T. Saitoh

Critical current properties in HTS tapes

Abstract The author’s recent studies on the critical current properties in Bi-2223 multifilamentary tapes as well as YBCO coated IBAD tapes have been reviewed. Extended electric field vs. current density ( E – J ) characteristics were studied over wide range of temperature, T , magnetic field, B , and angle, θ . Based on the analysis of statistic J c distribution and scaling, we derived an analytical expression for J as a function of E , T , B , and θ . This method allows us to describe and extrapolate the nonlinear E – J characteristics even in extremely low E region and/or high B region based on rather simple measurements.

High-speed deposition of high-quality RE123 films by a PLD system with hot-wall heating

We have studied the use of a hot-wall heating system for producing GdBa2Cu3O7?X (Gd123) films with large critical current (Ic), stably, at high speed, by a pulsed laser deposition (PLD) method. We achieved a material yield from the PLD system with hot-wall heating of about 50% through optimization of the evaporation conditions. With the PLD system, we were able to fabricate a Gd123 film about 20?cm long with a high Ic of 997?A (77?K, self-field) at about 6??m thickness. The maximum Ic that we achieved for a Gd123 film about 2?cm long was 1040?A (77?K, self-field) at about 6? ?m thickness. Furthermore, we were able to form a Gd123 film about 1? ?m thick and 20?cm long with an Ic of 352?A (77?K, self-field) at a high speed of 60? m?h?1 using high laser power. For the production of long conductors coated with RE123 (REBa2Cu3O7?X, where RE stands for rare earth) using hot-wall heating, it was found that RE123 coated conductors over 100?m long with Ic of over 200?A had uniform Ic distributions with ? 2% variations in the direction of the length.

Estimation of E–J characteristics in a YBCO coated conductor at low temperature and very high magnetic field

Abstract E – J characteristics in a YBCO coated conductor at low temperature and very high magnetic field up to 27 T have been estimated using experimental E – J characteristics below 12 T along with a scaling analysis based on the percolation model. 1 mm thick YBCO films was deposited on a biaxially aligned Y 2 O 3 /YSZ/Hastelloy substrate fabricated by the IBAD technique. The critical current I c for the 1 cm wide tape was 85 A at 77 K in self-field. We measured E – J characteristics in perpendicular field up to 12 T using 100 μm wide and 1 mm long bridge patterned in the tape. Statistic J c distribution and pinning properties were analyzed from the E – J characteristics. Using the pinning parameters determined by the scaling analysis, we can calculate the critical current properties as a function of temperature and magnetic field. E – J characteristics at low temperature and very high magnetic field up to 27 T were estimated analytically and compared with the experimental data. Estimated value of J c and n -value, and their error analysis were also discussed.

Percolative transition and scaling of transport E-J characteristics in YBCO coated IBAD tape

We have investigated extended electric field-vs.-current density E-J characteristics in YBCO coated IBAD tapes. The results of a Monte-Carlo study on the depinning in a random pin medium were compared with measurements. Using a low temperature scanning laser microscope, we examined the percolative behavior of the local resistive transition in the YBCO tape. It was also shown that the depinning probability, which is proportional to the dynamic resistance of the E-J curves, is scaled as a function of reduced current density with the aid of a power index. Consequently, the E-J characteristics in the tapes can be described by the combination of the two kinds of scaling laws: one is the scaling law of the depinning probability predicted in a random network and the other is the scaling law for the flux pinning force. These properties agree well with the percolation model of depinned clusters.

Development of 66 kV and 275 kV Class REBCO HTS Power Cables

A Japanese national project called “Materials & Power Applications of Coated Conductors (M-PACC)” started in FY2008. In this project, we are developing a 66 kV/5 kA large-current high-temperature superconducting (HTS) cable and 275 kV/3 kA high-voltage HTS cable, using rare-earth barium copper oxide (REBCO) tapes. These HTS cables are expected to offer a compact cable with a large capacity and low power transmission loss. After the cable design has been studied and elemental technologies for each component of the cable system, such as ac loss reduction, protection against over-current, and high-voltage electrical insulation have been developed, two cable systems will be constructed and verified to meet the required specifications in FY2012. This paper describes the progress and status of these HTS cable developments in the M-PACC project.

