Understanding how REBCO-based coated conductors (CC) are affected by neutron irradiation during service in the magnets of a tokamak power plant is key to optimising plant design and cost. This will require establishing a reliable and cheaper ion irradiation protocol on commercially available REBCO CC as a proxy to replace expensive neutron irradiation campaigns. In this work, 20 MeV oxygen ions combined with a pyramidal ridge style energy filter are used to irradiate several types of REBCO CC for comparison with neutron irradiation experiments, and to measure the response of each sample type to irradiation. The critical temperature (Tc), critical current density (Jc) and flux flow exponent (n) were all measured at varying temperatures (T) and magnetic fields applied perpendicular to the tape plane (B perpendicular to) as irradiation damage (& fcy;) is accumulated in steps up to and beyond the damage levels relevant to fusion power plant operating parameters. Our results on the changes in Tc, high-field Jc and n-values with oxygen irradiation show similar trajectories to published data on neutron irradiation, suggesting that filtered ion irradiation could be a good proxy for neutron irradiation of REBCO. Our results showed that Tc decreased linearly with increasing damage, but YBCO CC declined at a slower rate compared to (Gd,Y)BCO and EuBCO CC. The n-values also decreased linearly with increasing damage at a faster rate than Tc, suggesting irradiation effects REBCO superfluid density. Self-field Jc, the accommodation field (B alpha) and high-field Jc all change with & fcy; following a trajectory that is well-fitted using an empirical equation containing terms describing an increase in flux pinning efficiency combined with an exponential decay term to model the degradation due to faster flux creep and a decreasing depairing current.

Applied-field magnetoplasmadynamic (AF-MPD) thrusters have been proposed as highly propellant-efficient thrusters for satellites and spacecraft. The electromagnets for these devices have only been reducible to practical dimensions with the maturation of high-temperature superconductors. We report the development and ground testing of such a magnet which has been designed and constructed with the intention of deploying to the International Space Station and testing in orbit. The magnet is of dimensions suitable for accommodating a small thruster, will be cooled by a miniature space-compatible cryocooler and energized by a flux pump. It can generate a magnetic field of up to 760 mT operating at around 77 K in that configuration. A passive magnetic shield has been incorporated in order to comply with stray-field requirements of the ISS when operating at the target operational field of 300 mT.

Magnets using high-temperature superconducting (HTS) tapes have been proposed for use in several space-borne applications. In particular they are well suited for use in applied-field magnetoplasmadynamic (MPD) thrusters for satellites as they can achieve a high magnetic field intensity in a suitable volume and mass range and can be cooled by commercial-off-the-shelf space-heritage Sterling cryocoolers. However, the stray fields produced by such magnets can be problematic, in that they can generate undesirable torque through interaction with the Earth's magnetic field and can interfere with other on-board systems and electronics. We present the design and initial mass optimization of a passive DC magnetic shield for a small HTS magnet that could fit within a 12U cubesat but will in the first instance be tested on board the International Space Station (ISS) in 2025. The magnet has a 62 mm diameter clear bore, compatible with a small MPD thruster, and will generate the target central field of at least 300 mT. The stray field from this magnet would be 1.7 mT at a distance of 220 mm from the center of the magnet and a magnetic shield is required to reduce this to 0.3 mT to comply with ISS specifications. We describe active and passive shield options, and choose ultimately to construct a passive shield from magnetic low-carbon steel and confirm that this is able to achieve the required stray-field level.

