Katsuhiko Asano

Study on a mobile-type magnetic separator applying high-Tc bulk superconductors

Abstract We have developed a new water-treatment system that consists of a membrane separator and a magnetic separator that uses long high-Tc bulk superconductors (SCs) as permanent magnets. Basic tests were performed using a new prototype water-treatment system to verify the separator’s ability to remove water bloom. Based on the test results, we design the mobile-type magnetic separator system. The designed magnetic separator system consists of three components; a pre-process unit, a rotating membrane separator and a magnetic separator. The capacity of the magnetic separator is 100 ton/day. Removal rate is 90% or more. It is thought that the system is compact and high efficiency.

Cryogenic small-flaw strength and creep deformation of epoxy resins

Resin cracking, a cause of coil quenching in superconducting magnets, occurs when a resin contains small flaws and sustains high thermal stress. Seven epoxy resins were chosen in order to evaluate thermo-mechanical properties, small-flaw strength, and creep deformation at low temperatures. Although the plain specimen strengths consistently increase as the temperature decreases, the fracture toughness resulting from large cracks reaches a maximum at around 80 K and then decreases at 4 K. The loss factor during cyclic loading behaves similarly, because of low-temperature relaxation of the resin, and has a maximum value at around 150 K. The strengths resulting from decreasing flaw size level off to those of plain specimens; that is, they deviate from linear fracture mechanics. Thermal stress (caused by coil restraint) in the epoxy resin is experimentally measured by simulating the coil molding process. These thermal stresses are close to the calculated ones obtained by using the elastic moduli and the coefficients of thermal expansion of the resin. However, they are a little lower because of stress relaxation at around the glass transition temperature. Thermal stress and small-flaw strength at 4 K are used to calculate the critical volume-ratio of resin to conductor that produces coil quenching as well as the allowable flaw sizes. The calculated ratios and sizes of the resins vary considerably. Even at 77 K, creep deformation in the resins is significant and may cause a cryogenic delayed fracture of the resin and result in unexpected coil quenching during steady-state persistent-current operation. And the ratios of the creep strain to the initial strain (in all resins tested) decrease uniformly as the difference between glass transition temperature and test temperature increases.

Magnetization method for long high-T/sub c/ bulk superconductors used for magnetic separation

We have developed a new water-treatment system consisting of a membrane separator and a magnetic separator that uses long (>200 mm) high-temperature bulk superconductors (SCs) as permanent magnets. For magnetizing the long bulk SCs, we developed a new magnetization method. Namely, a field is applied, the bulk SCs are cooled below their transition temperature (T/sub c/), and during cooling, the bulk SCs are inserted into and withdrawn from the applied field repeatedly. As a result, the bulk SCs experience high and low fields under a gradually cooled ambient, and the trapped field gradually increases. The benefit of this method is that it can magnetize large bulk SCs by using a small field magnet. We fabricated an embedded-SC bulk magnet more than 200-mm long, and magnetized it by split-solenoid superconducting magnets with a diameter of approximately 100 mm. Field measurements showed that the developed method can magnetize a wider area over the bulk SCs than that possible with a conventional field-cooling method.

Connecting tests of superconducting persistent-current-switch in a type of current transformer to 1 kWh SMES system

A full system of 1 kWh/1 MW module-type SMES (superconducting magnetic energy storage) has been completed at a substation in Fukuoka City. There is a need for a PCS (persistent-current-switch) with a quick response and large current capacity for use with a SMES system. We have investigated a superconducting PCS in a type of transformer which works according to the principle of a current transformer. A 900 A class PCS has already been manufactured. This paper describes experimental results with connecting tests of the PCS to a SMES which consists of two modules, each comprising a converter of 500 kVA and three superconducting pulse coils.

