Hideaki Maeda

AC loss analysis on high-temperature superconductors with finite thickness and arbitrary magnetic field dependent voltage-current relation

A numerical model is developed for the current distribution in high-temperature superconductors (HTS conductors) subjected to self and applied field. This analysis is based on the two-dimensional Poisson equation for the vector potential. The vector potential is rewritten into an equation of motion for the current density J. The finite thickness of the conductor and the n-power law electric field-current density relations (E-J relations) of the conductor are included in the model. The magnetic field dependence of critical current density Jc is also included in this E-J relation. The self-field loss obtained with this analysis has good agreement with experimental data. Also the effects of aspect ratio and n value on ac loss are pointed out to be important on the ac loss.

Gamma‐ray‐induced conduction in polyethylene‐terephthalate under high electric fields

Radiation‐induced conduction in polyethylene‐terephthalate under high electric field (1×106 to 1.6×108 V/m) has been studied. At field strengths below 1×108 V/m, the induced current reaches its initial steady‐state value (primary component) soon after irradiation is initiated, then monotonically decreases with time thereafter. This decrease of current is due to a ’’polarization’’ effect. Above 1×108 V/m, the induced current decreases for a while then increases again to reach the equilibrium value (secondary component). This secondary component is due to the electron injection, enhanced by the fixed positive space‐charge accumulation near the cathode. The radiation‐induced conductivity of the primary component increases with electric field strength between 1×106 and 1×108 V/m and then shows a saturation tendency above 1×108 V/m. This may be due to geminate recombination in the carrier generation process. The primary component in the 50‐μm‐thick specimen is deduced to be dominated by a deep‐trapping‐limited...

Stabilization of superconducting dry solenoids

Premature quenches in superconducting solenoids wound with Formvar-coated NbTi conductors have been studied. Some model coils wound with various tensions were tested. The experimental results are discussed with attention given to the stress distribution for coil winding, cool-down to liquid helium temperature, and energization at 4.2 K. Some mechanisms of premature quench are classified according to the winding tension, and some stabilization methods are presented on the basis of these quench mechanisms. It is found that if a solenoid is wound loosely, macroscopic slips occur in inner layers due to low frictional force. Although such motions are removed if the coil is wound tightly, shear stress concentration at the interface between the winding and the bore tube then causes quenches by microslips. Inserted polymer films affect the sliding behavior of the conductor, which can improve the coil performance. >

Compact 17 T epoxy-impregnated magnet without bore tube

Abstract A compact 17 T magnet was successfully fabricated and energized using a flexibly supported epoxy-impregnated coil without a bore tube. This coil structure was developed with a magnet stabilization study on small Nbue5f8Ti and Nb 3 Sn coils. The 17 T magnet is composed of two sets of Nb 3 Sn coils and an Nbue5f8Ti coil graded into two sections. The magnet successfully attained the designed field, 17 T, in the 33 mm bore. The stabilization contributed to the performance of each coil. No quench occurred in the inner Nb 3 Sn coil. The stabilization enabled a current density as high as 260 A mm −2 , generating 8.8 T, to be achieved for the Nbue5f8Ti coil in a compact system operated in a 300 mm diameter cryostat.

Cryocooler directly cooled 6 T NbTi superconducting magnet system with 180 mmroom temperature bore

Abstract This paper describes a cryocooler cooled NbTi superconducting magnet system. The technical features of this magnet system are a 4K-Gifford-McMahon (GM) refrigerator using magnetic regenerator material and a high-Tc Bi 2 Sr 2 CaCu 2 O y superconducting current lead. The NbTi coil was directly cooled by the 4K-GM refrigerator without liquid helium and it took about 21 hours for the NbTI coil to be cooled from room temperature to below 4 K. The stable magnetic field of 6 T at 3.5 K and the maximum magnetic field of 6.45 T were obtained in the 180 mm room temperature bore.

Racetrack coil instability resulting from friction heat generation at fixtures

A series of experiments on the instability resulting from mechanical disturbance at the coil surface of a small racetrack coil is described, along with a preventive measure against its instability. Epoxy-impregnated racetrack coils sometimes experience premature quenches due to frictional heat produced by coil slides at fixtures. The first experiment confirmed coil slides during coil charging. These slides were about 10 mu m, an equivalent of 20 mJ in fractional heat generation. One effective preventive measure against this mechanical disturbance is the utilization of a thermal barrier method. The thermal barrier is an insulation layer at the interface between the coil and the fixtures. The second experiment examined the thermal barrier effect on the stability margin on the racetrack coil. A thicker insulation layer substantially increased the coil stability margin. The margin increased from 105 mJ to 200 mJ by thickening the insulation layer from 0.36 mm to 1.00 mm. The two experiments showed that the racetrack coil was stabilized if the thickness of the insulation layer exceeded 0.20 mm. Charging a racetrack coil with a 0.36 mm-thick insulation layer confirmed this criterion.<<ETX>>

A 4.7-T magnet with a 0.3 M bore for P-31 magnetic resonance spectroscopic imaging

A 0.3-m-bore, 4.7-T, high-homogeneity magnet, used for P-31 magnetic resonance imaging, has been constructed and installed. The magnets are comprised of a set of solenoids, which attained designed 94.8-A current (1.8 MJ) without quenching. Diodes in the cryostat resulted in a fast ramp rate of 3 T in 10 min. The field inhomogeneity for the bare magnet was -250 p.p.m./0.1 m dsv, which was finally reduced to 6 p.p.m./0.1 m dsv by shim coils. The field decay was less than 0.05 p.p.m./h. The magnet system is currently used for P-31 magnetic resonance spectroscopic imaging experiments on living animals in the laboratory. >

Mechanical disturbances for a cable-in-substructure superconductor

Abstract Frictional cable movement, due to the electromagnetic force, was studied for a cable-in-substructure conductor. Both axial extent and dissipation energy density for the movement were determined experimentally. The void fraction effect was also studied.

Stabilization of Cu-Ni based persistent current switch

Stabilization of a persistent current switch with a parallel connection is described. The current transfer behavior for a seven-strand conductor, wound as a persistent current switch, was studied. Each strand current was measured by using Rogowski coils. In the current ramp, negative current flowed into the center strand, which was inverted to a positive direction in about a 2-min time constant. Strand quench was induced by a heater wound in each strand. When one of the outer strands was quenched by its heater, the current was mainly transferred to the nearest three strands including the center strand in about 2 ms. These experimental results agreed well with the calculated results considering the self/mutual inductance for a parallel conductor and the joint resistance. The transferred current of several tens of amperes, which flowed with a few orders of 10000 A/s ramp rate, resulted in the degradation of the whole switch quench current from the conductor critical value.<<ETX>>

Instability of persistent current switch

The instability for a persistent current switch (PCS) wound bifilarly by a CuNi matrix conductor was studied experimentally and by numerical calculations. The temperature rise due to self-field AC loss during the ramping up of the current causes degradations. The quench current gradually decreases to the adiabatic value, if the current sweep rate is progressively increased. Current transfer from the current terminal to the winding limits the current capacity for the PCS wound by a thick conductor, such as 0.9 mm in diameter. A short ramp pulse causes premature quenches at extremely low currents, which may be due to heat concentration of the self-field AC loss in the conductor outer shell.