Particle irradiation offers a route to incorporating additional flux pinning centres in high-temperature superconducting wires with minimal disruption to the pre-existing defect landscape, thereby further enhancing the critical current in a controllable fashion. This work is a comprehensive study of the fluence-dependence of proton irradiation using protons of two energies, 2.5 and 1.2 MeV, in enhancing the critical current performance in commercially available (Y,Dy)Ba2Cu3O7-delta coated conductors. A sequence of fluences covering the range from 1 x 1015 to 5 x 1016 protons cm-2 was used in the irradiation process to study the flux pinning in this material. The resulting samples were characterized using field angle-dependent transport critical current measurements over a range of temperatures from 20 K to 77.5 K and magnetic fields up to 8 T, thus covering the wide range of operating conditions. Optimisation of fluence for highest performance at each energy resulted in a similar level of isotropic critical current enhancement, a factor 2.6 improvement at 20 K and 8 T, but with a significant difference in the optimised fluence in each case. The lower energy 1.2 MeV protons produce this enhancement at a three-fold lower fluence compared to 2.5 MeV protons, a result of their higher electronic energy loss. The different samples are analysed within the framework of the maximum entropy model, helping to understand the vortex dynamics before and after irradiation.

The Spherical Tokamak for Energy Production (STEP) requires high-field magnet designs and has therefore adopted the REBCO-based high-temperature superconductor (HTS) as its current carrier. The HTS enables the toroidal field (TF) coils to be remountable, which unlocks STEP's vertical maintenance approach; however, remountable joints, approximately 18 GJ of stored energy and limited space down the centre of a spherical tokamak, make the TF coils the most challenging. STEP has pursued a passive approach to TF coil quench protection in order to limit coil terminal voltage. Initial results suggest that a solution may rely on tuning internal coil resistance coupled with actively powered heaters. The pre-conceptual inter-coil structure demonstrates acceptable stresses and deflections under steady-state operating conditions and preliminary fault scenarios, and loads are distributed to limit the tensile force on the TF centre rod. Finally, the HTS must operate reliably in a high radiation environment and endure high neutron fluences, ensuring commercially relevant magnet lifetimes. Initial experiments indicate that instantaneous gamma irradiation of HTS has no negative impact on current carrying capacity. Experimental programmes are underway to cold irradiate HTS to fusion-relevant fluences and to develop a method of assuring tape irradiation tolerance using oxygen ions as an analogue for neutrons.This article is part of the theme issue 'Delivering Fusion Energy - The Spherical Tokamak for Energy Production (STEP)'.

As space development and exploration progress, the need for high-performance propulsion systems becomes increasingly crucial. Electric propulsion utilizing a strong magnetic field to enhance thrust performance emerges as a promising candidate. In this study, we propose a kilowatt-class plasma thruster incorporating a high-temperature superconducting magnet capable of generating a high magnetic field in the 1 T range. The thruster operates based on electrostatic ion acceleration, utilizing the potential difference across the magnetic field lines. To achieve high magnetic field strengths, the superconducting magnet is cooled by a cryocooler, enabling efficient power-saving operation up to a magnetic field strength of 0.8 T. During the experiment, it was observed that the ignitability of the thruster was compromised at high magnetic field strengths. However, through modification of the anode electrode shape, ignitability was improved. The experimental results demonstrate notable performance improvements with higher magnetic fields, culminating in a thrust efficiency of 26.5% with a specific impulse of 1930 s at 0.8 T. These findings underscore the potential of utilizing high-temperature superconducting magnets in plasma thrusters for achieving enhanced thrust performance. The successful development of this kilowatt-class plasma thruster represents a significant step towards realizing efficient propulsion systems for future space missions.

Increasingly ambitious space missions rely upon the advancement of propulsion systems, and innovations in electric propulsion represent a vital step toward potential solutions. Electric propulsion research has indicated improved thruster performance at higher magnetic fields, but real-world implementations have been encumbered by available magnet technology. In this study, a new class of superconducting magnet designed for such space applications is demonstrated. A central cathode electrostatic thruster was paired with a conduction cooled high temperature superconducting magnet operated in non-pulsed mode. Using Xe propellant, thrust of 38--73 mN was measured at a central bore magnetic field of 1.13 T and on-anode field of 1.28 T, and the thruster's specific impulse was throttled between 1050s and 1450s. The peak thruster efficiency was 18.7%, inclusive of the power consumption of the magnet subsystems. This was performed without the onset of plasma instabilities, and is the first case in the literature of a high temperature superconducting plasma thruster operating at > 1 T.

