High-temperature Superconducting Layer Research Articles

Understanding ac losses in CORC cables of YBCO superconducting tapes by numerical simulations

Alternating current (ac) losses in conductor-on-rounded-core (CORC) cables of YBCO high-temperature superconducting (HTS) tapes are a significant challenge in HTS power applications. This study employs two finite element analysis (FEA) models to investigate the contributions from different ac loss components and provide approaches for reducing ac losses in cables. An FEA model based on the T-A formulation treats the cross section of thin superconducting layers as 1D lines and, therefore, only can predict the ac loss generated by the perpendicular magnetic field. In contrast, the model based on H-formulation can be performed on the actual 2D rectangular cross section HTS tapes to provide the total ac losses generated by magnetic fluxes penetrating from both the edges and surfaces of HTS tapes, although this model requires more computing time and memory. The 1D and 2D simulation models were validated by cross comparing the results from both models and by comparing sub-section and full cross section models. Subsequently, two models relate cable design and operational parameters to the surface and edge losses of a two-layer CORC cable by considering the (1) relative contributions of edge and surface losses to the overall ac losses; (2) effect of the current distribution between inner and outer HTS layers on ac losses; (3) impact of the tape alignment on ac losses in each HTS layer; (4) influence of the thickness of HTS layers on ac losses; (5) effect of size and number of inter-tape gaps on ac losses; and (6) contribution frequency on the ac losses. The research results given in this paper are therefore not only valuable to suggest strategies for reducing ac loss in multi-layer cables but also for developing more accurate and effective methods to calculate ac loss in CORC HTS cables.

Magnetisation and demagnetisation of trapped field stacks in a superconducting machine for electric aircraft

This research presents a comprehensive and innovative approach to investigating the magnetisation and cross-field demagnetisation behaviour of high-temperature superconducting (HTS) coated conductors (CCs) in practical superconducting machines. This study introduces several novel contributions, including the operation of the machine in propulsion energy conversion mode, the exploration of harmonics interaction in a real electric machine environment involving CCs, and the extraction of these harmonics as cross-field components. A 2D electromagnetic-thermal coupled numerical model employing the finite element method has been developed and validated against experimental data to simulate a partially superconducting machine. Upon magnetisation, the HTS stacks effectively operate as trapped field magnets, generating rotor fields for motor operation. With a peak magnetic flux density of 462 mT of the trapped field stacks (TFSs) in the air gap, the average values of the fundamental and fifth harmonics of the tangential magnetic flux density experienced by the TFSs were observed to be 25 mT and 1.75 mT, respectively. The research has thoroughly examined the impact of cross-field demagnetisation parameters including amplitude and frequency on the demagnetisation of TFSs. Furthermore, the study has also investigated the magnetisation losses occurring in various layers of HTS tapes, encompassing the HTS layer, magnetic substrate layer, and silver stabiliser at different amplitudes and frequencies. Two tape structures, namely a semi-homogenised model and a multi-layered model, have been analysed in terms of magnetisation loss. Additionally, insights into the shielding effect and skin effect at high frequencies were obtained, offering valuable information on the performance of HTS TFSs exposed to high frequency scenarios especially in high-speed machines for electric aircraft. The research outcomes are anticipated to provide valuable knowledge for the design and optimisation of HTS rotors employing TFSs in superconducting machines, contributing to the advancement of superconducting machine technology.

Influence of Water Content on the Dielectric Characteristics of LN2-Impregnated PPLP for HTS Power Cables

