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A cryogen-free 20 T superconducting magnet with a 52 mm room-temperature bore was developed and installed in Tohoku University in 2013. This magnet consists of a Bi2223 insert coil, which generates 4.5 T, and outer low-temperature superconducting (LTS) coils. Both coils were cooled by a GM/JT cryocooler with 4.2 W-class cooling capacity at 4.3 K. To generate a higher magnetic field, a new cryogen-free 25 T superconducting magnet using a REBCO insert coil, which generates 11.5 T, and new outer LTS coils is now under development. The magnetic field contribution of this REBCO insert coil is considerably higher than that of the previous Bi2223 insert coil, and the ac-loss of the insert coil during field ramping rises to approximately 9.7 W. The LTS coils have to operate at about 4 K, but the REBCO coil can operate at various temperatures above 4 K. In addition, the cooling capacity of a GM cryocooler is greater than that of a GM/JT cryocooler around 10 K. Thus, the REBCO insert coil is cooled to about 10 K by using two GM cryocoolers, independently of the LTS coils, which are cooled by two GM/JT cryocoolers. To protect the cryocoolers from the leakage field of the magnet, the two GM cryocoolers cool circulating helium gas through heat exchangers, and the gas is transported over a long distance to another heat exchanger provided for the REBCO insert coil. The maximum temperature of the REBCO insert coil was calculated under the most severe condition where an insert coil ac-loss of 9.7 W was generated continuously. And it was confirmed to be less than the target maximum temperature of 12 K. [ABSTRACT FROM PUBLISHER]

We fabricated a cryocooler-cooled Bi2223 superconducting pulsed coil and experimentally studied the thermal runaway in dc or ac operation. We carried out numerical simulation of thermal properties of the coil in order to explain the thermal runaway of the coil. Firstly, we analyzed the total heat generation of flux-flow losses and ac losses inside the winding from the experimental results of the external field losses and the E - J characteristics for the Bi2223 strands. Secondly, we numerically simulated the thermal properties by using a 2-dimensional heat conduction equation with axial symmetry. The numerical simulation shows the relation between the initiation of thermal runaway and the temperature distribution with highly concentrated heat source in the winding. We have a semi-quantitative agreement between the numerical results and the experimental ones for the condition of the thermal runaway. [ABSTRACT FROM AUTHOR]

We studied the thermal and electromagnetic behavior of a conduction-cooled superconducting pulse coil. It was wound with a 4-strand parallel conductor composed of silver sheathed Bi2223 multifilamentary tapes and impregnated with an epoxy resin. We first investigated the temperature dependence of the thermal runaway current in dc operation. It was higher than the critical current by several tens amps at any temperature from 40 to 90 K. In order to study the mechanism of the thermal runaway, we carried out the numerical simulation of the coil behavior. We found out that the thermal runaway was caused when the heat generation at the winding end close to the cryocooler head where the flux-flow loss is the largest due to perpendicular magnetic field exceeded the cooling through thermal diffusion though the thermal runaway of the coil started at the other end of the winding. If the transport current is smaller than the thermal runaway current at the coil temperature just after applying current, the temperature distribution in the longitudinal direction of the coil varied into a monotonous gradient and the steady thermal flow was established. After that the coil temperature decreased to the balanced one. [ABSTRACT FROM AUTHOR]

We designed and fabricated a 1 T cryocooler-cooled oxide superconducting pulsed coil with Bi2223 multifilamentary flat wires. Previously we studied temperature dependence of the thermal runaway current of the pulsed coil in dc operation. The thermal runaway current was higher than the critical current that was defined as the current where the electric field 10-4 V/m was generated on average over the whole length of the conductor. In this paper, we studied the coil temperature and the ac loss dependence of the thermal runaway current in ac operation. We operated the coil at 0.5 to 5 Hz with a sinusoidal-waveform transport current. The initial coil temperature was set between 30 and 100 K. The thermal runaway current decreased monotonically with increasing temperature in a similar way to dc operation. However, in a low temperature region, the thermal runaway current was restricted to a lower level than that in dc operation due to the ac loss. We discuss the frequency dependence of thermal runaway current in ac operation. [ABSTRACT FROM AUTHOR]

At present, ball bearings (BBs) are generally used in spindles of hard disk drives, but it has been known that BBs generate high-frequency vibration and acoustic noise. Hence, other types of spindles using fluid dynamic bearings (FDBs) that have high rotational accuracy and less vibration characteristics have been developed and put into practical use. FDB spindles have be- come common in use; nevertheless, a conventional characteristics evaluation method based on rotational accuracy, such as repeatable runout and nonrepeatable runout, is still used for PUB spindles. Since the FDB mechanism is different from BBs, we think that not only rotational accuracy, but also the shaft stiffness and the damping effect of spindle, are important in evaluating the spindle. In this study, we propose a new evaluation method for damping characteristics and frequency characteristics of FI)B bearing spindles using a high-frequency vibration base. The presented method in this study is not a precious evaluation of bearing characteristics, but an evaluation method for basic mechanical comprehensive design considering tracking servo and the vibration of the disk and base in use for hard disk drives. [ABSTRACT FROM AUTHOR]

We fabricated a 1 T cryocooler-cooled oxide superconducting pulsed coil. It is a 16-layer solenoidal coil wound with an interlayer-transposed 4-strand parallel conductor composed of Bi2223 multifilamentary flat wires. The insulated strands were transposed only between layers to make all strands inductively equivalent. To suppress the temperature rise due to the ac loss, the aluminum nitride(AIN) plates were arranged between layers as heat drains. The pulsed coil was cooled down to 30 K by a single stage cryocooler. It can be operated continuously at 40 K in the 1 T triangular wave operation at 1 Hz. First we studied the temperature dependence of thermal runaway current of the coil under the application of direct transport current. The runaway current was higher than the critical current, which was defined as the current where the electric field of 10-4 V/m was generated on average over the whole length of the conductor. Next we studied the temperature variation of the coil in a sinusoidal operation of 1 Hz. The temperature increment was much more gradual than that in the dc operation. In this paper, we discuss the thermal properties of the cryocooler-cooled oxide superconducting pulsed coil, taking into account the heat generation of ac loss and flux-flow loss and the cooling capacity of a cryocooler. [ABSTRACT FROM AUTHOR]