We have used silver-ion irradiation and proton irradiation to produce point-like and spherical defects in REBa 2 Cu 3 O 7 coated conductors. We compare the resulting pinning landscape for optimized fluences and show that proton irradiation gives a slightly greater pinning enhancement at 20 K, but in the same samples silver irradiation gives significantly better pinning enhancement at 65 K. We attribute this to the relative sizes of the defects and to the distribution of defects resulting from the different ion collision rates.

We investigate changes to the pinning landscape in ion-irradiated coated conductors through comparisons of critical current and n -values as a function of field, field angle and temperature, where n is the power law exponent in the current density-electric field relation, ${\bm{E\ }} = {{\bm{E}}}_{\bm{c}}\ {({{\bm{J}}/{{\bm{J}}}_{\bm{c}}})}^{\bm{n}}$ . Many models of J c have been developed but models of n -value are rare in the literature. The n -value can be interpreted as the sensitivity of E to J or in the context of a maximum entropy analysis n can be related to a constraint on the vortex system. We investigated samples of (Y,Dy)BCO coated conductor tapes from AMSC irradiated with 50 to 150 MeV Ag ions, and measured J c and n -values at temperatures from 20 to 77 K and fields up to 8 T at all field angles. As n -values arise from vortex pinning behavior we model the angle dependence using the maximum entropy distributions we have previously used for J c (θ) analysis. Fitting with these distributions shows the n -values are the outcome of the combined defect structures interacting with vortices. The changes in n -value with field and temperature are not simply correlated with J c values, for example, n -values often have a broad c -axis centered peak with no correlation to a peak in J c . At higher temperatures n -values generally have no peak centered on the ab -plane. Irradiation along the c -axis changes n -values in a broad angular range. The n -value is sometimes interpreted as a measure of the energy barrier associated with thermal creep, but from our observations we propose that it is a measure of the dimensionality of the vortex system.

Particle irradiation using light ions and heavy ions is found to be an effective method to introduce flux-pinning centers into REBCO films and coated conductors. The degree of enhanced critical current at various conditions depends upon the size, morphology, and orientation of ion tracks. Proton irradiation to the optimised fluence results in greater isotropic enhancement at lower temperatures, the enhancement decreases as temperature increases. Silver ion irradiation on the other hand gives a greater enhancement at higher temperature but limited to particular angular ranges. We compare the results of these two types of irradiation and then produce a mixed pinning landscape with a combination of the two. We find a nearly isotropic enhancement in Ic at lower temperatures and an enhancement about the c -axis direction, similar but broader than silver irradiation alone, at higher temperatures.

A 6.5 MVA/25 kV high-temperature superconducting (HTS) transformer for the Chinese Fuxing high-speed train has been proposed to replace the oil-based transformers while achieving higher efficiency, lighter weight, and minimized volume. The high targeted efficiency (>99%) makes ac loss reduction a vital issue. HTS coated conductors generally exhibit asymmetric critical current characteristics as a function of magnetic field angle $I_{c}$ ( $B$ , $\theta$ ), leading to a nontrivial influence on the ac loss of coil windings. The fast computing $T$ – $A$ homogenization method is proposed to calculate the ac loss of the 6.5 MVA/25 kV traction transformer with large turn numbers. The variables, $T$ and $A$ , are the current and magnetic vector potentials, respectively. The ac loss of the transformer windings is analyzed for various coil configurations with and without flux diverters considering $I_{c}$ ( $B$ , $\theta$ ). At the rated current and 65 K, employing the flux diverters with a square-shape cross section, the total ac loss is decreased by 73.7% and an extra 150-W loss reduction was also obtained. Moreover, an additional reduction of 37 W is realized upon utilizing the asymmetric $I_{c}$ ( $B$ , $\theta$ ) characteristic. The reduced 187 W in ac loss at 65 K corresponds to a reduction in ambient power requirement of over 5.6 kW. Therefore, considering asymmetric $I_{c}$ ( $B$ , $\theta$ ) can lead to a nontrivial reduction in ac loss, even incorporating flux diverters.