High-Temperature Superconducting (HTS) power cables operate at low, medium, and high voltage levels for electrical power transmission. Polypropylene Laminated Paper (PPLP) is wrapped helically around the HTS layer and it is impregnated with Liquid Nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) to form a composite dielectric layer. During the installation and maintenance process of HTS cables, any leak of moisture in the cryostat will lead to immediate ice formation. Once the cable warms up, the ice will melt into water, which will be absorbed by the PPLP Kraft paper layer and is similar to the PPLP paper soaked with water. When the moist PPLP is cooled with LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> it will form ice and forms a composite material during operation. The objective of this paper is to investigate the influence of water content on the breakdown strength, relative permittivity (ϵ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ) and dielectric losses of LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> impregnated PPLP samples for different number of layers. Considering the worst-case scenario of moist samples, wet PPLP samples were tested for breakdown strength and dielectric properties at Room Temperature (RT) and LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> temperature and was compared with dry PPLP samples. Two different experimental setups were developed to measure the breakdown strength and dielectric properties respectively for stacked PPLP layers. The wet and dry samples were tested for the AC V-I characteristics, the effect of PPLP stack thickness on dielectric strength, statistical analysis using two parameter Weibull distribution and computation of relative permittivity and loss factor from capacitance measurement.

An increase in the critical current of superconducting composites upon implantation of iron ions

The results of studying the effect of ion irradiation (Fe2+ E=5.6 MeV) in the modes of creating radiation defects and implantation on the critical current of high-temperature superconducting (HTS) composites are presented. An analysis was made of both the integral critical current obtained from measurements of the total magnetization of the samples and the local critical current determined from the data of scanning Hall magnetometry. It is shown that at the same ion fluence Phi=2·1013 cm-2, an increase in the critical current Jc is observed in the ion implantation regime, while in the regime of radiation defects, a slight drop in Jc is observed. This circumstance indicates an enhancement of pinning due to the additional magnetic interaction of Abrikosov vortices with magnetic ions implanted in the HTS layer. Keywords: high-temperature superconductor, irradiation, radiation defects, critical current, magnetization.

Увеличение критического тока сверхпроводящих композитов при имплантации ионов железа

The results of studying the effect of ion irradiation (Fe2+ ions E=5.6 MeV) in the modes of creating radiation defects and implantation on the critical current of high-temperature superconducting (HTS) composites are presented. An analysis was made of both the integral critical current obtained from measurements of the total magnetization of the samples and the local critical current determined from the data of scanning Hall magnetometry. It is shown that at the same ion fluence Ф=2‧1013 cm-2, an increase in the critical current Jc is observed in the ion implantation regime, while in the regime of radiation defects, a slight drop in Jc is observed. This circumstance indicates an enhancement of pinning due to the additional magnetic interaction of Abrikosov vortices with magnetic ions implanted in the HTS layer.

Suitable Structure of Triaxial HTS Cable With Low Thermal Conductive Layer for Increasing Power Transmission Cable Length

We have investigated the effectiveness of the insertion of a low thermal conductive layer (LTCL) in a tri-axial High Temperature Superconducting (HTS) cable to realize a long-distance tri-axial HTS cable. This study analyzed the relationship among intrusion heat from outside of the cable, AC loss, temperature distribution, and the maximum temperature within a tri-axial HTS cable with the LTCL to clarify the suitable insertion position and thickness of the LTCL for a long-distance tri-axial HTS cable. Analysis results showed that inserting the LTCL inside the HTS layers was effective for the long-distance tri-axial HTS cable. In addition, there was a trade-off relationship between the effects of suppressing the thermal conduction from the outer refrigerant to the inner refrigerant and increasing the intrusion heat from outside of the cable by inserting the LTCL. The suitable thickness of the LTCL can be obtained by considering this trade-off relationship.

Temperature distribution of long-length high temperature superconducting cable cooled by slush-nitrogen

A numerical model is established to calculate the temperature and pressure distributions of the 3 km long high temperature superconducting (HTS) power cable. The model assumes that the configuration of HTS power cable is three phase co-axial shape with counter-flow cooling method. In the numerical model, the circulating nitrogen enters the inner channel of the HTS power cable for cooling at the one end, flows to the other terminal, returns through the outer channel, and exits from the HTS power cable. The numerical model considers the mass, the momentum, and the energy balances of nitrogen, and the heat diffusion inside the HTS layer to obtain the realistic temperature and pressure distributions of the cable. When the long HTS cable is cooled by subcooled liquid nitrogen, the result shows that the maximum temperature of the compact HTS layer cannot be kept under 80 K even though the subcooled liquid nitrogen is supplied at the melting temperature. In order to supply nitrogen with lower enthalpy for ensuring sufficient cooling through the whole cable region, we suggest using slush nitrogen, and analyze it in the detailed numerical model. Thermo-hydraulic equations for slush nitrogen are incorporated in the numerical model. The results show that the maximum temperature of the HTS layer decreases by 6.4 K when slush nitrogen with solid volume fraction of 30% is supplied to the HTS cable. The effect of the inlet enthalpy of nitrogen and the heat transfer characteristics to the temperature distribution is discussed and further elucidated in this paper.

