T. Naito

Simulation of temperature and magnetic field distribution in superconducting bulk during pulsed field magnetization

A theoretical simulation of electromagnetic and thermal fields was performed for a cryocooled superconducting bulk disc after applying a magnetic pulse. The results of the simulation qualitatively reproduced the experimental ones for the time and applied field dependences of the trapped field Bz and the temperature T on the bulk surface. For magnetic pulse application with a rise time of τ = 0.01xa0s, the magnetic flux propagation was about two orders of magnitude faster than the heat propagation because of the low thermal conductivity of the bulk. The results show that the intruding magnetic flux escaped because of the delayed temperature rise. For a longer magnetic pulse application with τ = 1 or 10xa0s, the flux propagation speed becomes slow and approaches the heat propagation speed. In this case, the magnetic flux escape attributable to the flux creep becomes small and a higher trapped field can be achieved. The method of exploring the enhancement of the trapped field using pulsed field magnetization is discussed.

Trapped magnetic field and vortex pinning properties of MgB2 superconducting bulk fabricated by a capsule method

We have studied the trapped magnetic field and the critical current density for an MgB2 superconducting bulk disc fabricated by a capsule method. The maximum trapped magnetic fields at the centre of bulk surfaces 20 and 30xa0mm in diameter were, respectively, 1.43xa0T at 13.4xa0K and 1.50xa0T at 16.4xa0K. The critical current densities, Jcs, were estimated to be 5.5xa0×xa0104 and 4.2xa0×xa0104xa0Axa0cm−2 at 20xa0K for the 20 and 30xa0mm diameter bulks, respectively, from the measured trapped field at the top surface, by using the analytical equation. The estimated Jc was as small as about 30–60% of the measured Jc in the self-field which was determined from the magnetization hysteresis loop of small pieces cut from the bulk. The results suggest that an MgB2 bulk magnet over 3xa0T can be realized at 20xa0K by means of the enhancement of both the Jc values and the mass density.

Pulsed Field Magnetization for GdBaCuO Bulk With Stronger Pinning Characteristics

The GdBaCuO superconducting bulk with stronger pinning characteristics (<i>B</i> _T = 1.8 T at 77 K by the field cooled magnetization (FCM)) has been magnetized at <i>T</i> _s = 70 - 20 K by the pulsed field magnetization (PFM) techniques; a sequential pulsed-field application (SPA) and a modified multi pulse technique with stepwise cooling (MMPSC). With decreasing <i>T</i> _s, the pinning force <i>F</i> _p increases especially at the growth sector boundaries (GSBs), and then the nonuniformity of the trapped field profile becomes more and more conspicuous on the bulk surface. At low <i>T</i> _s, the SPA technique is not necessarily a suitable technique to enhance the trapped field <i>B</i> _T and the total trapped flux <i>Phi</i> _T. However, these values can be enhanced by the MMPSC method. The properties of the bulk with stronger pinning characteristics during PFM are discussed.

Mechanism of magnetic flux trapping on superconducting bulk magnetized by pulsed field using a vortex-type coil

Numerical simulations of trapped field Bz and temperature T have been performed for a cryo-cooled superconducting bulk disc after applying a magnetic pulse using a vortex-type coil. Results of the simulation qualitatively reproduced experimental results reported by Ida et al (2004 Physica C 412?414 638). Using the vortex-type coil, the magnetic flux does not intrude into the bulk from the periphery but intrudes from both surfaces of the bulk disc. The temperature rise was less than that obtained when using a conventional solenoid coil. The effect of the trapped field enhancement using a long pulse application is small, which suggests that down-sizing of the condenser bank used for the pulsed field magnetization is possible and that the use of the vortex-type coil is desirable for practical applications.

