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Recent studies of pulsed-Zero Field Cool Magnetization (P-ZFCM) of high temperature superconductor (HTS) trapped field magnets (TFMs) have reported a new phenomenon, a sudden giant field leap (GFL), accompanied by a decrease in the activation ratio, BA/BT,max, from ∼4.0 to 1.0. Here, BA is the P-ZFCM pulse magnitude required to activate the TFM to its maximum magnetizable field, BT,max. Experiments are reported here on the temperature increase, ΔT, caused by P-ZFCM and on a new record trapped field of 4.54 T achieved by that method of magnetization. ΔT is measured indirectly to avoid difficulties inherent in direct methods. P-ZFCM heating, accompanied by GFL, is found to decrease BT,max by only 3.25% and increase the TFM temperature by less than 1.07 K. In addition, data presented here show that the GFL increase in field is not caused by an induced electric field, a mechanism used in all prior theories of HTS magnetization, but by a new phenomenon resulting from a high magnetic field, which has not yet been successfully described theoretically. The resulting TFMs are applicable to electromechanical devices, such as motors and generators, and should greatly reduce the volume and cost of such applications while sharply increasing torque density.

Recent experiments on pulsed-zero field cool magnetization of bulk high JcYBCO (YBa2Cu3O7-δ) have shown unexpected results. For example, reproducible, non-destructive, rapid, giant field leaps (GFLs) to higher penetrated field are observed. The observations are inconsistent with the critical state model (CSM), in several aspects. Additional experiments have been pursued in an attempt to clarify the physics involved in the observed anomalies. Here, we present experimental results for the Jc dependence of the anomalous features. It is found that the sudden field increase in the GFL is a monotonically increasing function of Jc. The ratio of required pulsed field amplitude, BA,max, to obtain maximum trappable field, BT,max, which CSM predicts to be ≥2.0, gradually approaches 1.0 at high Jc. Tests using values of pulsed, applied field BA,max just below the GFL exhibit two additional anomalies: (i) At high Jc, the highest trapped field is up to ∼6 times lower than predicted by CSM, and (ii) the measured Lorentz force as a function of Jc deviates sharply from CSM predictions. The data rule out heating effects and pinning center geometry as possible physical causes of these anomalies. A speculative cause is considered.

We have measured the magnetic hysteresis loops and temperature dependent trapped fields in melt‐textured YBa2Cu3O7−δ samples before and after p + and 3He++ irradiation using a Hall effect magnetometer (HEM) as well as a commercial vibrating sample magnetometer (VSM). For proper 3He++ fluence, the critical current density may be enhanced by a factor of 10. Calculations based on various critical state models show that before the irradiation, the hysteresis loops can be well accounted for by a critical current density of a modified power law field dependence J c (T,B)=J 0(T)/(1+B/B 0) n with n=1/2; after the irradiation, the best fit has been achieved by using an exponential form such as J c (T,B)=J 0(T)exp(−B/B 0), where B 0 is a model dependent parameter. J c and its field dependence deduced from HEM hysteresis loops are in good agreement with those deduced from the VSM loops, suggesting that the Hall effect magnetometer can be conveniently used to characterize bulk high T c oxide superconductors.

Spatial distributions of persistent magnetic field trapped by sintered and melt-textured ceramic-type high-temperature superconductor (HTS) samples have been studied. The trapped field can be reproduced by a model of the current consisting of two components: (1) a surface current Js and (2) a uniform volume current Jv. This Js + Jv model gives a satisfactory account of the spatial distribution of the magnetic field trapped by different types of HTS samples. The magnetic moment can be calculated, based on the Js + Jv model, and the result agrees well with that measured by standard vibrating sample magnetometer (VSM). As a consequence, Jc predicted by VSM methods agrees with Jc predicted from the Js + Jv model. The field mapping method described is also useful to reveal the granular structure of large HTS samples and regions of weak links.

Bi2Sr2Ca2Cu3Ox (Bi-2223) tapes were processed by the standard powder-in-tube technique, but small amounts of UO4 were added prior to processing. Both U-doped and undoped tapes were subsequently exposed to a reactor spectrum of neutrons. Whereas the undoped materials show moderate enhancements of the critical current densities Jc due to the pinning action of the fast-neutron-induced collision cascades, Jc in the U-doped tapes increases by factors of 10–20 due to thermal-neutron-induced fission of 235U and the subsequent formation of fission tracks. Since the enhancements are particularly pronounced, when the magnetic field is applied perpendicular to the tape surface, the characteristic Jc anisotropy is strongly reduced in a certain field range. At the same time, the irreversibility field at 77 K is doubled for both field orientations.

