Abstract:A torsional twist strain effect on the critical current of a thin HTS tape has been found to be well described by a longitudinal strain model taking into account the internal shortening compressive strains accompanied with the tensile longitudinal strains due to a torsional twist. The critical current of a twisted tape is given by the integration of the critical current densities corresponding to the strain distribution over the tape cross-section using axial strain data of the tape. The model is supported wit… Show more
“…The torsional strain takes the form (Takayasu et al. 2010) for a position along the tape width , where is the angular rate of change of the winding angle per unit length along the coil trajectory (). Here takes an offset-parabolic form and also peaks at the tape edge (), with a value of .…”
“…Two strain components are possibly severe in non-planar coils made with NI-HTS tape, as illustrated in figure 1. These are the hard-way bending strain () and the torsional strain () (Takayasu, Minervini & Bromberg 2010). Figure 1.Visualization of the strain components considered in the optimization.…”
“…The torsional strain ( tor ) does not depend on the local radius of curvature but instead is related to the angular rate of change of r C along the coil trajectory. The torsional strain takes the form (Takayasu et al 2010) tor…”
The rapidly emerging technology of high-temperature superconductors (HTS) opens new opportunities for the development of non-planar non-insulated HTS magnets. This type of HTS magnet offers attractive features via its simplicity and robustness, and is well suited for modest size steady-state applications such as a mid-scale stellarator. In non-planar coil applications the HTS tape may be subject to severe hard-way bending strain (
$\epsilon _{\textrm {bend}}$
), torsional strains (
$\epsilon _{\textrm {tor}}$
) and magnetic field components transverse to the HTS tape plane (
$B_{\perp }$
), all of which can limit the magnet operating space. A novel method of winding angle optimization is here presented to overcome these limitations for fixed input non-planar coil filamentary geometry. Essentially, this method: (i) calculates the peak
$\epsilon _{\textrm {bend}}$
and
$B_{\perp }$
for arbitrary winding angle along an input coil filamentary trajectory, (ii) defines a cost function including both and then (iii) uses tensioned splines to define a winding angle that reduces
$\epsilon _{\textrm {tor}}$
and optimizes the
$\epsilon _{\textrm {bend}}$
and
$B_{\perp }$
cost function. As strain limits are present even without
$B_{\perp }$
, this optimization is able to provide an assessment of the minimum buildable size of an arbitrary non-planar non-insulating HTS coil. This optimization finds that for standard 4 mm wide HTS tapes the minimum size coils of the existing HSX, NCSX and W7-X stellarator geometries are around 0.3–0.5 m in mean coil radius. Identifying the minimum size provides a path to specify a mid-scale stellarator capable of achieving high-field or high-temperature operation with minimal HTS tape length. For coils larger than this size, strain optimization allows use of wider (higher current capacity) HTS tapes or alternatively permitting a finite (yet tolerable) strain allows reduction of
$B_{\perp }$
. Reduced
$B_{\perp }$
enables a reduction of the HTS tape length required to achieve a given design magnetic field or equivalently an increase in the achievable magnetic field for fixed HTS tape length. The distinct considerations for optimizing a stellarator coilset to further ease compatibility with non-insulated HTS magnets are also discussed, highlighting relaxed curvature limits and the introduction of limits to the allowable torsion.
