High-temperature Superconductor Technology Research Articles

Characterizing anomalous peaks in the resistance–temperature profile of Bi2Sr2CaCu2O8+δ flakes featuring surface degeneration

Abstract This study focuses on an observed anomalous resistance peak in the temperature-dependent resistance (RT) curves of Bi2Sr2CaCu2O8+δ (BSCCO), attributed to surface degradation and pronounced electrical resistance anisotropy. Employing a standard four-point probe technique on the ab-plane, this research circumvents conventional c-axis testing limitations, enhancing the understanding of BSCCO’s electrical behavior by avoiding contact resistance and etching issues. A comprehensive three-dimensional model, developed using the finite element method, captures the strong resistive anisotropy and correlates the depth of surface degradation with the anomalous resistance peaks, explaining this phenomenon from a quantitative perspective, providing a more specific reference for future analysis of relevant signals. The fabrication process involved pre-patterning and mechanical exfoliation techniques to minimize atmospheric exposure and ensure device integrity. Despite these efforts, surface degradation impacting the superconductivity of surface layers was inevitable. The study’s experimental results, complemented by numerical modeling, reveal the intricate relationship between surface layer thickness and the anomalous resistance peak, providing an approach to gauge the extent of degradation in BSCCO devices. Moreover, it underscores the potential necessity of employing some critical techniques to avoid degradation, such as low-temperature exfoliation in other literatures where degradation signal is notably absent from RT curves. This work advances the understanding of BSCCO’s electrical properties and highlights the critical need for precise fabrication and environmental controls in developing high-temperature superconducting technologies.

Development of a superconducting undulator cryostat based on the thermosiphon effect

As a specific device for light production, undulators have been researched and developed since the third generation of synchrotron photon sources. Nowadays, superconducting undulators become a research hotspot. However, the cryostat, which is used to create a liquid helium temperature environment, often causes the failure of the superconducting undulator. In this paper, a cryostat for a superconducting undulator is designed and investigated. First, a new refrigeration distribution is proposed that can provide more excess cooling capacity at 4.2 K temperature. For the cooling of the superconducting magnet, a liquid helium circulation loop based on the thermosiphon effect is designed, which has no pump or any other moving parts. Next, based on high-temperature superconducting technology, six binary current leads are used to decrease the heat load. The beam chamber is cooled below 20 K to reduce possible effect on the magnet. In addition, for the tubes that connect the 4.2 K helium tank to the room temperature component, the thermoacoustic oscillation is studied. In the experiment, the superconducting magnet could be cooled and maintained at 4.2 K depending on the thermosiphon loop. No helium was discharged when the magnet went through a quench which generated large heat in a short time. There was no liquid helium consumption and the excess cooling capacity reached 2.2 W. The maximum magnet current is as high as 470 A. This study can provide a valuable reference for the development of superconducting undulator cryostats.

Fabrication of neutron absorbing metal hydride entrained ceramic matrix shield composites

With significant improvement in High Temperature Superconductors (HTS), several projects are adopting HTS technology for fusion power systems. Compact HTS tokamaks offer potential advantages including lower plant costs, enhanced plasma control, and ultimately lower cost of electricity. However, as compact reactors have a reduced radial build to accommodate shielding, HTS degradation due to radiation damage or heating is a significant and potentially design limiting issue. Shielding must mitigate threats to the superconducting coils: neutron cascade damage, heat deposition and potentially organic insulator damage due x-rays. Unfortunately, there are currently no hi-performance shielding materials to enable the potential performance enhancement offered by HTS. In this work, we present a manufacturing method to fabricate a new class of composite shields that are high performance, high operating temperature, and simultaneously neutron absorbing and neutron moderating. The composite design consists of an entrained metal-hydride phase within a radiation stable MgO ceramic host matrix. We discuss the fabrication, characterization, and thermophysical performance data for a series of down-selected composite materials inspired by future fusion core designs and their operational performance metrics. To our knowledge these materials represent the first ceramic composite shield materials containing significant metal hydrides.