Thermo-mechanical properties of epoxy GFRPs used in superconducting magnet winding ☆

Abstract A tightly constructed rigid structure of a conductor winding and GFRP spacers in a coil case is an essential element for a superconducting magnet, that is to operate at a cryogenic temperature and sustain large magnetic forces. However, the contact pressures on the conductor, the spacers and the coil case, which are imposed in fabrication process, may be greatly reduced by relaxation at room temperature and by thermal contraction in the GFRP spacers during cool-down process. We therefore studied 61 kinds of commercial and test GFRPs and established a basis for suitable GFRP spacer material to be used in superconducting magnet windings. Glass transition temperature, T g , of the impregnating resin plays an important role in transverse creep deformation of GFRPs. GFRP spacers with T g above 423 K can maintain 80% of the initial pressure in a winding for two years at room temperature. This result was obtained by utilizing the transverse creep moduli of GFRPs at different temperatures and the time–temperature superposition principle. Transverse thermal contraction from 293 to 4 K decreases uniformly in all GFRPs as the resin weight content, R c , decreases. Also, contraction is smaller than that of the 304 stainless steel used for the coil case when R c is less than 15%. As a result, pressure decrease in the winding can be prevented during cool-down. The elastic modulus in the transverse direction of a GFRP is calculated by dividing the elastic modulus of the impregnating resin by R c . Fractures of the GFRPs at low temperatures are primarily in shear mode under four-point bending, in-plane compression, transverse compression, or interlaminar shear loading. Moreover, the transverse-compressive fatigue limit of plain-woven GFRP at low temperature depends on the maximum compressive stress of the cyclic loading under high mean-compressive stress and the stress range under low mean-compressive stress.

A diagnosis method for properties of superconducting magnet using fast current discharge

The authors studied a diagnosis method to investigate the properties of superconducting magnets throughout their life. The magnet current, initially held constant, is damped with a short time constant. One can evaluate the properties of the magnet using the normal voltage, which may appear during the damping phase. A bridge circuit, which can adjust both amplitude and phase with over five orders of accuracy, was developed to measure unbalance voltages. Experiments were carried out using a small solenoidal NbTi magnet. By adjusting bridge parameters and numerically processing the voltage waveform, the authors measured the normal transition voltage with the S/N ratio of over 10/sup -4/ in the damping phase. The applicability of the diagnosis method was studied in both numerical simulations and experiments.

Losses in superconducting coils of 1 kWh/1 MW SMES

The in-situ measurements of pulse losses are carried out for the total system of ESK (Experimental SMES (superconducting magnetic energy storage) of Kyushu Electric Power Co.) with a capacity of 1 kWh/1 MW which was completed in 1998. ESK has six toroidal coils wound by Rutherford cables, each coil being contained in a fiber reinforced plastic (FRP) vessel set up in a Dewar. The loss was calculated by using a method of measuring the decrement of liquid helium (LHe) level in each vessel. The form of current variation for the measurement is a repeated triangular waveform. The charge-discharge rate was changed from 10 A/s to 140 A/s. Although there are considerable errors in the measurements, agreement between the measured results and theoretical calculations is satisfactory in view of the limited conditions of the in-situ measurement for the SMES coils of this scale. The origins of the loss are discussed in detail.

DESIGN AND CONSTRUCTION OF COIL SUPPORTING STRUCTURE AND CRYOSTAT VESSEL FOR LHD

The Large Helical Device is a fusion experimental machine with all superconducting coils. Electromagnetic force generated by the coils is supported by the torus shaped “supporting structure” of 100 mm thick 316 stainless steel. This structure consists of 20 fan shaped parts that were welded together on site. The coils needed to be positioned within 2 mm accuracy in the absolute coordinates so that the supporting structure had to be assembled within 5 mm accuracy prior to the coil setting. After completion of the welding, the overall accuracy of < 5 mm was achieved.

Test Results of Elementaly Coils for Toroidal SMES

Our experimental SMES of 1kWh/1MW named “ESK” (Experimental SMES of Kyushu Electric Power Co., Inc.) is going to be completed in 1997. It has a toroidal coil system with 6 elementary coils (~ each 1/6 kWh) made by 3 manufactures independently, each of which has its own type of coil and conductor: Compacted Rutherford type for conductors, and pancake and layer-winding type for coils. The newly designed coil shape “modified D” has two straight parts in-between D parts.

Experimental Results of Elementary Coils for 1kWh/1MW Experimental SMES System

We have already reported on a plan for development of an experimental SMES system for kyushu (ESK)1). This SMES system is now being installed at substation in Fukuoka and is going to be tested in order to confirm the stabilizing effect for actual electric power system. This coil system is consisted of 6 modified D shaped coils. We manufactured 2 coils and have finished shop assembly and test of each coils. We confirmed that these coils have inductance almost corresponding to design value, and that these coils can be charged at rated current and under the condition of actual electromagnetic force that is generated in case of charging 6 coils in stable.