This study investigates crystal orientation evolution at microscopic level, and changes in the superconducting properties at macroscopic level of rare-earth-barium-copper-oxide (REBCO) superconductor tape under severe deformation. At the microscopic level, in-situ electron backscatter diffraction is used for real-time observation of the crystal orientation evolution during the entire tensile process. At the macroscopic level, the critical current (Ic) value and Ic angle dependence performance are systematically measured under different applied tensile stresses. By comparing the microscopic and macroscopic results, the synergetic evolution mechanism between them is successfully established. Stress at the initial descent point of the Ic value excellently fits the one where the (001)[100] orientation sharply decreases, thereby revealing the real Ic degradation mechanism of REBCO tapes. The application of a moderate applied tensile stress can increase the in-field Ic value and pinning force because new generated defects and strains act as pinning centers. A higher tensile stress can destroy the biaxial texture of the REBCO layer and become a significant barrier to the supercurrent flow. This research elucidates the REBCO degradation mechanism and may assist manufacturers in improving the electromechanical properties of commercial REBCO tape.

Interplanetary transport of payloads of unprecedented mass, as envisaged beyond the lunar gateway, will require thrusters with high specific impulse as well as high thrust. To achieve this, innovations in propulsion are critical. Many classes of electric thruster utilise a magnetic applied field module to accelerate charged particles. Magnetoplasmadynamic thrusters exhibit improved performance with increasing field, at least up to the limit of around 0.5 T able to be provided by permanent magnets or copper electromagnets. However, superconducting magnets can generate much stronger magnetic fields. In this study, we utilised a space-relevant cryocooled high temperature superconducting magnet as the applied field module for a central cathode electrostatic thruster (CC-EST). A convex anode enabled ignition at high magnetic fields, and in this configuration the thruster’s performance was characterised in the power range of 1 kW to 2.5 kW and at steady applied fields ranging from 0.6 T to 0.8 T, representing a significant advance in achievable field strength. In combination, these operating parameters enabled the achievement of a magnet-inclusive thruster efficiency of 19%, while the cryocooled magnet was demonstrated to be thermally stable in the presence of the kW-scale plasma, demonstrating the viability of such a design for space flight applications.

Heavy-ion irradiation of solids produces damage tracks with radii typically of the order of 1 nm, depending on the ion species and energy. In cuprate superconductors this is close to the coherence length, which makes these defects highly effective flux pinning centers. Varying the ion-beam energy allows tuning of the dimensionality of the defects created, with higher-energy ions tending to produce columnar tracks and lower-energy ions tending to produce point-like defects. Starting with consistent production-standard REBCO tape from American Superconductor we have explored the energy-dependence of silver-ion irradiation and characterized the irradiated samples with angle-dependent transport critical current measurements. Using silver ions with energies in the range 50 MeV to 150 MeV and fluence of 4 × 1011 ions/cm2 we have been able to tune the irradiation-induced damage from point-like defects to columnar tracks, manifesting in changes to the pinning landscape ranging from isotropic critical current enhancement to the production of strong peaks in the angle dependence of critical current.

Normal-incidence irradiation by 100 MeV Ag ions is used to improve flux pinning in previously optimised commercial REBCO tapes from the American Superconductor Corporation. We observe distinct critical-current anisotropy enhancements below and above 40 K. Above 40 K a strong c -axis peak appears in the angular dependence of the critical current, as is usually expected upon the introduction of columnar defects. The critical current is enhanced significantly but only for a limited range of field angles. Close to the parallel-field direction there is no enhancement or even a reduction in critical current. Below 40 K, on the other hand, the enhancement is much broader with respect to field angle, creating an almost isotropic response at 20 K, 3 T. The absence of a prominent c -axis peak does not indicate a lack of pinning, since the absolute value of the critical current still increases by a factor of 2.8 compared to an unirradiated sample. Instead, we postulate that pre-existing point-like pinning centres act to mediate an interaction between the existing planar and newly-introduced columnar pins, broadening both contributions. The point-like pins become less effective with increasing temperature as the coherence length increases, leading to a reduction in this interaction and a separation of the individual peaks relating to planar and columnar pins. At 20 K, we achieve an enhancement in the angular-minimum critical current by a factor of 2.7, in a material that had already been process-optimised for low-temperature pinning.

