C. Beduz

Quantum rotation of ortho and para-water encapsulated in a fullerene cage

Inelastic neutron scattering, far-infrared spectroscopy, and cryogenic nuclear magnetic resonance are used to investigate the quantized rotation and ortho–para conversion of single water molecules trapped inside closed fullerene cages. The existence of metastable ortho-water molecules is demonstrated, and the interconversion of ortho-and para-water spin isomers is tracked in real time. Our investigation reveals that the ground state of encapsulated ortho water has a lifted degeneracy, associated with symmetry-breaking of the water environment.

THE INFLUENCE OF GEOMETRY ON SELF-FIELD AC LOSSES OF AG SHEATHED PBBI2223 TAPES

We measured axial fields along an Ag sheltered PbBi2223 superconducting tape 3 mm wide and 0.03 mm thick carrying 47 Hz alternating currents. Tte critical current of the tape at 77 K was 1 5 A corresponding to a critical current density of 16 000 A/cm2. The axial electric field was measured by pairs of voltage leads brought out in a plane perpendicular to the tape axis, turned axially to meet and so led out to a voltmeter. The spatial variation and the current dependence of the electric field, obtained from several pairs of taps closed at different radial positions, were consistent with the prediction for a uniform superconducting thin rectangle with a critical current Ic rather than a superconductor of elliptical cross-section. Our results also showed that the hysteretic losses at currents below Ic can be measured with an accuracy of about 5% using a pair of taps closed at a radial distance about three times the tape half-width.

Experimental study of loss mechanisms of AgAu/PbBi-2223 tapes with twisted filaments under perpendicular AC magnetic fields at power frequencies

AC losses under perpendicular AC fields have been measured at 77 K and power frequencies for multifilamentary AgAu (10 wt.%)/Bi-2223 tapes with filaments twisted at different pitches. Using simultaneous measurements of the first and higher harmonics of the voltage induced in the pick-up coil, the main loss contributions (superconductor and coupling current losses) have been obtained separately. At power frequencies, twisting produces the desired uncoupling of the filaments at fields lower than the coupling field, which has also been determined experimentally. In the uncoupled-filament regime, the superconductor losses are reduced strongly with respect to the untwisted tapes. The reduction of the total loss with twisting is also observed. However, due to the important contribution of the coupling current losses for this field orientation, a very small pitch (<5 mm) is necessary for a considerably lower loss than that of untwisted tapes. The dependence of the coupling field and coupling current losses on the twist pitch has been analysed and compared with the theoretical predictions.

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.

Irreversibility and hysteretic aspects of granular YBCO

Abstract Data showing the dependence of the transport critical current of polycrystalline YBCO on applied magnetic field are presented. At low applied fields the critical current versus applied field path is reversible. The large hysteretic behaviour of the critical current observed at higher fields in field cooled and zero field cooled cases is explained in terms of the local intergranular field. Further data are presented emphasizing the relationship between the local field and the applied field.

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.

Experimental study on AC losses in Ag sheathed PbBi2223 tapes with twist filaments

Abstract Experimental measurements of AC losses were carried out on Ag sheathed PbBi2223 tapes with twisted and untwisted filaments. Losses were measured at 77 K as function of frequency and magnetic field parallel and perpendicular to the tape surface, using appropriate pick-up loops. Both the first and third harmonics of the signal were measured, in order to distinguish between the hysteresis loss and other types of loss. The effect of filaments uncoupling by twisting was clearly identified. For a tape with a twist pitch of 10 mm and Ic=40 A (∼20 kA cm−2) operating at 43 Hz, the filaments are uncoupled in fields less than 40 mT, which is greater than the full penetration field for both the filaments and the tape. Hence, a reduction in the hysteretic loss of the superconducting core is realised at power frequency between 10 and 40 mT. Results form the self-field loss measurement implies the uncoupling of twisted filaments at relative low transport current (I