Andrew Twin

Strain response of fibre Bragg grating sensors at cryogenic temperatures

The temperature and strain responses of fibre Bragg grating (FBG) sensors were assessed in comparison with resistive foil gauges over a temperature range of 300 K to 2.2 K. The temperature sensitivity of the FBGs was observed to decrease significantly with decreasing temperature, offering the prospect of temperature insensitive strain measurement in cryogenic environments. The FBG strain response was found to be independent of temperature.

Cryogenic temperature response of fibre optic long period gratings

The thermal response of the attenuation bands of an optical fibre long period grating was monitored over a temperature range of 4.2–280 K. A linear dependence of the central wavelength of the band, of gradient 0.2 nm K−1, was observed over the range 77–280 K. A measurable wavelength shift was observed at temperatures as low as 20 K.

High Field Insert Coils From Bi-2212/Ag Round Wires

Bi-2212/Ag round wire is a promising and practical material for extending high field superconducting magnets beyond the limits of Nb3Sn. Efforts to develop superconducting magnets in the 25 to 30 T range include fabrication and test of practical size insert coils using this wire. Recent studies have focused on improvements in wire performance, wire insulation, and coil fabrication for wind-and-react coils. Continued improvements in the engineering critical current density (JE) and the critical current density (Jc) performance have been achieved by optimizing the starting precursor composition, and the heat treatments. The highest Je of 1580 A/mm2 at 4.2 K, 0 T and 420 A/mm2 at 4.2 K, 31 T were obtained in 0.81 mm wire. In particular, significant progress on braided insulation has been made for enabling a robust procedure for wind-and-react Bi-2212 solenoid coils. Performance of three of these coils has been measured in background fields up to 19 T, showing good prospects for high field magnet application of this conductor.

Present and Future Applications for Advanced Superconducting Materials in High Field Magnets

Advances in high field magnets are driven primarily by the availability of high current density conductors. The restack rod process (RRP), internal Sn superconductors have achieved engineering current densities nearly five times that of bronze route conductors at high fields. Careful utilization of this low temperature superconductor (LTS) enables the production of magnets beyond the previous benchmark of 21 Tesla without an associated increase in magnet and cryostat volume. Steps to realize extremely compact high field magnets for a variety of applications are described. The next significant challenge is to produce magnetic fields beyond 25 Tesla solely using superconducting solenoids. High temperature superconductors (HTS) will be required and, to this end, Bi-2212/Ag matrix wires are at an advanced stage of development. The tangible objective is a new generation of compact, ultra-high field magnets.

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.

Multi-component strain development in superconducting magnet coils monitored using fibre Bragg grating sensors fabricated in highly linearly birefringent fibre

The commissioning of superconducting magnet coils was monitored using embedded optical fibre Bragg grating sensors (FBG) fabricated in highly linearly birefringent (HiBi) fibre. The HiBi FBG sensors monitored the internal strain developed in the coils during the energization of the coils. The development of multiple components of strain in the coils when the magnet was energized and quenched was monitored, revealing phenomena that it had not been previously possible to measure using other sensor technologies.

Development Trends in High Field Magnet Technology

The production of high magnetic fields using low temperature superconductors (LTS) has become common place. However, large magnet sizes and associated high cooling costs have often precluded the full utilization of these research capabilities. Recent advances in internal Sn superconductors and cryogen free technology have opened up a new era in superconducting magnet development. Ultra-compact, laboratory sized magnets producing fields up to 22 Tesla are available. This new class of high field magnet weighs under 200 kg and is suitable for general laboratory installation. In addition, extremely compact, high field, split pair magnets with open access are now operating at the elevated temperature of T = 4.2 K. Beyond conventional wet magnet technology, there is a growing trend to utilize cryogen free technology. Cryogen free magnets do not require liquid Helium baths and, with the addition of active shielding, both the experimental sample access and siting availability is improved. The influence of enabling technologies required to realize the above practical applications for high field, superconducting magnet systems is described.

Simultaneous laser vibrometry on multiple surfaces with a single beam system using range-resolved interferometry

A novel range-resolved interferometric signal processing technique that uses sinusoidal optical frequency modulation is applied to multi-surface vibrometry, demonstrating simultaneous optical measurements of vibrations on two surfaces using a single, collimated laser beam, with a minimum permissible distance of 3.5 cm between surfaces. The current system, using a cost-effective laser diode and a fibre-coupled, downlead insensitive setup, allows an interferometric fringe rate of up to 180 kHz to be resolved with typical displacement noise levels of 8 pm Hz-0.5. In this paper, the system is applied to vibrometry measurements of a table-top cryostat, with concurrent measurements of the optical widow and the sample holder inside. This allows the separation of common-mode vibrations of the whole cryostat from differential vibrations between the window and the sample holder.

Multicomponent strain development in superconducting magnet coils using optical fibre grating sensors fabricated in highly linearly birefringent fibre

The manufacturing and commissioning of superconducting magnet coils was monitored using embedded optical fibre Bragg grating sensors (FBG) fabricated in highly linearly birefringent (HiBi) fibre. Strain developed in the coils was monitored from the resin impregnation and curing process, through cool down of the magnet in a helium cryostat from room temperature to 4 K, to energising the coils. The development of multiple components of strain in the coils when the magnet quenched was monitored, revealing phenomena that it had not been previously possible to measure using other sensor technologies.

Characterisation of a cryostat using simultaneous, single-beam multiple-surface laser vibrometry

A novel range-resolved interferometric signal processing technique that uses sinusoidal optical frequency modulation is applied to multi-surface vibrometry, demonstrating simultaneous optical measurements of vibrations on two surfaces using a single, collimated laser beam, with a minimum permissible distance of 3.5 cm between surfaces. The current system, using a cost-effective laser diode and a fibre-coupled, downlead insensitive setup, allows an interferometric fringe rate of up to 180 kHz to be resolved with typical displacement noise levels of 8 pm · Hz−0 5. In this paper, the system is applied to vibrometry measurements of a table-top cryostat, with concurrent measurements of the optical widow and the sample holder target inside. This allows the separation of common-mode vibrations of the whole cryostat from differential vibrations between the window and the target, allowing any resonances to be identified.