Ongoing research has increased achievable field in trapped field magnets (TFMs) to multi-Tesla levels. This has greatly increased the attractiveness of TFMs for applications. However, it also increases the already very difficult problem of in situ activation and reactivation of the TFMs. The pulsed zero-field-cool (ZFC) method of activation is used in most applications because it can be accomplished with much lower power and more modest equipment than field-cool activation. The critical state model (CSM) has been a reliable theoretical tool for experimental analysis and engineering design of TFMs and their applications for over a half-century. The activating field, BA, required to fully magnetize a TFM to its maximum trappable field, BT,max, using pulsed-ZFC is predicted by CSM to be R ≡ BA/BT,max ≥ 2.0. We report here experiments on R as a function of Jc, which find a monotonic decrease of R to 1.0 as Jc increases. The reduction to R = 1.0 reduces the power needed to magnetize TFMs by about an order of magnitude. This is a critical advantage for TFM applications. The results also indicate the limits of applicability of CSM, and shed light on the physics omitted from the model. The experimental results rule out heating effects and pinning center geometry as causes of the decrease in R. A possible physical cause is proposed.

We have examined levitation forces of field cooled (2.3 T) and zero field cooled YBa2Cu3O7−δ superconductors opposed by a SmCo magnet. We have achieved unusually large levitation forces (140 N) and pressures (44.6 N/cm2) in field cooled measurements. 5–10 subsequent cycles, of approach and retreat of the magnet, result in a 10–20% decrease in force, which then stabilizes.

We report on persistent field in quasi-permanent magnets of high temperature superconductors. Magnets composed of irradiated Y(1+)Ba2Cu3O7 trapped field Bt = 1.52 T at 77 K and 1.9 T at lower temperature. However, the activation magnet limited Bt at lower temperature. We present data on Jc(H,T) for unirradiated materials, and calculate Bt at various T. Based upon data at 65 K, we calculate Bt in unirradiated single grains at 20 K and find that 5.2 T will be trapped for grain diameter d about 1.2 cm, and 7.9 T for d = 2.3 cm. Irradiated grains will trap four times these values.

Advances on permanent high‐Tc superconducting magnets are reported. Materials are tested in the form of small tiles. An accurate phenomenological model of the currents in a magnetized tile predicts that the maximum trapped field BT,max∝Jcf(d), where Jc is the critical current density, d is the diameter of the single‐grain tile, and f(d) is a function which increases monotonically with d. Results are reported of increasing Jc via chemical additives, and via bombardment by high‐energy light ions and fission fragments. Increases in d via chemical and temperature gradients are also reported. Methods, data, and most recent results are presented. Present values are d=2 cm, and Jc=85 kA/cm2, at 77 K. A six tile minimagnet, 1.2×1.2×1.5 cm3, fabricated from earlier tiles with d∼1 cm, Jc∼45 kA/cm2, retains 1.52 T at 77 K. It is calculated that the more recent values of Jc and d will result in fields of 3 T at 77 K. BT,max also increases rapidly with T−1, and approximately doubles at 60 K.

The trapped magnetic field of melt‐textured Y‐Ba‐Cu‐O samples has been studied. The measured field distributions can be well reproduced by a simple two‐current model consisting of a constant surface current and a constant volume current. If the sample is maximally magnetized (saturated case), the remnant sample magnetic moment can be calculated from the two‐current model, and the results agree with vibrating sample magnetometer measurements to typically 10%. Jc of the sample can then be obtained. The application of the two‐current model to the analysis of the nonsaturated case is also reported, for both field‐cooled and zero‐field‐cooled samples. The experimental data show the general features predicted by the Bean model, which, however, has to be modified to account for the data quantitatively.

A study of persistent, trapped magnetic field has been pursued with high-temperature superconducting (HTS) materials. The main effort is to study the feasibility of utilization of HTS to fabricate magnets for various devices. The trapped field, when not in saturation, is proportional to the applied field. Thus, it should be possible to replicate complicated field configurations with melt-textured YBa2Cu3O7 (MT-Y123) material, bypassing the need for HTS wires. Presently, materials have been developed from which magnets of 1.5 T, at 77 K, can be fabricated. Much higher field is available at lower operating temperature. Stability of a few percent per year is readily attainable. Results of studies on prototype motors and minimagnets are reported.

A study of field trapping by the incomplete Meissner effect was performed on YBa2Cu3O7−x at 77 K on both sintered and oriented grain samples. Fields of up to BT=1280 G have been trapped in small, oriented grain, planar samples. Circulating persistent current densities of up to 4800 A/cm2 are calculated. Based on these values hollow cylinders of 2 cm i.d. will trap fields of 6000 G. Initial creep is about 10%/week for both types of samples. Creep rate varies as (BT/BT,max)2. BT, due to an external field B0, is BT∝(1−e−cB0/BT,max). Lamination and mosaic assembly steps for producing practical magnets have been performed.

We report on persistent field in a quasipermanent magnet made of high temperature superconductor. The material has an average of 40% molar excess of Y, relative to Y1Ba2Cu3O7 and has been irradiated with high energy light ions at 200 MeV. The magnet, which traps 1.52 T at 77.3 K, traps nearly 4 T at 64.5 K. No evidence of giant flux jump or sample cracking was observed.