AC Loss Reduction in HTS Coil Windings Coupled With an Iron Core

Rapid-cycling synchrotron (RCS) magnets based on a superferric design consist of high temperature superconducting (HTS) coil windings coupled with iron cores. However, the presence of iron cores significantly increases AC loss in HTS coil windings, making AC loss reduction a critical issue for applying HTS technology in RCSs. Enlarging the distance between iron cores and coil windings may reduce AC loss. In addition, magnetic materials with different saturation magnetic fields also may influence AC loss in HTS coil windings coupled with iron cores. To investigate the distance dependence, AC losses of 1DPC (double pancake coil)-, 2DPC-, 4DPC-, and 8DPC assemblies with various distances are simulated using the 3D <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> - <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> homogenization method with/without an iron core. Two magnetic materials with different saturation magnetic fields are chosen as the iron core to evaluate AC loss in HTS coil assemblies. The results show that the AC loss values in HTS coil assemblies coupled with the iron core decrease significantly with growing distance. For a given distance and current, when the iron core has a lower saturation field, the AC loss values of the 4DPC assembly are smaller than those with a higher saturation field.

High Temperature Superconductivity Application Readiness Map - Energy Delivery - Transmission, Substation and Distribution

The International Energy Agency's High Temperature Superconductivity Technology Collaboration Program (IEA HTS TCP) analyzed energy delivery applications for HTS. The product was a Readiness Map that illustrates the Technology Readiness Levels (TRL) over time of HTS applications. Assessing these TRL levels is important since the electric power system that supplies our residential, commercial and industrial sectors is experiencing a transition to safer, more efficient and cleaner power supply. The international experts assessed the present degree of technological development of the main transmission, substation and distribution HTS applications in the energy delivery sector and estimated the pathway to commercialization phase. Instead of using a specific TRL number 1-9 for each of the applications, they are categorized into low, medium and high TRL levels. Superconducting fault current limiters and medium voltage AC cables to interconnect substations at the low side of the transformer are currently at a high TRL as they can be purchased in the market. High voltage AC and DC cables are at a medium TRL and could reach a high level TRL level in 2030-2035. Superconducting transformers are at a low TRL and they could reach a high TRL level by 2030-2035.

Miniaturized Quadband High-Temperature Superconducting Bandpass Filter with a Wide Frequency Ratio

This paper presents a miniaturized quadband high-temperature superconducting (HTS) bandpass filter based on a multistub-loaded resonator. Owing to the symmetrical structure of the proposed resonator, the odd- and even-mode analysis method can be used to derive its resonance frequencies. First, the frequency ratio between passbands may be significantly changed by adjusting the impedance ratio and fold spacing of the resonator. Moreover, the proposed filter exhibits well-controlled center frequencies and bandwidths for the four passbands, resulting in a considerably enhanced design possibility of quadband bandpass filters. A low insertion loss within each band is obtained using HTS technology. Additionally, the proposed filter was etched on an MgO substrate with a thickness of 0.5 mm and combined with YBCO thin films for demonstration. The results obtained were highly consistent with those of the simulation, validating the effectiveness and viability of the proposed method.

A High-Temperature Superconducting Bandpass Dual-Mode Filter with Tunable Relative Bandwidth and Center Frequency.

This study proposes a high-temperature superconducting (HTS) bandpass filter with a continuously tunable bandwidth and center frequency. The proposed filter combines several gallium arsenide varactors and a dual-mode resonator (DMR). The even and odd modes of the DMR can be tuned simultaneously using a single bias voltage. The capacitive value of varactors in the circuit is tuned continuously under continuous voltage and frequency tunability. External couplings and the interstage can be realized using an interdigital coupling structure; a fixed capacitor is added to the feeder to improve its coupling strength. A low-insertion loss within the band is obtained using HTS technology. Additionally, the proposed filter is etched on a 0.5 mm-thick MgO substrate and combined with YBCO thin films for demonstration. For the as-fabricated device, the tuning frequency range of 1.22~1.34 GHz was 9.4%; the 3-dB fractional bandwidth was 12.95~17.39%, and the insertion loss was 2.28~3.59 dB. The simulation and experimental measurement results were highly consistent.