“…The torsional strain takes the form (Takayasu et al. 2010) for a position along the tape width , where is the angular rate of change of the winding angle per unit length along the coil trajectory (). Here takes an offset-parabolic form and also peaks at the tape edge (), with a value of .…”
“…Two strain components are possibly severe in non-planar coils made with NI-HTS tape, as illustrated in figure 1. These are the hard-way bending strain () and the torsional strain () (Takayasu, Minervini & Bromberg 2010). Figure 1.Visualization of the strain components considered in the optimization.…”
“…The torsional strain ( tor ) does not depend on the local radius of curvature but instead is related to the angular rate of change of r C along the coil trajectory. The torsional strain takes the form (Takayasu et al 2010) tor…”
The rapidly emerging technology of high-temperature superconductors (HTS) opens new opportunities for the development of non-planar non-insulated HTS magnets. This type of HTS magnet offers attractive features via its simplicity and robustness, and is well suited for modest size steady-state applications such as a mid-scale stellarator. In non-planar coil applications the HTS tape may be subject to severe hard-way bending strain (
$\epsilon _{\textrm {bend}}$
), torsional strains (
$\epsilon _{\textrm {tor}}$
) and magnetic field components transverse to the HTS tape plane (
$B_{\perp }$
), all of which can limit the magnet operating space. A novel method of winding angle optimization is here presented to overcome these limitations for fixed input non-planar coil filamentary geometry. Essentially, this method: (i) calculates the peak
$\epsilon _{\textrm {bend}}$
and
$B_{\perp }$
for arbitrary winding angle along an input coil filamentary trajectory, (ii) defines a cost function including both and then (iii) uses tensioned splines to define a winding angle that reduces
$\epsilon _{\textrm {tor}}$
and optimizes the
$\epsilon _{\textrm {bend}}$
and
$B_{\perp }$
cost function. As strain limits are present even without
$B_{\perp }$
, this optimization is able to provide an assessment of the minimum buildable size of an arbitrary non-planar non-insulating HTS coil. This optimization finds that for standard 4 mm wide HTS tapes the minimum size coils of the existing HSX, NCSX and W7-X stellarator geometries are around 0.3–0.5 m in mean coil radius. Identifying the minimum size provides a path to specify a mid-scale stellarator capable of achieving high-field or high-temperature operation with minimal HTS tape length. For coils larger than this size, strain optimization allows use of wider (higher current capacity) HTS tapes or alternatively permitting a finite (yet tolerable) strain allows reduction of
$B_{\perp }$
. Reduced
$B_{\perp }$
enables a reduction of the HTS tape length required to achieve a given design magnetic field or equivalently an increase in the achievable magnetic field for fixed HTS tape length. The distinct considerations for optimizing a stellarator coilset to further ease compatibility with non-insulated HTS magnets are also discussed, highlighting relaxed curvature limits and the introduction of limits to the allowable torsion.
Fiber Bragg grating (FBG) sensors were used to measure the torsional strain distribution in a second-generation (rare-earth) Ba 2 Cu 3 O y high-temperature superconducting sample tape (SST) at 77 K. Two SST samples were prepared. A slit was fabricated in each sample to allow an optical fiber to be embedded. In the first sample, a 10-mm-long single grating element was inserted into the slit. In the second sample, three grating elements of 10-mm length each were put into the slit. These grating elements were fabricated along a single-mode fiber and could be addressed individually using the wavelength division multiplexing (WDM) technique. Finally, the samples were recoated with a primary and secondary coating to form an embedded sensing system. When the high-temperature superconducting (HTS) tape is subjected to torsion, there is a strain induced in the FBG sensor, which varies the grating period accordingly. It was interrogated using a tunable laser in a WDM scheme. Based on the wavelength shift and the photoelastic properties of the FBG sensor embedded to the HTS tape, the torsional deformation of the HTS tape can be obtained. In this paper, the design and technology requirements to embed the single and WDM FBG sensors into the coated conductors are presented. Initial experiments to measure the torsional strain distribution at 77 K are reported.
“…It has led to the tapes in CORC® cables being wound at 45 ° to minimise the suppression of with that type of strain [1,5]. However, there have been comparatively fewer studies on the effect of the other types of strain on [3,[13][14][15][16] and even fewer on the effect of having strains along multiple axes of a tape simultaneously [13,16]. Single crystal data suggest that the in-plane strain dependence of the critical temperature (and of ) is large (along the a-axis and b-axis) but very weak along the c-axis of (RE)BCO [17].…”
Recently, we designed and commissioned a 'crossboard' sample holder which can apply biaxial strains in the plane of a (RE)BCO coated conductor. It allows us to measure the critical current density for arbitrary combinations of x-and y-strain. Understanding the in-field, in-plane, biaxial strain dependence of a tape's ( , , ) is crucial for applications such as CORC® or Roebel cables, as the cables are subjected to multiaxial strains during manufacturing and operation. Here we present experimental data for ( , ,) on a SuperPower SCS4050 APC tape in magnetic fields up to 0.7 T, at 77 K. We also outline a theoretical model for the biaxial strain dependence of and use it to parameterise our data and show that the fraction of A-domains and B-domains are roughly equal ( = 0.49 ± 0.03) and that the strain sensitivity of the critical temperature is 1.8 ± 0.1 K% -1 and -1.3 ± 0.1 K% -1 along their a-and b-axes respectively, for all the domains in this (RE)BCO tape. For the first time, we show both parabolic and linear strain dependencies of in a single tape by changing the angle between the applied strain direction and the twin boundaries in the (RE)BCO layer. Index Terms-Critical current, strain measurement, 2G HTS conductors, cuprates.
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