Recent advances in commercial miniaturised cryocoolers and high-temperature superconductors (HTS) have revived the discussion of using HTS electromagnets to enhance the thrust and efficiency of electric thrusters for space applications. An HTS applied-field magnetoplasmadynamic (AF-MPD) thruster is currently being developed. While the thruster is operating, there will be a large time-variable heat load on the cryogenic environment. The operation of a low-power cryocooler and energised HTS coils (operating at 70 K) adjacent to streams of hot plasma and large electrical discharges (on the order of 1 kW) represents a significant thermal management problem. The electromagnetic and thermal behaviour of non-insulated (NI) coils under these conditions, and how resilient they are to quenching during thruster operation, is not well understood. In this paper, a model is formulated to study the transient electromagnetic and thermal behaviour of an HTS-AF-MPD thruster with NI coils. The thruster and a conduction cooled cryogenic design are coupled via surface-to-surface radiation heat transfer. The model predicts current flow within the HTS, copper stabiliser and between turns, with the contact resistivity being a key input variable. Critical current is determined locally using temperature, magnetic field, and field angle in combination with a measured data set for a specific conductor. This model reveals conditions where the cryocooler can passively compensate for large instantaneous heat loads on the coils, demonstrating quench resistance.

Detailed measurements of the critical current of a commercial 2G HTS REBCO wire as a function of temperature, magnetic field, and field angle, performed at the Robinson Research Institute of Victoria University of Wellington.

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.

Ion irradiation of REBCO films and coated conductors, in which the ions pass completely through the REBCO film, produces damage tracks which form near-ideal flux-pinning defects. The radius and aspect ratio of the tracks depends on the mass and energy of the incident ions. We have investigated the effect of Ag ion irradiation, at different incident energies and incidence angles, on REBCO production-quality coated conductors from American Superconductor Corp. Transmission electron microscopy and in-field transport critical current anisotropy analysis indicates that the ion-energy threshold for the formation of elongated tracks is around 50 MeV. At this energy tracks are not readily identifiable in low-resolution TEM, and enhancement of critical current is nearly isotropic. For a higher ion energy of 100 MeV, on the other hand, clear elongated (but still not fully continuous) tracks are visible in TEM, and the critical current is anisotropic with strong enhancement occurring when the applied field is parallel to the ion incidence angle. We particularly analyze the case of 60° inclined irradiation. This produces a clear peak in the field-angle dependence of critical current for 100 MeV irradiation, but only an incipient peak for 50 MeV irradiation. The incipient peak can be identified by curve fitting using a minimal number of maximum-entropy functional components.

The magnetic-field anisotropy of the critical current is an extrinsic property of superconducting wires that is of greatest relevance to the design of high temperature superconducting (HTS) devices. It is also a highly useful diagnostic tool to understand the dominant flux-pinning mechanisms active in different temperature and field regimes. REBa2Cu3O7 (REBCO) coated conductors typically exhibit a large peak in critical current when the field is aligned with the REBCO a-b plane. The commercial pinning-enhanced American Superconductor Corporation (AMSC) REBCO coil wire used for this study is unusual in having a relatively small a-b plane peak at higher temperatures due to an inherently low density of stacking faults. Stacking faults can then be introduced by annealing the wire in oxygen to create the more commonly observed strong a-b plane peak. Complementary c-axis columnar defects can be added by silver ion-irradiation producing an additional peak in the critical current at 0°. The resulting complex critical current anisotropy is studied using transport critical current measurements over a temperature range from 20 K to 77.5 K and under magnetic fields up to 7 T. Through systematic studies on pristine and annealed samples, in the presence and absence of columnar defects created by silver irradiation, we investigated the flux-pinning interactions between stacking faults and columnar defects.