Evaluation of Electromechanical Properties in Multiple HTS Layered Conductors at 77 K

High engineering critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> ) is demanded in high-performance large-scale applications such as power cables, high-magnetic-field magnets, accelerators, and fusion reactors. In developing high current capacity and low vulnerability under a high magnetic field, high-temperature superconducting (HTS) rare-earth barium copper oxide coated conductors (CCs) have undergone various trials, and CC tape characteristics have ultimately been greatly improved. The Korea Electrotechnology Research Institute recently developed a “multiple HTS layers on one substrate” (MHOS) conductor that can carry high critical current, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , based on the number of superconducting layers deposited on one substrate. The superconducting layers in a MHOS conductor are much thicker than in a single-layered CC tape, possibly affecting the degradation behaviors of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> under a mechanical load. To ensure the performance and reliability of MHOS conductors, it is necessary to evaluate the strain/stress response of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . In this study, the electromechanical properties of MHOS conductors were investigated using the uniaxial tension test method as well as the single-sided bending test at 77 K and self-field and at 0.5 T using a pair of neodymium permanent magnets.

Dynamic loss and magnetization loss of HTS coated conductors, stacks, and coils for high-speed synchronous machines

Dynamic loss is an essential parameter to consider for the design of high temperature superconducting (HTS) synchronous machine windings. For aerospace electric propulsion systems, the fundamental frequency component and harmonics in electric machines can attain kHz level because of the high rotating speed. However, for HTS coated conductors (CC), the existing definition of dynamic loss only considers the HTS layer, the validity of which at high frequencies is questionable. Besides, the variation of dynamic loss and magnetization loss under skin effect due to high frequency is still unknown. Additionally, the influence of shielding effects among distinct turns on the dynamic loss of HTS stacks and coils remains unclear. In response to the above concerns, by use of the H-formulation based numerical multilayer modelling method which considers all layers of a CC, the frequency dependence of dynamic loss and magnetization loss of HTS CCs, stacks and coils over a wide range up to 20 kHz has been investigated. Results show that the existing definition of the dynamic region is no longer valid at kHz level, which shrinks rapidly with increasing frequency and magnetization loss plays a progressively important role due to skin effect. Meanwhile, the shielding effect in HTS stacks and coils can enhance the significance of dynamic loss. This paper clarifies the characteristics of dynamic loss and magnetization loss of HTS CCs, stacks, and coils over a wide frequency band, which can serve as a useful reference for accurate loss controlling of machine windings in future aerospace HTS propulsion systems.

Study on critical current of a flexible quasi-isotropic HTS conductor with high engineering current density

This paper presents a flexible quasi-isotropic High Temperature Superconducting (HTS) conductor with high engineering current density. The conductor consists of three components: the inner core is a twisted quasi-isotropic strand (Q-IS) with a circular cross section achieved by symmetrically stacking four sub-stacks of HTS wires and four arc aluminum fillers, the middle component consists of copper layers constructed by copper wires spirally winding around the inner core, and the outer component consists of HTS layers constructed by HTS wires spirally winding around the middle section, which is similar to the manufacturing method used for HTS cable or CORC cable. After the three-dimensional finite-element simulation model of the proposed HTS conductor is established, the dependence of the magnetic field and critical current distributions on the twisting pitch of the inner Q-IS and the winding pitch of the outer HTS cable layers are analyzed. Consequently, the model HTS conductor is designed and fabricated, and the critical current of this HTS conductor is measured in liquid nitrogen. The results show that the simulated results are in good agreement with the experimental ones. It is predicted that this type of HTS conductor has potential applications in fusion magnet and power transmission.

