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Staff Profile

Mr Jack Greenwood, MPhys

Postgraduate Student in the Department of Physics
Postgraduate Student in the Centre for Materials Physics

(email at jack.r.greenwood@durham.ac.uk)

Biography

After achieving a first class degree in Physics (MPhys, Hons) from the University of Durham in 2016 and gaining experience working in the European Fusion Reference Laboratory here at Durham I have embarked upon an experimental PhD under the supervision of Professors Damian Hampshire and Ray Sharples. I am a member of the Department’s Superconductivity Group and the EPSRC Centre for Doctoral Training in the Science and Technology of Fusion Energy, based in York (http://www.fusion-cdt.ac.uk/). The Fusion CDT is an institution that pulls together world class expertise in the field of nuclear fusion, with an emphasis on research into designing fusion reactors for research and eventually, power generation. The CDT consists of collaborations between 5 major UK universities: The University of York, University of Durham, University of Oxford, University of Liverpool and University of Manchester, as well as other key partners in the fusion sector such as the Culham Centre for Fusion Energy (CCFE), at which the Joint European Torus (JET) is housed. 

Research Interests

Figure 1: A typical schematic of a commercial high temperature superconductor, in a coated conductor form. The superconducting layer is shown in black. This coated conductor is manufactured by SuperPower http://www.superpower-inc.com/content/2g-hts-wire.

Introduction

High temperature superconductors have the potential to replace traditional low temperature superconductors in future applications such as magnetic confinement fusion (MCF), magnetic resonance imaging (MRI), particle accelerators, magnetic levitation and power transmission. They have much higher values of upper critical magnetic field (Bc2), critical temperature (TC) and critical current density (JC). Many commercial high temperature superconductors are fabricated as ‘coated conductors’ which contain a micron thick layer of superconducting (RE)BaxCuyOz, with (RE) being a rare earth element such as Yttrium or Gadolinium.

In high field magnet systems such as those used for MCF, a superconductor is subjected to large Lorentz type forces, due to the high magnetic fields and large currents that are present. If coated conductors are used, the Lorentz forces induce strains in the superconducting (RE)BaxCuyOz layer and the presence of strain (ε) causes the superconductor’s values of Bc2, TC and JC to change. The main focus of my work is to determine the mechanisms responsible for the strain dependence of JC in coated conductors, which will allow manufacturers to optimise their fabrication processes and engineers to improve their magnet designs. The scope of the work is summarised below.

Figure 2: The bending beam apparatus used to apply uniaxial strains to a coated conductor [1].

Experimental Apparatus & Facilities

Bending beams (springboards) are used to apply both compressive and tensile uniaxial strains to (RE)BaxCuyOz coated conductors along the direction of current flow. We can apply strains ranging from -1.2% ≤ ε ≤ 0.6%. The bending beam is attached to a bespoke probe which can be used in our world class, 15 T horizontal magnet system. Using the horizontal magnet system we are able to investigate the angular dependence of JC also. Critical currents are measured using a standard 4-terminal technique. We have also developed a variable temperature cup which can be used to measure JC for temperatures ranging from 4.2 K to 90 K.

Recently, we have developed a biaxial sample holder which can be used to apply biaxial strains to a (RE)BaxCuyOz coated conductor. Some early JC(εxεy) results are discussed below. 

Figure 3 (L): The uniaxial strain dependence of Jc for a (RE)BaxCuyOz coated conductor, normalised to the peak value of JC [2]. Figure 4 (R): The biaxial sample holder, known as the ‘crossboard’, which is used to apply biaxial strains to (RE)B

Key Results

It is well established that there is a parabolic relationship between JC and applied uniaxial strain ε [2]. The peak in JC(ε) may occur at a non-zero value of applied strain. The standard explanation for the location of the peak is that it is influenced by the thermal strains that the (RE)BaxCuyOz layer is subjected to upon a temperature change. This is due to the fact that the different layers within the coated conductors have different coefficients of thermal expansion. It is argued that the maximum value of JC occurs when the (RE)BaxCuyOz layer has zero net strain [3].

However, we have shown that when we apply an additional compressive strain in the direction orthogonal to current flow, as well as a strain in the direction of current flow, the peak position of JC(ε) changes from to compressive strain to a tensile strain and the value of JC at the peak increases by ~11%. These results cannot be explained by considering the thermal strains induced in the (RE)BaxCuyOz layer alone. A paper has been submitted to the EuCAS2017 special issue of IEEE Transactions on Applied Superconductivity and is due for publication in June 2018 [4].

Future Work

I am currently working on:

  • Designing a bespoke probe that can be used to investigate the biaxial strain dependence of JC in our 15 T horizontal magnet system.
  • Improving the range of strains that the biaxial sample holder can apply to a coated conductor.
  • Relating the biaxial strain dependence of J­C to strain measurements on single crystals of (RE)BaxCuyOz.

 

References

[1]Sunwong, P., Higgins, J.S. & Hampshire, D.P. (2014). Probes for investigating the effect of magnetic field, field orientation, temperature and strain on the critical current density of anisotropic high-temperature superconducting tapes in a split-pair 15 T horizontal magnetReview of Scientific Instruments 85(6): 065111.

[2] Sunwong, P., Higgins, J.S., Tsui, Y., Raine, M.J. & Hampshire, D.P. (2013). The critical current density of grain boundary channels in polycrystalline HTS and LTS superconductors in magnetic fieldsSuperconductor Science and Technology 26(9): 095006

[3] Osamura, K., Machiya, S. & Hampshire, D. P. (2016). Mechanism for the uniaxial strain dependence of the critical current in practical REBCO tapesSuperconductor Science and Technology 29(6): 065019.

[4] ­­Greenwood, J.R, Surrey, E. and Hampshire, D.P. Biaxial Strain Measurements of JC on a (RE)BCO Coated Conductor. Submitted to IEEE Transactions on Applied Superconductivity, scheduled for June 2018 (vol. 28, issue 4).

Presentations

Poster Presentations
  • 13th European Conference on Applied Superconductivity, 17th - 21st September 2017, Geneva, Switzerland

  • Culham PhD Showcase, 11th - 12th July 2017, Oxford, UK

  • Joint CDT Nuclear Energy Event, 24th May 2017, York, UK;

  • Fusion Frontiers and Interfaces Workshop, 8th - 10th May 2017, York, UK

Oral Presentations
  • Applied Superconductivity for Fusion Technology, 21st March 2017, Oxford, UK

Is supervised by