Genetic algorithms molecular effect model optimization computational method for high temperature superconductors

Genetic algorithms software was applied in 3D Graphical and Interior Optimization methods for two High Temperature Superconductors (HTSCs) classes. Namely, Tin (Sn) class with [TC > 0°] and Thallium (Tl) one subject to [TC ˂ 0°, TC > 0°] in Molecular Effect Model (MEM). Results comprise Tikhonov Regularization Functional mathematical algorithms for these HTSCs group without using logarithmic changes. Results also show the contrasts between these two classes for Molecular Effect Model (MEM) hypothesis. Solutions show a series of 2D/3D imaging process charts complemented with a group of numerical results. Electronics Physics applications for Superconductors and High Temperature Superconductors and Medical Technology are specified for MEM and presented.

News From Japan

In the “News from Japan” column in the November/December 2022 issue, the development of a 6-kV/3000-A triaxial high-temperature superconducting (HTSC) cable as well as its verification test conducted at a site of a chemical company was reported [1]. As another Japanese example of the HTSC technology, the following reports the development of a HTSC magnet with the highest trapped field of 17.89 T. This type of HTSC magnet is expected to be usable for a tabletop nuclear magnetic resonance (NMR) spectrometer. Its future applications are also anticipated for field magnets of motors for electric mobility systems such as electric airplanes and magnetic levitation bearings. The magnet can retain a practically acceptable high magnetic field level as long as a sufficiently low cryogenic temperature is maintained, although its field intensity gradually decays slowly after the magnetization.

Study the Impact of Magnetic Field on Dosimetry of Proton Therapy Using Monte Carlo Simulation

<h3>Purpose/Objective(s)</h3> Using Monte-Carlo simulation to study the dosimetry of proton therapy under the environment of magnetic field. The results of this study will provide knowledge for us to design and build the MRI system that is integrated to proton therapy to form an MRI-guided proton therapy system using our new developed medium temperature superconductors (MTS) and high temperature superconductors (HTS). <h3>Materials/Methods</h3> We performed Monte Carlo calculations using version 3.26 of the PHITS code to simulate and investigate various proposed configurations of MRI-guided proton therapy. The first thing to study is the dosimetry under the environment of magnetic field. In simulation we used a cylindrical water phantom 60 cm in diameter and 2 m in length to imitate a patient and irradiated it with the proton beam with an energy of 250 ± 5 MeV under the environment of a dipole magnetic field. We started the study first with simple and uniform magnetic fields, and then with a more realistic magnetic field that is close to or similar to what we are going to design: 1) simple and uniform magnetic fields with different strength varies from 0.5 to 3.5 Tesla along X, Y or Z direction in respectively; 2) a more realistic magnetic field of a designed magnet using MTS and HTS technology. The track of particles and the dose distribution were investigated. <h3>Results</h3> For magnetic field in only X or Y direction (Bx≠0 or By≠0), the proton beam was bent and the coordinates of the maximum absorbed dose (Bragg peak) position shifted. The displacement of Bragg peak position increases with the magnetic field strength. For magnetic field along Z direction (Bz≠0, Bx=By=0), the change in Bragg peak position and the absorbed dose are small. For the magnetic field using MTS and HTS technology and the magnet was aligned along the beam direction, our simulation shows no significant change in Bragg peak position as well as the absorbed dose such as the maximum dose D_max. <h3>Conclusion</h3> Using Monte Carlo simulation to study the dosimetry of the proton beam interacting with medium (water) under the environment of magnetic fields with different directions and various strengths allowed us to understand the trajectories of proton beam and other secondary particles as well as the impact to the absorbed dose, which provide us important information to assist the design of MRI-guided proton therapy system.

