Tadatoshi Yamada

Minimum quench energy measurement for superconducting wires

We have developed a new method of measuring minimum quench energy (MQE) of superconducting wire. There have been conventional methods using heating wires, whose diameters are /spl sim/0.1 mm and are glued by epoxy resin. When we induce a pulse heat to superconducting wires, a duration of the pulse must be several hundreds micro seconds to demonstrate actual disturbances (for instance wire motion) in a superconducting magnet. In spite of this fact, thermal diffusion time constants of the conventional heaters are larger than the duration of the actual disturbance, because of these electrical insulator of the heating wire and the epoxy bind. Therefore, this kind of heater is not able to demonstrate the actual disturbance. To solve this problem, we have proposed a new constitution of a heater. In our method, heat generation is introduced in high resistive layer on a surface of a superconducting wire. The high resistive layer is consist of carbon paste. The thickness of the carbon paste layer is /spl sim/20 /spl mu/m, thus a time constant of this heater is expected to be small enough to demonstrate the actual disturbance. Adopting our new method to our MQE measurement, we successfully evaluate MQE of superconducting wires with high precision. Several results are introduced in this paper.

30 MVA superconducting synchronous condenser: Design and it's performance test results

30 MVA superconducting synchronous condenser has been designed, fabricated, and tested successfully. The test suggested that the new approaches applied to the 30 MVA machine were reasonable. The superconducting field winding composed of six Nb-Ti and two Nb 3 Sn coils, supported in torque-tube slots by wedges, was excited up to the rated operating current without training. The amplitudes of the rotor vibration were small and special balance at cryogenic temperature was not needed. In this paper, the studies on the key design aspects, namely, superconducting field winding, rotor structural materials, and rotor structures including the connection means between torque-tube(cold) and damper(warm) are described together with the experimental verifications. Moreover, the stator design and the electro-magnetic design including an analysis of the operational performance with regard to double damper system are presented. Machine constants obtained from the theoretical investigations using three-dimensional electro-magnetic field calculations are in good agreement with test results.

Initial magnetic field decay of the superconducting magnet in persistent current mode

The initial magnetic-field decay in the persistent current mode of a magnetic resonance imaging magnet is greater than the steady field decay due to joint resistances of conductors. Imaging experiments cannot be carried out during periods which last ten or more hours. The current distribution in the multifilamentary conductor is nonuniform just after energization. It is suggested that the change of the current distribution causes the initial magnetic-field decay. A sixth-order superconducting magnet was prepared for experiments (central field=0.35 T, inner diameter=1 m, length=1.86 m). The stability state magnetic-field decay was 7*10/sup -8//h. The initial magnetic-field decay was 3*10/sup -6//h. Overshoot currents (101 and 105% of the rated current) were applied to the magnet and the current reduced to the rated current to improve the initial decay. The energizing and de-energizing rate of the field was 1.8 G/s. No initial decay was observed when 105% current pattern was applied to the magnet. >

Superconducting bending magnets for compact synchrotron radiation source

A 180 degrees superconducting magnet has been constructed for a synchrotron radiation source. The authors describe the superconducting magnet design and some component test results for the compact synchrotron radiation source. The magnetic field at the electron orbit is 4.5 T. The bending radius is 0.593 m. The magnet has several features such as low liquid helium consumption ratio, persistent current operation, high homogeneous magnetic fields, and leakage flux shielding by the iron core surrounding the cryostat. Component tests of persistent current switches of 15 Omega and 3 Omega and a superconducting joint test have been performed.<<ETX>>

Field correction of a high-homogeneous field superconducting magnet using a least squares method

A 6th order superconducting solenoid coil has been developed for a whole body magnetic resonance imaging. Nine sets of shim coils are equipped on the solenoid coil. A least squares computer program was applied to determine shim coil currents analytically. After the field correction using shim coils, the homogeneity of 7.6 × 10-6/30cm DSV was obtained. The effects of ferromagnetic objects on the homogeneity was studied. The error field caused by iron spheres agreed well with the theoretical results. It was found that ferromagnetic objects generated mainly the 1st order error field and this could be corrected by shim coils. The effects of heat and charge/discharge cycles of the magnet on the homogeneity was studied.