Loss characteristics of HTS coated conductors in field windings of electric aircraft propulsion motors

High-temperature superconducting (HTS) coated conductors (CCs) are widely regarded as a promising candidate to enable very high power density motors. These machines operate at high rotational speeds, with some designs going up to 12 000 rpm. HTS CCs are applied to the field windings of these motors to increase the magnetic loading and hence the power density. Even though the superconducting field windings operate with a DC current, due to the magnetic field environment, losses are present. This paper examines the dynamic and total loss characteristics of YBCO-coated conductors in the frequency range relevant to high-speed motors for electric aircraft propulsion. A multi-layer model was created using the H-formulation and the losses for each layer were highlighted. For the first time, it was shown that the DC transport current region in the HTS layer shrinks as the frequency of the applied field increases due to the increased magnetisation current around the edges of the CC, which reduces the dynamic loss per cycle as the frequency increases. To fully understand the loss distribution in the HTS CC, the total loss in the conductor was investigated. For an applied magnetic field of 100 mT and 800 Hz, more than 30% of the total loss occurs in the copper layer due to the decreased penetration depth of the magnetic field and the skin effect. Results show that to accurately model and understand the losses in superconducting field windings, a multi-layer model should be used, since a significant proportion of loss shifts towards the copper stabilizers. Over all, it was shown that both the dynamic loss as well as magnetisation loss play a crucial role in the estimation of the loss in superconducting field windings.

Modelling of electromagnetic loss in HTS coated conductors over a wide frequency band

Electromagnetic loss is a crucial factor to evaluate the performance of high temperature superconducting (HTS) coated conductors (CCs). Although the loss characteristics around power frequency have been well studied, it is still unclear how loss varies towards high frequencies above kHz level, which is important to a wide range of applications, such as wireless power transfer. This paper is to investigate the frequency dependence of both magnetization and transport current loss in HTS CCs and therefore, to provide comprehensive analyses through a detailed multilayer model reflecting their actual structures. In this paper, a T-formulation based multilayer numerical model for HTS CCs, consisting of HTS layer, substrate, silver overlayer, and copper stabilizers, has been developed. Both magnetization loss and transport current loss over a wide frequency range, from 50 Hz to 1 MHz, have been simulated and discussed. The results obtained by the existing thin film model based on T-formulation, multilayer model and homogenization model by H-formulation are also presented and compared. The proposed multilayer model has been validated by experimental measurements, which has proven the widely adopted HTS thin film model to be inapplicable for frequencies above 100 Hz. In addition, most magnetization losses occur in the copper stabilizers above 1.2 kHz due to skin effect. Results can be used to study the performance of HTS devices towards high frequencies at kHz or even MHz level.

Computation of Superconducting Stacks Magnetization in an Electrical Machine

Superconducting technology offers the prospect of sharply increasing power density of rotating electrical machines, especially in the low-speed, high-torque range, with impact in applications such as wind energy and aircraft propulsion. Among the enabling technologies, stacks consisting of piled up layers of high-temperature superconductors may provide a source of magnetic flux density for torque production, without the complexity of superconducting wound rotor poles. For this to happen, careful designs optimizing electromagnetic, mechanical, and thermal aspects at the same time must be developed. In that sense, this paper applies a recently developed combined electromagnetic formulation to compute the magnetization level of high-temperature superconductor stacks installed in the airgap of an electrical motor after field cooling magnetization. The results, congruent with the applied field, show a strong interaction between the teeth and stacks and provide a way of initializing the state of the machine prior to operation.

The Influence of the Voltage Rise Time on Transient Processes during Current Overload in Stabilized Second-Generation HTSC Wires

The effect of spontaneous reverse transition of a stabilized high-temperature superconductor (HTSC) tape from the normal to resistive state during rapid (jumplike) voltage application and alternating current overload has been observed. It is established for the first time that, at a stabilizing layer thickness above 13 μm, the main heat evolution takes place in the copper stabilizer and this heat is effectively removed to the refrigerant, which significantly decreases the HTSC layer’s overheating. This is accompanied by a drop in the resistance of the secondary resistive state and increase in the duration of its existence. These data allow using the observed effect to increase the time of current limitation and improving reliability of HTSC current limiters.