Estimation of temperature rise from trapped field gradient on superconducting bulk magnetized by multi-pulse technique

Trapped field profiles of the GdBaCuO superconducting bulk of 65xa0mm in diameter have been measured as magnetized using a successive pulsed-field application (SPA) and a subsequent iterative pulsed-field magnetization with reducing amplitude (IMRA). The trapped field gradient dBT/dx on the bulk periphery increases concomitantly with increasing pulse number in the IMRA part. The dBT/dx value on the bulk magnetized by field-cooled magnetization (FCM) was also measured at each temperature TF. The maximum temperature Tmax for each pulse was estimated by comparing the dBT/dx value for the IMRA part with that for the FCM at each temperature. Furthermore, Tmax was measured directly for the SPA part in the drilled hole in the bulk. The estimated Tmax was nearly equal to the measured Tmax, which thereby confirmed that Tmax after applying the pulsed field can be estimated using the dBT/dx value in the bulk periphery. The enhancement of the total trapped flux ΦTP in the IMRA part with increasing pulse number results from enhancement of the critical current density Jc because of the reduction in Tmax.

Analysis of Temperature and Magnetic Field Distribution in Superconducting Bulk During Pulsed Field Magnetization

We have constructed the framework of theoretical simulation for magnetic field and temperature in the superconducting bulk during pulsed field magnetization (PFM) using finite element method (FEM). Three successive pulsed field applications with identical strength (SPA) were performed to the cryo-cooled bulk and time and spatial dependence of local field Bz(t,(z,r) and temperature T(t,(z,r) were calculated. The trapped field Bz increased and the maximum temperature rise Tmax decreased with increasing pulse number. The Bz enhancement results from the decrease in temperature rise because of the already trapped flux. The results of the simulation well reproduced the experimental ones qualitatively.

Direct Jc measurements and trapped field profiles using an identical superconducting bulk

We measured the trapped field profiles on a ∅45xa0mm GdBCO superconducting bulk plate of 2xa0mm thickness magnetised using pulsed field magnetisation (PFM), zero-field-cooled magnetisation (ZFC) and field-cooled magnetisation (FCM). The profiles were compared with the distribution of the absolute value of the critical current density (Jc) estimated by the magnetisation measurements using small pieces cut from the bulk plate. The Jc value was enhanced about 20% below the seed crystal compared with that in the growth sector regions (GSRs). The Jc value was also slightly enhanced on the growth sector boundaries (GSBs). For a lower applied pulsed field than that for full magnetisation for PFM, a small amount of the magnetic flux was preferentially trapped at the lower Jc region around the bulk periphery, which was not necessarily similar to that by ZFC. For a higher applied pulsed field, the magnetic flux was finally trapped below the seed crystal and at the GSBs.

Trapped magnetic field between double stacked MgB2 bulks magnetized by pulsed field

Pulsed field magnetization (PFM) was performed at Ts=14 K for the double stacked MgB2 bulks (bulk pair) 55 mm diameter fabricated by a reactive liquid Mg infiltration (Mg-RLI) method, compared with that for the single bulk. The trapped field of Bz=0.80 T was achieved between two bulks and Bz at the bulk surface was enhanced from 0.42 T to 0.50 T by stacking of the bulks. The trapped field characteristics of the bulk pair can be qualitatively explained by the model analyses.

Size Effect of Magnetizing Solenoid Coil for Pulsed Field Magnetization on the Trapped Field on Superconducting Bulk

We have investigated experimentally and numerically the trapped field B_z and temperature rise ΔT on the superconducting bulk with homogeneous critical current density, J_c magnetized by the pulsed field using the solenoid coils with different inner and outer diameters. The B_z was experimentally enhanced and the ΔT was reduced using the coil with a smaller inner diameter. However, the size effects of the coil on the numerically estimated B_z and ΔT were pretty small and the results of the simulation cannot reproduce the experimental ones. The inhomogeneous distribution of the J_c certainly exists in the real bulk, and the in- homogeneous flux intrusion influences on the experimental results of B_z and ΔT.

Vortex pinning properties of (Y1-xLax)–Ba–Cu–O and (Y1-xPrx)–Ba–Cu–O superconducting bulks

We have studied the effect of a small amount of Y-site substitution by La or Pr ions on the vortex pinning in the Y–Ba–Cu–O system. (Y1-xLax)(Y1-xLax)–Ba–Cu–O and (Y1-xPrx)(Y1-xPrx)–Ba–Cu–O bulks were fabricated by the melt-textured growth, in which x was varied from 0 to 0.01. The critical current density JcJc at 77xa0K is improved in magnetic fields parallel to the c-axis above 2–4.5xa0T and the corresponding irreversibility field, HirrHirr, shifts to the higher value in both bulks.