Test and Analysis of Laboratory-Scale D-Shaped Co-Wound No-Insulation HTS Single Pancake Coil for TF Coil Application

No-insulation (NI) high temperature superconductor (HTS) toroidal field (TF) coil or its variations have been regarded as potential options for a compact, high magnetic field tokamak due to compactness in size and robustness in operational reliability. However, its real application still faces multiple challenges, e.g., limited experience in the design, construction, and operation of “D-shaped” NI HTS coils for toroid magnets. Here we report, as the first attempt in Korea, fabrication, test, and analysis of a reduced size D-shaped NI HTS single pancake coil to explore the feasibility of NI HTS technology in TF coil. The D-shaped NI HTS coil is designed to be 1/20 of the Korea Superconducting Tokamak Advanced Research (KSTAR) TF coil in height. One of the three test coils is wound with single HTS tape, and the other two test coils are co-wound with double and triple tapes, respectively. Current charging/discharging tests and sudden discharge tests were conducted for each coil, and the results are analyzed by the well-known lumped circuit model of a typical NI HTS coil. The results show possible variations in the contact resistance during charging and sudden discharge within a coil.

Highly Regular Hexagonally-Arranged Nanostructures on Ni-W Alloy Tapes upon Irradiation with Ultrashort UV Laser Pulses.

Nickel tungsten alloy tapes (Ni—5 at% W, 10 mm wide, 80 µm thick, biaxially textured) used in second-generation high temperature superconductor (2G-HTS) technology were laser-processed in air with ultraviolet ps-laser pulses (355 nm wavelength, 300 ps pulse duration, 250–800 kHz pulse repetition frequency). By employing optimized surface scan-processing strategies, various laser-generated periodic surface structures were generated on the tapes. Particularly, distinct surface microstructures and nanostructures were formed. These included sub-wavelength-sized highly-regular hexagonally-arranged nano-protrusions, wavelength-sized line-grating-like laser-induced periodic surface structures (LIPSS, ripples), and larger irregular pyramidal microstructures. The induced surface morphology was characterized in depth by electron-based techniques, including scanning electron microscopy (SEM), electron back scatter diffraction (EBSD), cross-sectional transmission electron microscopy (STEM/TEM) and energy dispersive X-ray spectrometry (EDS). The in-depth EBSD crystallographic analyses indicated a significant impact of the material initial grain orientation on the type of surface nanostructure and microstructure formed upon laser irradiation. Special emphasis was laid on high-resolution material analysis of the hexagonally-arranged nano-protrusions. Their formation mechanism is discussed on the basis of the interplay between electromagnetic scattering effects followed by hydrodynamic matter re-organization after the laser exposure. The temperature stability of the hexagonally-arranged nano-protrusion was explored in post-irradiation thermal annealing experiments, in order to qualify their suitability in 2G-HTS fabrication technology with initial steps deposition temperatures in the range of 773–873 K.

Overview of the SPARC physics basis towards the exploration of burning-plasma regimes in high-field, compact tokamaks

The SPARC tokamak project, currently in engineering design, aims to achieve breakeven and burning plasma conditions in a compact device, thanks to new developments in high-temperature superconductor technology. With a magnetic field of 12.2 T on axis and 8.7 MA of plasma current, SPARC is predicted to produce 140 MW of fusion power with a plasma gain of Q ≈ 11, providing ample margin with respect to its mission of Q > 2. All tokamak systems are being designed to produce this landmark plasma discharge, thus enabling the study of burning plasma physics and tokamak operations in reactor relevant conditions to pave the way for the design and construction of a compact, high-field fusion power plant. Construction of SPARC is planned to begin by mid-2021.

ФОРМИРОВАНИЕ ОСТРОЙ КУБИЧЕСКОЙ ТЕКСТУРЫ В ЛЕНТАХ-ПОДЛОЖКАХ ИЗ СПЛАВОВ (Cu+Ni)–Me (Me=Mo, Mn, Nb) ДЛЯ ВЫСОКОТЕМПЕРАТУРНЫХ СВЕРХПРОВОДНИКОВ ВТОРОГО ПОКОЛЕНИЯ