Design and Experimental Research on Self-Shielding DC HTS Cable Model With Large Current Capacity

DC high-temperature superconducting (HTS) cable has advantages over the ac HTS cable in the power transmission due to its zero Joule loss, higher current capacity, and controllability. To minimize the magnetic field influence and critical current degradation in dc HTS cable, we have ever proposed a new structure of dc HTS cable that is, self-shielding dc HTS cable which has interleaved winding configuration. In this paper, in order to solve the difficulty of current lead connection with HTS layer in the cable terminal with multilayers, we put forward a new kind of terminal configuration which can make the current in each layer being controlled individually and avoid soldering in terminal as well as be convenient for installing and disassembly. With this kind of terminal connection method, a dc HTS model cable with eight layers and 1900 mm in length is fabricated and tested in liquid nitrogen temperature. Uniform current distribution among layers is realized by adjustable resistance connecting with each layer. The experimental results show that the critical current degradation of self-shielding dc HTS cable is only 7% compared with the critical current in self-field while the traditional structure of dc HTS cable is about 20%, which indicates that the self-shielding cable can promote the current capacity effectively with lower critical current degradation and save more HTS tapes in the application of power transmission. So, this kind of cable with a thinner insulation layer is suitable for mid- and low-voltage situations, especially for industrial and ship situations where large current and low voltage are required.

Long-length YBCO coated conductors for ultra-high field applications: gaining engineering current density via pulsed laser deposition/alternating beam-assisted deposition route

Gaining engineering critical current density (Je) in high-temperature superconducting (HTS) coated conductors based on double-disordered YBCO for operation at high fields is one of the key requirements in upcoming magnet/accelerator projects. Currently, the development of a tape with advanced Je-s is performed based on an alternating beam-assisted deposition–pulsed laser deposition (ABAD–PLD) manufacturing route. An obvious way to improve Je-s is to reduce substrate thickness. A 40% Je increase is expected owing to the thickness reduction from 100 to 50 μm. Nevertheless, the reduction of substrate thickness in case of the applied processing technology that employs relatively thick (2–3 μm) yttria-stabilized zirconia buffer layers leads to the manifestation of considerable strain in the tape resulting in strong tape bowing. Advanced processing routes have been developed to suppress this effect. The highest engineering current density was recorded at well above 1000 A mm−2 under an ultra-high field of 18 T at 4.2 K, B//c. Influence of tape bowing and the impact of longitudinal defects are studied via V–I measurements at 77 K and a moderate magnetic field with intrinsic edge gradients. Potential for further gaining of Je was found employing mechanisms of (i) film nucleation from lateral flows that provide material transfer during the PLD process. Further steps include suppressing the instability of instantaneous temperature via quasi-adiabatic pulsed heating of the growing HTS film via (ii) control of laser plume energy, and (iii) energy release during film condensation (condensation enthalpy). These effects disturb the instantaneous temperature of the growing HTS layer, which exhibits very limited capability of heat transfer. Temperature pulses reaching 30–40 K were evaluated via heat transfer modeling. Stabilization of the level of pulsed temperature during layer growth is anticipated to result in a further increase in Je.

Stainless Steel as Substrate Material for High-Temperature Superconducting Tapes Processed Via the ISD MgO Route

Second-generation high-temperature superconducting (HTS) tapes strongly rely on the substrate material, in many cases Hastelloy C276 (“H-C276”). In order to ameliorate performance (and reduce the price) of superconducting tapes, replacement of H-C276 has been investigated. In this paper, the potential of the readily available austenitic stainless steels X2CrNiMo17-12-2 (“CrNiMo”) and X11CrNiMnN19-8-6 (“CrNiMnN”) as substrates is assessed. It was found that CrNiMnN is not suitable as a replacement for H-C276. Tensile stresses occurring at elevated temperatures during the HTS tape production result in plastic deformation of the CrNiMnN substrate. In the case of CrNiMo, however, plastic deformation was not observed after processing of the HTS tape. Electrical losses as a result of alternating currents beyond those of H-C276 did not occur due to the paramagnetic nature of CrNiMo. It is demonstrated that, for the very same processing parameters, HTS films deposited on electrochemically polished and buffered CrNiMo substrates exhibit similarly high critical current densities as on polished and buffered H-C276 substrates. The application of CrNiMo stainless steel reduces substrate costs considerably. More importantly, tapes processed on CrNiMo substrates withstand higher tension strains with respect to the reduction of the critical current compared to H-C276 substrates. This effect is interpreted to be a result of the matching coefficients of thermal expansion of the HTS layer and CrNiMo, resulting in lower residual tensile stress in the HTS layer after processing.

