Drew Parks

Is it really possible to increase Jc by reducing the pinning potential

We discuss an experiment on large-grain YBCO, the intent of which is to compare the effectiveness of continuous columnar pinning centres (CCPC) to that of discontinuous aligned pinning centres, in their ability to increase Jc. We find that discontinuous pinning is far superior. The motivation and design of the experiment are reviewed. High-energy heavy ions were utilized to produce pinning damage of controlled morphology. The resulting increase in Jc is presented as a function of the variable Se, which is closely related to the discontinuity and diameter of the damage. The dependence of Jc on magnetic field, temperature, and Fi (the number of ion tracks per cm2) is presented and discussed. The dependence of Hirr on Se and Fi, and the temperature and fluence dependence of a Jc fishtail effect, are presented. The experimental results indicate that multiple-in-line-damage (MILD), despite its discontinuities, is far more effective in increasing Jc than CCPCs are.

Very high values of Jc obtained in NdBa2Cu3Ox by use of the U/n process

Abstract The U/n process has been used to introduce quasi-columnar pinning centers into Nd123, textured in 1% O 2 , 99% Ar. The process involves adding 235 U to precursor powders of the superconductor, texturing, and irradiating with thermal neutrons. The two resulting ions, from each nuclear fission, cause aligned discontinuous electronic damage of length ∼5.4 μm, which acts as a pinning center. Best results for J c require U deposits spaced by s ⩽5.4 μm. This has been achieved by deposits containing two foreign elements, U and Zr, and three native elements. The deposits of UZrNdBaO and Nd422 refined by the Zr, act as additional non-aligned pinning centers. The U/n process increases J c by a factor R J ⩾10, to values of J c ∼274 kA/cm 2 at T =77 K, B =666 G.

A significant advantage for trapped field magnet applications—A failure of the critical state model

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.

Self-assembling nano-diameter needlelike pinning centers in YBCO, utilizing a foreign element dopant

Although pinning centers created by irradiation presently produce the highest Jc, it is probable that ultimately these will be emulated by chemical pinning centers. The best pinning centers produced by irradiation nevertheless provide guidelines for desirable morphology of chemical pinning structures. The highest Jc produced earlier in textured HTS was obtained using isotropic high-energy ions produced by fission of 235U. This so-called U/n process produces pinning centers of diameter ≤ 4.5 nm, with an effective length of ~2.7 µm. Maximum Jc occurs for pinning center density of ~1010 cm−3. We use this as a model for desired chemical pinning centers. Our approach to introducing chemical pinning centers has been to produce precipitates within the HTS containing elements not native to the HTS, and to seek needlelike (columnar) deposits of small diameter. We report here on the formation of needlelike or columnar deposits in textured Y123 containing a dopant foreign to Y123. It serves as a demonstration that self-assembling nanometer diameter columns utilizing a dopant foreign to the HTS system are a feasible goal. These deposits, however, do not fully meet the ultimate requirements of pinning centers because the desired deposits should be smaller. The self-assembling columns formed contain titanium, are ~500 nm in diameter, and up to 10 µm long. The size and morphology of the deposits vary with the mass of admixed Ti dopant. Jc is decreased for small dopant mass. At larger dopant masses needlelike precipitates form, and Jc increases again. A small range of mass of admixed Ti exists in which Jc is enhanced by pinning. In the range of admixed Ti mass studied in these experiments there is a negligible effect on Tc. Magnetization studies of Jc are also reported.

Anomalous results observed in magnetization of bulk high temperature superconductors—A windfall for applications

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.

Experimental contradiction of the conventional wisdom that continuous columnar pinning centers result in maximum Jc

Continuous columnar pinning centers result in maximum pinning potential, Upin. Most scientists have assumed maximum Upin would maximize Jc. However, several clues pointed to high Jc despite pinning discontinuities. An experiment was performed to directly compare continuous and discontinuous pinning, using high-energy ion damage to create the pinning centers. Results in YBCO show that Jc for discontinuous pinning is much higher than for continuous pinning. At high pinning density (20 Tesla equivalent field), Jc is 60 times higher for discontinuous pinning, and at 5 Tesla equivalent field, it is 4 times higher. Record Jc ≈ 300 kA/cm2 at 77 K was achieved in large grain textured HTS for pinning which was 67% discontinuous. This resulted without improving the texturing or purity. Despite reduced Upin, Hirr for discontinuous pinning is as high as for continuous pinning. The superior Tc, and percolation, achieved by discontinuous pinning, far outweighs the decrease in Upin.