After cold-rolling reduction with the shrinkage of more than 97 % and recrystallization annealing, the edged cubic texture develops in some fcc lattice metals with the high and medium values of stacking fault energy such as Ni, Cu, Al, Pt, and some alloys on their base. The extended bands of metals and fcc lattice alloys can be used to apply multilayer functional compositions. The authors studied the structure and crystallographic texture in bands of three-component copper-nickel-based alloys. The study showed the crucial possibility of creating multi-component alloys based on the Cu+40% Ni binary alloy doped with such elements as Mo or Nb. The paper considers the formation of an edged cubic texture in bands of Cu–Ni–Mn, Cu–Ni–Nb, and Cu–Ni–Мо alloys produced through cold deformation with rolling and recrystallization annealing performed at different temperatures. The study identified that annealing during one hour at 1050 °С was an optimal recrystallization annealing mode when on the surface of bands made of (Cu+40 % Ni)–Me alloys (where Me=Mn, Mo, Nb) deformed at ~99 %, the most perfect cubic texture was realized. According to the data obtained, after such annealing mode, from 94% to 98% of grains with orientation {001}&lt;100&gt; developed in the Cu–40% Ni–1.3% Mn, Cu–40% Ni–0.8% Mo, and Cu–40% Ni–0.5% Nb alloys. It opens the prospect of using these alloys as epitaxial substrates in the technology of high-temperature second-generation superconductors. The evaluation of mechanical characteristics showed that alloying contributed to an increase in the yield strength of Cu–40% Ni–1.3% Mn, Cu–40% Ni–0.8% Mo, and Cu–40% Ni–0.5% Nb alloys by 3–4 times compared with the yield strength value of a textured copper band.

Charging an HTS Coil: Flux Pump With an HTS Square Bridge

In recent years, along with the unceasing development of High-temperature Superconductor (HTS) technology, the performance of HTS tape has been improved significantly, including the higher critical current, lower joint resistance, and longer tape length etc. To use HTS instead of Low-temperature Superconductor (LTS) in Magnetic Resonance Imaging (MRI) applications, the flux pumping method with travelling wave has been investigated. In this paper, our new experimental results were regarding to the frequency, amplitude, and wavelength characteristics of charging an HTS coil with an HTS square bridge, using an electromagnetic flux pump designed at the University of Cambridge.

On similarity scaling of tokamak fusion plasmas with different aspect ratio

Previously, tokamak research has been focused mainly on large aspect ratio devices where the vessel/plasma major radius is about a factor three larger than the plasma radius. This research culminated in the design and construction of the international thermonuclear experimental reactor, ITER. Spherical tokamaks (ST), with aspect ratio below two, represent an attractive alternative to large aspect ratio tokamaks as, in our opinion, provide a faster, more economical and compact solution on the path to a fusion reactor. STs are the focus of research at Tokamak Energy Ltd with its present device ST40 in operation and the first ST reactor being designed, taking advantage of the high temperature superconductor (HTS) technology. HTS allow to design a ST with magnetic field/comparable or exceeding that of present-day large aspect ratio tokamaks. However, plasma studies carried out so far on compact, low aspect ratio tokamaks have been limited to small, low magnetic field, low plasma-current devices and therefore the data available for extrapolating to large scale ST plasmas is limited. This paper addresses the problem of scaling the results of large aspect ratio tokamak and existing ST plasmas to a high field ST reactor using plasma-similarity arguments in order to mitigate its design and operational risks. The role of the plasma aspect ratio in scaling burning plasmas as well as conventional experiments in deuterium is highlighted. We find that the scaling for fusion-reactor plasmas exhibit a stronger dependence on the magnetic field and aspect ratio than the one of conventional non-burning plasmas. The parameters of a ST having the same fusion gain Q fus of ITER under different confinement assumptions and for different aspect ratios are presented and discussed.

Orbital and thermal modelling of a 3U CubeSat equipped with a high-temperature superconducting coil

High-temperature superconductor (HTS) based devices have the potential to be useful technologies for space applications, allowing very high current transfer and magnetic field generation. However, HTS technology requires cryogenic temperatures (<90 K) to operate, and it is not well understood if and how this can be achieved for HTS devices integrated into space vehicles. In this study, the thermal and power performance of a hypothetical 3U CubeSat equipped with an HTS magnetic coil is explored over a range of orbits around the Earth. After eliminating the possibility of passive cooling this close to a planetary body, a cryocooler was deemed necessary to maintain the required temperatures and was included in the simulations. The results show that the best strategy for maintaining the cryogenic operating environment is to maximise the power availability to the cryocooler from solar panels. This approach increases the volume-averaged temperature of the satellite, but the benefits of increased power outweigh the cost of a decreased cryocooler efficiency. This work demonstrates that a 1 T magnetic field can be generated with an HTS electromagnet in a space environment on a small satellite, enabling the use of HTS for space applications such as electric propulsion and energy storage.

