E.A. Young

The Development of High Field Magnets Utilizing Bi-2212 Wind & React Insert Coils

Wind & react Bi-2212 inserts have been manufactured and tested inside a wide-bore NbTi - Nb3Sn magnet providing a background field up to 20 T at 4.2 K. A pair of six-layer concentric coils both achieved critical currents of 350 A (JE = 200 A/mm2) in a 20 T background field. A thicker 14-layer insert made from 119 m of round wire had a critical quench current IQ of 287 A (JE = 162 A/mm2) at the same field and contributed to a combined central field of 22.5 T. This is a record for a fully superconducting magnet at 4.2 K. The 14-layer coil, equipped with an external protective shunt, was used for an extensive series of quench measurements and endured > 150 quenches without damage. Minimum quench energies were found to be in the range of 200-500 mJ in background fields of 15-20 T when the coil carried 70-95% of its critical quench current.

Quench Characteristics of a Stabilizer-Free 2G HTS Conductor

The prospect of medium/high field superconducting magnets using second generation (2G) HTS tapes is approaching reality with continued enhancement in the performance of these conductors. While the cryogenic stability and quench propagation are fundamental issues for the design and safe operation of superconducting magnets, there is insufficient understanding and experimental data for 2G HTS conductors, in particular for the high field scenario at low temperature (<77 K) where the current sharing regime is much larger than in low temperature superconductors. The present work includes a systematic characterization of the relevant thermal-electrical properties used for both qualitative discussion and numerical analysis. Direct measurements of one dimensional adiabatic quench initiation and propagation of a stabilizer-free 2G conductor have been carried out with spatial-temporal recording of temperature and voltage following the deposition of varying local heat pulses to the conductor at different temperatures between 30 K and 77 K carrying different transport currents. The minimum quench energy, and the heat generation in the minimum propagation zone (MPZ) have been obtained as a function of temperature and transport current. The results show quench features unique to HTS such as an increasing MPZ with transport current and higher quench energies at lower temperatures. The experimental results are discussed in the context of current sharing over a large temperature range.

The Removal of Lindane from Aqueous Solution using a Fungal Biosorbent: The Influence of pH, Temperature, Biomass Concentration, and Culture Age

A heat treated non viable cell suspension of the fungus Rhizopus oryzae was used for the removal of low concentrations of lindane from aqueous solution in a series of shake flask experiments. Experimental design was such as to allow the data to be tested against the Freundlich adsorption isotherm model thus quantifying the effect of experimental variables on both the adsorption capacity and intensity of adsorption exhibited by the fungus. The effects of temperature (5–45°C), pH (2.0–10), biomass density (1–12 g l−1) and biomass age (1–7 days) were studied. The results indicated that the mechanism of adsorption was by physical bonding of the negatively charged lindane molecule to the negatively charged fungal cell wall with hydrogen ions acting as the bridging ligand. Adsorption was most effective at low temperature and pH and was shown to be influenced, to a lesser extent, by cell density and biomass age. Data are presented in the form of adsorption isotherms and the Freundlich parameters associated with ...

Quench Characteristics of bi2212 Solenoid Insert Coils in Background Field up to 20 T

The use of wind and react bi2212 wire for high field ( >;20 T) insert coils has been demonstrated to be a promising technology. While cryogenic stability and quench propagation are fundamental issues for the design and safe operation of superconducting magnets there is little data for the high field application. The present work shows data from systematic quench measurements at 4.2 K on two 6-layer 300 mm high bi2212 solenoid coils at different currents I between 50% and 95% of IC in different background fields of 15, 18 and 20 T from a wide bore compact NbTi-Nb3Sn magnet developed by Oxford Instruments. The quench was induced by localized heat pulses (20-100 ms) and recorded with temporal-spatial resolved voltage taps and thermometers. By precise control of the power delivered in the heat pulse, the minimum quench energy MQE and the time constant for the quasi-stationary minimum propagation zone MPZ was obtained for each test condition. MQE was found to follow a scaling law of IC2/I4 in three different coils. The MPZ exhibited the expected anisotropy, extending predominately in the tangential direction and confined in a single winding layer. The propagation was also predominantly along the winding layer with a relatively slow velocity less than 50 cm/s, with an even lower radial propagation velocity of ~ 3 mm/s. In the present study, the interplay between the HTS coils and with the LTS background field were also measured and the results are highly relevant to the design of 20 T plus high field magnets with integrated LTS and HTS coils. The results were analysed in the context of a wide current temperature range for current sharing in the bi2212 wire and the T3 dependence of heat capacity at low temperatures.

Characteristics of AC Loss in Multifilamentary MgB

AC losses of stabilized multifilamentary MgB2 tapes in applied and self magnetic field were studied systematically at different temperatures, self and external ac fields (les0.3 T), and frequency (3-2000 Hz) to assess the contribution of the magnetic sheath and its interplay with the superconducting filaments. The self-field losses were dominated by the hysteretic losses of the ferromagnetic Ni sheath while the contribution from the superconducting filaments was negligible. Although a considerable loss reduction was obtained with the use of NiCr alloy sheath, the resulting losses were still significantly higher than that of the superconductor as the NiCr sheath was weakly magnetic after thermal processing. In applied ac field, a complex loss behavior was found with loss contributions from the magnetic hysteresis of the sheath, the critical state of the superconducting filaments, and the coupling current across the stabilizing matrix inside the filaments. A long time-constant coupling current was greatly enhanced by the co-existence in the inner matrix of a highly magnetic iron buffer layer and a very low resistive stabilizing copper core. Compared to the normal state losses above the critical temperature TC, the overall losses in the superconducting state were increased at low frequencies by the coupling current loss but reduced at high frequencies due to the magnetic shielding by the coupling current.

