Wendell Bailey

Performance Test of a 100 kW HTS Generator Operating at 67 K–77 K

A systematic test program is in progress to fully characterize a 100 kW HTS synchronous generator which was successfully constructed in 2004. The machine was one of the first HTS synchronous generator/motors to operate at liquid nitrogen temperatures while achieving a power rating relevant to practical application. It has a conventional 3-phase stator and a cold rotor with a magnetic core and a superconducting winding consisting of 10 HTS Bi2223 pancake coils separated by magnetic flux diverters. The test program includes a series of tests at various speeds, field currents and temperatures (65 K-77 K) with the machine in open circuit to determine the critical currents of the HTS rotor, the waveform and harmonic characteristics of generated voltage at different levels of iron saturation. Stationary measurements of the rotor critical current are carried out using dc current in the stator windings to quantify the influence of stator field on the performance of the superconducting winding. The voltages and temperatures of the rotor are measured using a radio frequency telemetry system.

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.

The Design of a Lightweight HTS Synchronous Generator Cooled by Subcooled Liquid Nitrogen

A final design of a dasiacorelesspsila 100 kW HTS synchronous generator, to be built at the University of Southampton is under way. The new generator will use the same conventional 2-pole, 3 phase stator used by for the dasiairon coredpsila generator. The new HTS rotor has no central core and the rotor winding is built with 15 double pancake coils. Each coil has 38 turns of BiPb2223 superconducting tape, with nominal current of 180 A at 77 K. In this design, only two flux diverters are used at the top and bottom of the winding to help shape the magnetic field. The coils are located on the inner wall of the cryostat. The construction of the cryostat poses our greatest mechanical challenge. Although we have avoided problems with differential thermal contractions by making the cryostat entirely from stainless steel, the cryostat requires considerable reinforcements. Additional supports and stiffening ribs must be welded to the thin-walled structure. Distortion/rippling during welding must be eliminated to ensure the final welds to the ring flanges can be made. The pole-pieces, which help to improve the waveform characteristics of the machine, are located in the warm space between the cold flange of the cryostat and the inner wall of the vacuum vessel. Unfortunately, the overall magnetic performance has been affected by the parallel stacking of the coils. Some suggestions for altering the current stator have been presented which show that improvements to the voltage waveforms can be achieved.

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

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

Boiling heat transfer to a liquid nitrogen pool from Ag sheathed BiPb2223 tapes carrying over-current

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.

Tm:fiber laser in-band pumping a cryogenically-cooled Ho:YAG laser

Cryogenically-cooled diode-pumped lasers have received significant interest in recent years for their demonstrated orders of magnitude improvement in output radiance using simple laser resonator configurations, with respect to their room temperature counterparts. Here we present a technique that offers the potential for a further order-of-magnitude radiance improvement utilising the in-band pumping hybrid-laser architecture, which employs high-power fiber lasers to excite cryogenically-cooled bulk gain media. The ability to exploit the quasi-four-level nature of a two-level laser system at very cold temperatures enables the operation of very low quantum defect transitions, thus providing reduction in the required thermal dissipation per unit power for the in-band pumped Ho:YAG laser, compared to diode-pumped Yb:YAG. Preliminary results will be discussed for a narrow linewidth Tm:fiber laser system operating in the 100W regime, pumping a cryogenically cooled Ho:YAG gain element, and employing a simple cavity configuration. Low quantum defect operation and power-scaling potential will be discussed.

Development of a coreless HTS synchronous generator operating at sub-cooled liquid nitrogen temperatures

This paper presents the current design concepts for a 100kW high temperature superconducting synchronous generator currently being designed at the University of Southampton, UK. The new generator will use the same conventional 2-pole 3-phase stator that was used in the HTS synchronous generator previously constructed at Southampton. The windings consist of 18 pancake coils made from BiPb2223 superconducting tape with a nominal current of 180A at 77K, provided by Sumitomo Electric Industries, Ltd. Tests were performed to determine the mechanical strain limits of the tape. The rotor has no central core, but magnetic pole pieces are used to improve the waveform of the generator. The coils are separated by magnetic diverter rings to reduce the normal field in the tape. The coils and diverter rings are supported by fibreglass formers that extend to the centre of the rotor so that they can be located by the through bolts that hold the stack together. A stainless steel tube encapsulates the rotor to provide a chamber for liquid nitrogen to flood the rotor.

Transport IV characterisation of MgB2 conductor at a bend radius of 50mm

Performance of state of the art MgB2 multifilamentary conductor at a required bend radius is essential for many applications including but not limited to magnets and motors. The characterisation is generally done with benchmark transport Ic but further detail can be seen in IV characteristics which are undertaken in this paper. Two conductors with the same architecture but different diameters, 0.89 and 0.45 mm were measured from 32 K to 20 K in self-field in conditions of as received and deformed to a 50 mm bend diameter, corresponding to strains of 1.4 % and 0.7 % respectively. The qualifying 0.45mm conductor was further measured in background fields up to 3 T. The smaller diameter wire was found to have no signs of degradation of critical behaviour in Ic or IV characteristics.

Cryogenically-cooled Ho:YAG laser in-band pumped by a Tm fibre laser

Scaling of laser power and brightness to meet the needs of ever-demanding applications is a demanding task which continues to preoccupy many within the laser community. In conventional “bulk” solid-state lasers the main obstacle is heat generation in the laser medium and its associated detrimental effects. Methods for combating these problems have been the focus of much research, resulting in many novel laser geometries with improved thermal management and reduced thermal lensing, but often at the expense of increased complexity and reduced flexibility. An alternative approach, which is beginning to attract a great deal of interest, is to operate the laser with the laser medium maintained at cryogenic temperatures (∼77 K), where the effects of heat loading are dramatically reduced due to a large increase in thermal conductivity and a large decrease in the temperature coefficient of refractive index (dn/dT) and expansion coefficient [1]. In host materials such as YAG the net reduction in thermal effects can be over 50 times compared to operation at room temperature. In the case of diode-pumped Yb:YAG lasers, the combined effect of a massive reduction in thermo-optic aberrations and lower re-absorption loss has allowed very impressive results to be achieved in terms of output power and beam quality from relatively simple laser resonator configurations [2]. In this paper we report on preliminary work ultimately aimed at achieving a further reduction in thermal effects by combining the advantages of cryogenic cooling with a very low quantum defect fibre-laser-pumping of bulk solid-state lasers. Here we describe preliminary results for a cryogenically-cooled Ho:YAG laser in-band pumped by a high-power Tm-doped silica fibre laser.

Temperature-dependent analytical thermal model for end-pumped solid-state lasers

Analytical expressions for the temperature distribution and thermal-lens power in end-pumped solid-state lasers are reported. Enabled by including a temperature-dependent thermal conductivity, applicable from cryogenic to elevated temperatures, these prove insightful for practical systems.