Quench protection of persistent current switches using diodes in cryogenic temperature

Transient characteristics of power diodes at cryogenic temperatures and their application to the protection of persistent current switches (PCS) in superconducting magnet systems are presented. The observed forward voltage drop was 9 V at 4.2 K and was six times larger than that at room temperature. The voltage was quickly reduced to 1 V after the diode was turned on. The transition time was less than 1 ms. A PCS with a normal state resistance of 1.5 Omega was adequately protected from quenching by the diodes.<<ETX>>

Flow visualization of coolant in cable-in-conduit conductor

Flow visualization to observe the flow behavior of a coolant in a cable-in-conduit conductor was performed by nuclear magnetic resonance (NMR) imaging. A dummy conductor in which many vinyl strings were inserted into an acrylic tube was used for visualization. Regions of both high and very low flow velocity, in comparison with the average velocity, existed in a cross section of the conductor. This fact shows that the flow behavior of the coolant in the cable-in-conduit conductor is not homogeneous but heterogeneous. Thus it is not possible to directly apply a numerical code based on homogeneous flow behavior to stability analysis of the cable-in-conduit conductor.

Field measurements of superconducting bending magnets for compact storage ring

We have constructed 180 degree superconducting bending magnets for compact electron storage rings and tests of magnetic field qualities of the magnets have been performed. Measured field qualities are; (1) high field homogeneity (normalized field errors are less than 1/spl times/10/sup -3/ over the electron beam aperture), (2) high field stability (the decay of the field is less than 0.1%/year) at persistent current mode of 3.5 T, (3) small gradient of coil axis (the averaged value is 0.5 mrad), and (4) small difference of the fields between two magnets (the difference is less than /spl plusmn/1/spl times/10/sup -3/). From the tests of field qualities, we confirm that the magnets have good performance for electron beam storage. An electron beam of 600 MeV can be stored.<<ETX>>

Super-Stable Superconducting MRI Magnet Operating for 25 Years

This paper describes a superconducting magnet performance continuing persistent current operation for 25 years. The central magnetic field is 0.5 T for whole-body magnetic resonance imaging (MRI). The observed stability of the magnetic field was extremely stable compared with the specification. Because of the ultrafine electrical resistances in persistent current mode operation, it was difficult to observe an accurate magnetic field stability at the factory test. By long-term operation, we are able to observe the magnetic field stability and the joint resistance precisely. The magnet was started to energize on January 1988 and has been operated consecutively without re-excitation. At that time, a superconducting whole-body MRI system was appeared in the world among a normal conducting magnet and a permanent magnet system. We made superconducting joints to connect superconducting conductors between superconducting coils. We carried out the measurement of the central magnetic field by the nuclear magnetic resonance method. Because the device is used for the medical examination at the hospital, the magnetic field data acquisition is in a limited situation. The stability of the last magnetic field becomes 0.0006 ppm/h and gets the good performance that is more than 100 times than the specification. The total resistance of the superconducting joints is 4 ×10-12 Ω and supports good superconducting connection.

Comparison of measured value and calculated value of multiple stability in forced-flow cooled superconductors

The multiple stability observed exclusively in forced-flow cooled superconductors is numerically calculated, and the result is quantitatively compared with the value measured by J.W. Lue et al. (1980). The calculated and measured values agreed well in certain cases, and did not in others. Based on this comparison, the effects of the transient heat transfer coefficient and ohmic heat generation on the quantitative prediction of stability are discussed. From this comparison, it is learned that a precise understanding of the transient heat transfer coefficient is essential for reliable predictions, and also that the ordinary evaluation method of ohmic heat generation, which considers the flux-flow resistance, tends to overevaluate the situation.<<ETX>>