Remarkable progress in fabricating RE123 coated conductors by IBAD/PLD technique at Fujikura

Increase of production rate and improvement of quality for RE123 coated conductors have been tried. In-plane texturing of MgO was attempted by the IBAD system with the world largest ion source. As a result of optimizing condition in large deposition area, the dramatically high throughput of 1000 m / h was realized to obtain the IBAD-MgO with Δ 600 A except some locations was obtained at the throughput of 15 m / h. In addition to the speed-up, the very high Ic of 1040 A was also achieved by the hot-wall heating PLD system.

Characteristics of Impregnated Pancake Coils Fabricated Using REBCO Coated Conductors

REBCO coated conductors are expected high performance in superconducting applications because of their high current density and high mechanical strength. However, in order to use these conductors in superconducting applications, a technique is required for fabricating impregnated REBCO coils without degradation.In this work, we fabricated six pancake coils without degradation using wounded REBCO coated conductors. These conductors were laminated with a copper tape as a stabilizer, and were fabricated by the IBAD/PLD method. In addition, we evaluated the V-I characteristics and n-values of the coils in liquid nitrogen and compared the calculated and measured values for the critical current (Tc) of the REBCO coated conductors. The measured Tc of the coils were in good agreement with the calculated ones. Furthermore, we fabricated a stacked coil using an approximately 190 m long REBCO coated conductor and evaluated the stability of the stacked coil at 77, 60 and 50 K under conduction-cooled condition. The thermal runaway current of the stacked coil was 166.4 A, and the central magnetic field was 1.27 T at 50 K.

Long-length GdBCO coated conductors with IBAD/PLD method

We fabricated a long-length GdBCO(GdBa2Cu3O7-?) coated conductor by the ion-beam assisted deposition (IBAD) and the pulsed laser deposition (PLD) methods. The GdBCO tape was over 400 m in length with 1.5-2.0 MA/cm2 of the critical density. The critical currents of the tape were over 304.8 A/cm along 368 m in length. So we have achieved Ic ? L = 112,116 Am. For the IBAD process, we have already developed 500 m-length IBAD-Gd2Zr2O7 (IBAD-GZO) tapes with highly aligned in-plane texture, and continuously fabricate 250 m-length tapes at a rate of 0.5~1 km/month. In addition, we have also developed a new type template for the IBAD-GZO, to enable the production rate fast. Structure of the template is IBAD-GZO/IBAD-MgO/Y2O3. It is note that MgO (111) is parallel to the substrate normal and three fold symmetry. For this structure, the film thickness of the IBAD-GZO could lessen from 1.2 ?m to 0.3 ?m. By the new IBAD buffer layer, a production rate is expected to be over 30 m/h.

Total AC Loss Characteristics in a Stacked YBCO Conductor

Four 5 mm-wide YBCO conductors with identical critical current were assembled face to face to obtain a stacked YBCO conductor. The magnetization losses without transport current, the transport losses without external magnetic field, and the total AC losses of the stacked YBCO conductor as well as a single YBCO conductor carrying various transport currents in transverse magnetic fields with various orientations were measured. The difference between the AC loss characteristics of the stacked YBCO conductor and those of the single YBCO conductor was clarified.

Progress in Scale-Up of RE-123 Coated Conductors With

By an IBAD/PLD method, a 500 m-length high-performance GdBa₂Cu₃O_x (GdBCO) coated conductor with BaZrO₃ (BZO) was fabricated. End-to-end I_c of the 503.5 m tape was 349.6 A/cm at 77 K, corresponding I_c times L value of 176,023 Am. New approaches to decrease the thickness of IBAD buffer layers were developed by combination of IBAD-MgO(111) and IBAD-Gd₂Zr₂O₇ (GZO). The maximum Jc of the GdBCO on the buffer layer was 550 A/cm. We also started to investigate IBAD-MgO(OOl) by using a large IBAD system with a 110 cm times 15 cm ion source. A 50 m-length IBAD-MgO tape with DeltaPhi of 13deg was fabricated at a 100 m/h-1 pass. Insulated REBCO wires with Cu or Ni-Cr lamination were developed using 5 mm-wide conductors sliced from 10 mm-wide IBAD/PLD tapes. Practical long insulated wires were supplied for several prototypes of motors, transformers and fault current limiters in fiscal 2007.