Bending of CORC® cables and wires: finite element parametric study and experimental validation

A conductor on round core (CORC®) cable is composed of several layers of helically wound high-temperature superconducting (HTS) tapes on a round core with the winding direction reversed in each successive layer. The cable is flexible but the flexibility is limited by the critical strain value causing breakage of the HTS layer when this strain level is exceeded. The cables for magnets in fusion reactors experience large mechanical and electromagnetic loads. These loads arise from the cabling, coil manufacturing, cooling, and magnet operation. In order to optimize the manufacture and operating conditions, the mechanical behavior of CORC® cables must be understood for the different relevant loading conditions. The cable configuration with many contact interactions between tapes and the non-linear behavior of the materials during the production and operating conditions makes the modeling challenging. Detailed finite element (FE) modeling is required to account for these complexities. The FE modeling allows an accurate calculation of the stress–strain state (SSS) of the cable components under various loads and avoids time-consuming large-scale experimental optimization studies. This work presents a detailed FE modeling of the 3D SSS in a CORC® wire under bending load. The elastic–plastic properties of the individual tape composite materials and its temperature dependence are taken into account. The FE model is experimentally validated by a multilayer CORC® bending test performed by Advanced Conductor Technologies LLC. A critical intrinsic tensile strain value of 0.45% is taken as the threshold where the individual tape performance becomes irreversibly degraded. The proposed FE model describes the bending test of the CORC® wire adequately and thus can be used to study other types of loads. A parametric study is ongoing with dependent variables to pursue a further optimization of CORC® cables and wires for various applications.

Correlation Between Resistances of Face-to-Face Soldered Joints and Interface Resistance Between Layers in Superconducting Tapes

It is known from the literature that the resistance of soldered joints between two high-temperature superconducting (HTS) tapes is different not only for different tape producers, but also within the same producer for different tape batches. The resistance of joint can be considered consisting of several contributions: Resistances of solder and stabilizing layers covering superconducting layers, and interface resistances between solder and metals and between metal and superconducting layer within the tape. If the same solder is used, and metals are similar for the same manufacturer, different interface resistances between metallic layers and the superconducting layer within the tape can be a reason for the observed scattering in joints resistances. The interface resistance is difficult to measure directly. It can be indirectly evaluated from the measurement of the current transfer length (CTL) from a metal layer into the HTS layer within a superconducting tape. In this paper, we report on a systematic study on CTL and interface resistance between the HTS layer and stabilizing layers for selected commercial rare earth-barium-copper-oxide (R EBCO) tapes, and resistance of face-to-face joints made from the same tapes, in order to find a correlation between the evaluated interface resistance and the resistance of the corresponding lap joints.

Effect of Mechanical Loading on Coated Conductor Tapes Due to Winding Onto Round Cables

The influences of stresses/strains on critical current, I c in REBCO high-temperature superconducting (HTS) tapes wound onto a round cable were investigated. In the first step, the HTS tapes were helically wound on copper tubes with various diameters in the range of 2.95-8 mm. Considerably better results were achieved in case the superconducting layer was positioned during the winding at the inner side of the helix. The cables with tape wound on the tube with a diameter of 6.35 mm were subjected to bending tests with various bending radii in range from 21 to 60.5 mm. The I c degradation as a function of both the tube diameter and bending radius has been assessed from a current- voltage curve, from which the critical current was determined. The I, degradation in all samples was confirmed by microstructural investigation of an HTS layer by scanning electron microscopy.