Design of a superconducting machine and its cooling system for an aeronautics application

The transition to electric propulsion aircraft requires electrical motors or generators with high power density. The “zero resistivity” of the superconducting materials could be used in electrical machines to produce high magnetic fields and reduce the use of heavy components such as the ferromagnetic parts. The discovery and recent developments in High Temperature Superconductors (HTS) technology make the superconducting machine a serious candidate in the future of aircraft. The design of a superconducting machine is strongly dependent on its electromagnetic and thermal behavior. In this paper, the design of a 50 kW superconducting aircraft generator is presented. The mass of the cryogenic cooling system is included into the design in order to optimize the entire superconducting system. The study shows that the choice of the cooling temperature to reduce the mass of the superconducting machine and its cooling system will depends on the input power of the machine.

Recent development of iron-based superconducting films

<p indent="0mm">The discovery of F-doped LaFeAsO in 2008 brought a worldwide research surge of iron-based superconductors. As a new type of high-temperature superconductors, iron-based superconductors have attracted much attention due to their unique properties and significant differences from copper oxide and traditional superconductors. Iron-based superconductors have small anisotropy, high superconducting transition temperature and upper critical field, which are preferred in film preparation and applications. At the same time, the features of iron-based superconductor films, such as higher superconducting transition temperature in epitaxial films and the ability to stabilize metastable equivalents, are interesting to the researchers. The studies on films are very useful for the basic research on superconducting physics and the search of new superconducting materials. The iron-based superconductors discovered so far can be divided into four systems: (1) 1111 system: LnFeAs (O, F) (Ln= lanthanides); (2) 122 system: AEFe₂As₂ (AE = alkaline earth metals); (3) 111 system: LiFeAs and NaFeAs; (4) 11 system: FeCh (Ch=chalcogenides). 1111 system is the first reported iron-based superconductor. It has a high critical transition temperature up to <sc>58 K</sc> as well as a higher upper critical field. The disadvantage of 1111 superconductors is the lager anisotropy and the broadened superconducting transition under the magnetic field. The critical transition temperature of 122 system is lower than 1111 system, but it can still reach <sc>38 K,</sc> and its anisotropy and elemental stability is better than 1111 system. The structure of 11 system is the simplest in iron-based superconductor family. It contains no toxic element As, but the transition temperature is low <sc>(8 K).</sc> 111 system is unstable in the air, limiting its application prospects. In addition, the critical current density is very important for the application of superconductors, which has been obtained in 122-system superconducting tapes. Meanwhile, the 11-system superconducting film stands out because of its easy growth, simple structure and high transport current density, which can exceed <sc>10⁵ A/cm²</sc> even at <sc>30 T.</sc> During the past years, researchers around the world have made great progress in the fabrication and synthesis of iron-based superconducting single crystals, bulk materials, tapes and thin films. Among them, the development of thin films cannot be ignored. The fabrication of high-quality thin films has three purposes: Basic research, tape application and electronic application. Basic and application-oriented research is developing quickly. Increasing the critical current density is an important strategy for realizing commercialized and cost-effective high-temperature superconductor technology, which can be improved by microstructure engineering. Naturally grown pinning centers and artificial pinning centers are able to pin on magnetic vortices in magnetic fields, which can effectively prevent the dissipation of energy and raise critical current density. Based on the results achieved in 1111 system, doped 122 system, and 11 system, we analyzed the possible connections between the growth technique, microstructure and superconducting properties of iron-based superconducting films.

Non-planar coil winding angle optimization for compatibility with non-insulated high-temperature superconducting magnets

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.