_{2}

AC loss measurements were carried out on MgB/sub 2/ wire and bulk conductors at temperatures between 25 K and 40 K in applied field up to 0.2 T transverse to conductor length. For bulk conductors in the shape of a flat plate (3 mm /spl times/ 0.5 mm), loss behavior of a thin strip Q/spl prop/H/sub 0//sup 4/ was observed. A reduction of width to 1 mm leads to a change in loss behavior similar to a round wire with Q/spl prop/H/sub 0//sup 3/. For the ex-situ MgB/sub 2/ wire of Ni sheath, losses were totally dominated by the hysteresis of ferromagnetic Ni. While the losses in the superconducting core is greatly reduced by the strong magnetic shielding by the Ni sheath, a small increase in the Ni loss was observed upon superconducting transition in the core. This is attributed to the compression of magnetic flux due to the diamagnetic superconducting core. Such an interpretation is supported by measurements of PbBi2223 tape in a cavity of an iron block.

Tapes

To assess their suitability in enhanced cooling at near ambient temperatures, the pool boiling heat transfer performance and critical heat flux, (CHF), of pentane, methanol and water was measured for a flat, face-up, polished surface. By controlling the vapour pressure the working temperature range of the fluids is extended down to 30degC and up to 180degC. The optimum working temperature range of each fluid was determined for the same condition. Below 80degC the CHF of pentane is the highest at 45W/cm2, from 80degC to 110degC, methanol is highest at 65W/cm2, and above 110degC water with a CHF as high as 260W/m2 at 180degC. The CHF of all three liquids was seen to follow a power law dependence with saturation pressure, and agrees satisfactorily with the Kutateladze/Zuber equation using an adjusted constant K of 1.7 - compared to the original value of 1.3. The higher heat flux is explained by the roughness of the boiling surface condition, which enhances the CHF of all three liquids to the same degree

Measurement of AC losses in MgB/sub 2/ wire and bulk conductors at different temperatures

The prospect of medium/high field superconducting magnets using 2G HTS tapes is approaching to reality with continued enhancement in the performance of these conductors. Direct measurements of 1D adiabatic quench initiation and propagation of a Cu-stabilized 2G conductor have been carried out with spatial-temporal recording of temperature and voltage following the deposition of various local heat pulses to the conductor at different temperatures between 40K and 64K carrying different transport currents. It was found that the stabilizer-free 2G tape maintains the unique characteristics previously measured in non-stabilized tape of increasing minimum propagation zone, (MPZ), with transport current and higher quench energy at lower temperatures. The minimum propagation zone length is investigated at low temperatures and in a background field of ~ 0.1T. The change in MPZ size is discussed in the context of the temperature dependent physical properties including the E-J characteristics. The results add more detail to help understand the unique characteristics of increasing MPZ with transport current and lower temperatures.

Pool boiling study on candidature of pentane, methanol and water for near room temperature cooling

Measurements were carried out on boiling heat transfer to a liquid nitrogen pool due to self-heating of Ag sheathed BiPb2223 tapes with increasing over-currents up to 300 A, focusing on its difference from increasing heat flux in conventional heated surfaces. Large spontaneous oscillations of the surface temperature were observed and attributed to the interplay between the activation/deactivation of nucleate boiling and the highly nonlinear heat generation of the superconductor as a function of temperature. Two distinct steady states of 8 and 3 K superheating were also found for samples carrying 300 A, depending on the current cycle history.

Quench Characteristics of a Cu-Stabilized 2G HTS Conductor

Promising initial results on bulk Mg(B1-xCx)2 prepared with carbon doped boron are presented. Carbon doping is achieved by reaction of ethylene gas on boron powder using a stainless steel tube furnace, a technique suitable for industrial scale processing. The nominal amount of doping was controlled by varying the reaction time with a fixed volume of ethylene gas, and the actual carbon uptake was determined by weight change after the reaction. The amount of carbon substitution x in the Mg(B1-xCx)2 was found using the angular shift in the (100) X-ray reflection. Carbon substitution by the full nominal content in the C doped precursor boron was obtained for doping up to 7.2 at%, as shown by a o-axis compression consistent with that of carbon doped single crystals. The critical current density of the 4 at% C doped sample for temperatures at 20-30 K and fields up to 4 T, relevant to high temperature applications, was significantly higher than those in the published literature. The JC of a 10 wt% nano-SiC doped sample, used as a comparative benchmark, was found to be lower than the C doped sample at field below 2 T, but to reduce slower at higher fields. Structure analysis of the SiC doped sample revealed a coexistence of two C substitution levels of 2.25% at and 5.25% at.