Paul Noonan

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.

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.

A completely self-contained cryogen-free dilution refrigerator, the TritonDRTM

Oxford Instruments have developed a new dilution refrigerator for ultralow temperatures down to below 35mK. The TritonDRTM system is a continuous cryogenic cycle dilution refrigerator. The refrigerator is driven by a closed cycle cryocooler and hence requires no liquid cryogens. The system has a dedicated electronic control unit and software that provides full control of operation.

Chapter 11 – Cryogen Free Refrigeration to 270mK by Adiabatic Expansion of He3 Gas

Publisher Summary This chapter discusses the cooling performance and other features of two new types of He3 refrigerators that have been built at OIS. These refrigerators are constructed using the well-know phenomena of adiabatic expansion of gas and adsorption pumping. The first type operates in a liquid helium environment at 4.2K. The second design is cryogen free and uses a Pulse Tube Cryocooler. This ULT system is embodied in a commercial He3 Cryofree refrigerator Heliox AC-V, developed by Oxford Instruments. It is important to mention the reliability of the expansion refrigerator. Because current cryocoolers produce cooling down to <3K, this causes an extra liquid fraction by direct condensation into the He3 pot. However this temperature is close to the He3 critical point so any failure in the pulse tube and subsequent deterioration of the minimum temperature achieved by the PTR will completely stop the operation of a traditional He3 refrigerator. For these He3 systems this limit is much higher at 4.2K or even 5K, where adiabatic expansion of He3 still produces liquefaction of He3 and hence ultra low temperatures..