M.K. Al-Mosawi

Construction of a 100 kVA high temperature superconducting synchronous generator

This paper reports on the design and construction of the 100 kVA High Temperature Superconducting generator which is successfully built at the University of Southampton. The generator is a 2-pole synchronous machine with a conventional 3-phase stator and a HTS rotor which is constructed from stacks of 9 wt% Ni steel plates. The rotor winding is made of ten single vacuum impregnated pancake coils connected in series and operated in the temperature range 73-77 K. The relatively high operating temperatures are made possible by the use of metallic core as well as flux diverters between the coils. These coils have been successfully manufactured and tested over several cooling cycles. These provide the necessary mechanical support whilst minimizing the heat intake to the rotor core. The coils generate an air gap flux density of about 0.6 T at 77 K, while producing a perpendicular magnetic field of only 0.038 T to board face of the superconducting tape. Furthermore, the design and construction of two novel fiberglass torque tubes is also presented.

Design of a 100 kVA high temperature superconducting demonstration synchronous generator

The paper presents the main features of a 100 kVA high temperature superconducting (HTS) demonstrator generator, which is designed and being built at the University of Southampton. The generator is a 2-pole synchronous machine with a conventional 3-phase stator and a HTS rotor operating in the temperature range 57–77 K using either liquid nitrogen down to 65 K or liquid air down to 57 K. Liquid air has not been used before in the refrigeration of HTS devices but has recently been commercialised by BOC as a safe alternative to nitrogen for use in freezing of food. The generator will use an existing stator with a bore of 330 mm. The rotor is designed with a magnetic core (invar) to reduce the magnetising current and the field in the coils. For ease of manufacture, a hybrid salient pole construction is used, and the superconducting winding consists of twelve 50-turn identical flat coils. Magnetic invar rings will be used between adjacent HTS coils of the winding to divert the normal component of the magnetic field away from the Bi2223 superconducting tapes. To avoid excessive eddy-current losses in the rotor pole faces, a cold copper screen will be placed around the rotor core to exclude ac magnetic fields.

The effect of flux diverters on AC losses of a 10 kVA high temperature superconducting demonstrator transformer

In this paper the design, construction and characterization of a 50 Hz single phase high temperature superconducting (HTS) transformer is presented. The transformer is designed to deliver 10 kVA at 77.4 K and 20 kVA at 65 K. The transformer is made of two windings positioned at the middle limb of a three-limb iron core. The secondary winding is made of nine double pancake superconducting coils, using silver sheathed BPSCCO-2223 multifilamentary tapes with nominal critical current of 20 A (at 77.4 K and self field). The primary winding is made of copper to reduce the cost of HTS material. Utilizing a superinsulated nonmetallic doughnut shaped cryostat with low background heat leak, the AC losses in the superconducting winding have been measured by calorimetric method. Two powdered iron flux diverters have been used inside the cryostat at both ends of the superconducting winding in order to reduce the radial component of leakage flux density in the HTS tapes. The use of these flux diverters is shown to be very effective in reducing the AC losses by 40% at the nominal current.

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.

An NMR thermometer for cryogenic magic-angle spinning NMR: the spin-lattice relaxation of 127I in cesium iodide

The accurate temperature measurement of solid samples under magic-angle spinning (MAS) is difficult in the cryogenic regime. It has been demonstrated by Thurber et al. (J. Magn. Reson., 196 (2009) 84-87) [10] that the temperature dependent spin-lattice relaxation time constant of (79)Br in KBr powder can be useful for measuring sample temperature under MAS over a wide temperature range (20-296 K). However the value of T(1) exceeds 3 min at temperatures below 20K, which is inconveniently long. In this communication, we show that the spin-lattice relaxation time constant of (127)I in CsI powder can be used to accurately measure sample temperature under MAS within a reasonable experimental time down to 10 K.

Coreless HTS Synchronous Generator Operating at Liquid Nitrogen Temperatures

In this paper we report on our new design of a liquid nitrogen/air cooled 100 KW synchronous generator with core-less rotor. This follows our successful completion of 100 KW generators with a 9 wt% Ni steel core operating at 77 K. In the new design, we demonstrate that a coreless rotor using commercial BSCCO tape is a realistic choice while maintaining the cooling at 57-77 K rather than 25-30 K and still achieving reasonable air-gap flux density. This is made possible by a combination of improved HTS wire technology and a careful optimization of HTS winding and flux diverters. The maximum flux density normal to the broad face of the tape is kept below 0.13 T, even when the air-gap density is raised to 0.5 T. The superconducting winding consists of 22 pancake coils with a total length of around 1250 meters of HTS tape. The required temperatures down to 57 K are achieved by a purpose-designed refrigeration system where liquid cryogen is circulated via a network of well insulated pipes and a stationery-rotating liquid coupling junction.

The synthesis and characterization of (Tl0.6Pb0.2Bi0.2) (Sr1.8Ba0.2)Ca2Cu3O9+x powder and Ag-sheathed tape

We have prepared a single-phase superconducting Tl-1223 powder with Tc above 110 K and used this promising material to produce Ag-sheathed tape. The powder was synthesized by partially substituting Pb and Bi for Tl, and substituting Sr for Ba in a stoichiometry (Tl0.6Pb0.2Bi0.2)(Sr1.8Ba0.2)Ca2Cu3O9+x suggested by Liu et al. The accelerated formation and stabilization of the Tl-1223 phase was observed. The non-agglomerated powder was found to have a homogeneous platelike morphology with average grain size 4.5 mu m and maximum grain size 18 mu m. The intragrain critical current density was determined to be 6.3*107 A cm-2 at 5 K, 1 T, and 6.5*104 A cm-2 at 77 K, 1 T. Ag-sheathed tapes were fabricated with this powder and magnetic and transport critical current densities of as-rolled tapes and heat-treated tapes were measured. A transport critical current density of 5.6*103 A cm-2 was obtained at 77 K, O T. The relationship between processing conditions, microstructure, and superconducting properties of the tape were also investigated.

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.

Effects of the time sequence of the cold rolling on the processing time of Ag-alloy sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes

Abstract The effect of the time sequence of cold working/sintering on the transport J c has been studied. The results on the variation of J c with sintering time between the first and second intermediate cold rolling, t s2 , are presented. We have found that J c peaks sharply at an optimum t s2 . For example, tapes with a total sintering time of 140 h reach an optimum J c of 2.1 × 10 4 A cm −2 (77 K, self-field) at t s2 = 27 h . In general, there is a narrow window of optimisation in t s2 t s2 ; in this examples, J c drops by 50% within 7 h of the optimised value. We also found that the optimum total singering time ( t tot ) for a tape increases gradually with t s2 e.g. t tot t s2 = 22 h and t tot > 180 h for t s2 = 35 h . X-ray diffraction analysis (XRD) was used to correlate the phase conversion prior to the second intermediate rolling with the J c . The results suggest that the total processing time can be adjusted by t s2 . This has important consequences on reducing the long processing time of BSCCO tapes.

Full Cryogenic Test of 600 A HTS Hybrid Current Leads for the LHC

For full cryogenic test of CERN 600 A high temperature superconducting (HTS) current leads prior to integration into the Large Hardron collider (LHC), a dedicated facility has been designed, constructed and operated at the University of Southampton. The facility consists of purpose-built test cryostats, 20 K helium gas supply, helium gas flow and temperature control systems and quench protection system. Over 400 such leads have already been successfully tested and qualified for installation at CERN. This paper describes various design and operation aspects of the test facility and presents the detailed cryogenic test results of the CERN 600 A current leads, including steady state 20 K flow rates.