which the magnetic field trapped within a precooled TFM is increased, i.e., pumped up incrementally. A boundary element formulation is employed to illustrate what happens to the current density within the TFM during this pumping procedure. This is followed by a rigorous analytical formulation to simulate flux pumping within a 2-cm diameter TFM; the simulation is compared to an experiment using a NdFeB magnet as the pumping source.
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ABSTRACT: High-temperature superconducting trapped field magnets (TFMs) offer great potential as an alternative to second-generation YBCO wire, both in cost and performance. Attention is given to the calculation of current distribution within YBCO disks at partial and full activation and comparing this to experimental values. The best results are obtained by treating the current as a sequence of nested current rings. The fields are computed by integrating the elliptic integrals representing the fields from these rings and using variable metric optimization to choose the ring radii to best match the activation field over the unactivated material. A technique for treating the subregions of the TFM as voltage fed coils appears most expeditious for computing forces.IEEE Transactions on Magnetics 07/2008; · 1.42 Impact Factor
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ABSTRACT: Recent progress in material processing has proved that high temperature superconductors (HTS) have a great potential to trap large magnetic fields at cryogenic temperatures. For example, HTS are widely used in MRI scanners and in magnetic bearings. However, using traditional ways to magnetize, the YBCO will always need the applied field to be as high as the expected field on the superconductor or much higher than it, leading to a much higher cost than that of using permanent magnets. In this paper, we find a method of YBCO magnetization in liquid nitrogen that only requires the applied field to be at the level of a permanent magnet. Moreover, rather than applying a pulsed high current field on the YBCO, we use a thermally actuated material (gadolinium) as an intermedia and create a travelling magnetic field through it by changing the partial temperature so that the partial permeability is changed to build up the magnetization of the YBCO gradually after multiple pumps. The gadolinium bulk is located between the YBCO and the permanent magnet and is heated and cooled repeatedly from the outer surface to generate a travelling thermal wave inwards. In the subsequent experiment, an obvious accumulation of the flux density is detected on the surface of the YBCO bulk.Superconductor Science and Technology 09/2009; 22(10):105011. · 2.76 Impact Factor
Measurements of Flux Pumping Activation of
Trapped Field Magnets
Roy Weinstein, Drew Parks,
Ravi-Persad Sawh, and Kent Davey
Texas Center for Superconductivity and
University of Houston
This research was supported by the U.S. Army Research Office, the State of Texas via the Texas Center for Superconductivity at
the University of Houston, and The Welch Foundation (Grant E-1380).
• Trapped field magnets (TFMs) of type II superconductor have high
density of pinning centers (PCs). These hold in place fluxoids (ϕ0 ≈ 2 ×
10-7 G-cm2), which are formed when field penetrates the
• TFMs require good percolation paths, high quality PCs, and large size.
Btrap,max ∝ (in-field Jc) × (diameter of grain)
• At the present time Jc for large grains is limited by the quality of the
PCs, not by weak links, cracks, voids, . . .
• Present in-field Jcs for large grain HTS, and for coated conductor HTS
are nearly equal, for equal number and quality of PCs.
e.g., For BA = 1 Tesla, # PCs/cm2 = 1012
Large grain Jc = 340 kA/cm2
Coated conductor Jc = 500 kA/cm2
In-field Jc, at BA = 1 T, for large grain YBCO and 3 µm coated
conductor. Both contain same #/cm2 of optimum PCs based upon
• Trapped field magnets (TFMs) have advantages and disadvantages
compared to coated conductors (CC).
Consider, e.g., a 2 cm × 0.8 cm YBCO TFM.
Pinning Centers (PCs)
Advantages T Chem only Irrad. + Chem
• High field(1) 77 K 4-5,000 G 21,000 G
67 K 12,000 G 50,000 G
• Low cost(2)
• 50 to 100 × Engineering Jc of CC(3)
~$90. ~ $160.
• Very shock resistant
(1)Trapped fields up to Bt = 180,000 G have been demonstrated
(2)TFMs have been sold for as little as $20.
(3)This is because TFMs are self-supporting
Disadvantages: Small area; Requires a magnet to activate.
Applications presently being developed for TFMs include:
Motor; Generator; Flywheel energy storage; Magnetic gears; Water
purification, and more…
All of these would benefit from a method to ease the requirements of
Traditional activations are by:
• Field-cooling: TFM is cooled in applied field, BA. To achieve
trapped field, Bt, BA ≥ Bt
• Zero-field-cooling: TFM is cooled in uniform field with BA = 0.
Then, to achieve trapped field, Bt, BA ≥ 2Bt.