A novel heater-triggered HTS semi-persistent current switch with high switching-off resistance

The REBCO (where RE = Gd) tape used in this study is the TPL4600 type manufactured by THEVA. The tape has a layered structure of Cu stabilizer (20 μm), Ag layer (1 μm), GdBCO film (∼3 μm), MgO buffer layer (∼3.5 μm), Hastelloy-C276 (100 μm). The structure of the tape is shown in figure 1, and the detailed parameters of the tapes are shown in table 1. In addition, copper stabilizers were applied through an electroplating process.

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The processing steps of the HTS tape are shown in figure 2 and can be divided into three steps. In the first step, Kapton tapes are used to cover the copper layers that need to be retained, leaving the rest of the copper layers uncovered. As shown in figure 2, the superconducting side of the HTS tape is covered with two segments of Kapton tapes, exposing the center and periphery, and the non-superconducting side is covered entirely with a Kapton tape. In addition, the width of the Kapton tapes needs to be slightly narrower than the width of the HTS tapes to expose the copper layer at the edges. In this study, 5 mm wide Kapton tapes were used for the 6 mm wide HTS tape. Then, a FeCl₃ alcohol solution is used to remove the uncovered Cu layer on the tape surface and edges. In the second step, a mixture of NH₃·H₂O and H₂O₂ is used to remove the uncovered silver stabilizing layer of the tape. The critical current of HTS tape subjected to chemical etching can remain unchanged [15]. The FeCl₃ alcohol solution is prepared by mixing and dissolving FeCl₃ powder with a mass fraction of 98% and alcohol solution with a mass fraction of 99.7% at a ratio of 1 g:20 ml. The NH₃·H₂O and H₂O₂ mixed solution is prepared by mixing NH₃·H₂O with a mass fraction of 28% and H₂O₂ with a mass fraction of 30% [16]. The two steps are both cost-effective and highly efficient for removing Cu and Ag materials that maintain electrical connections between superconducting and non-superconducting surfaces.

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The buffer layer of the HTS tape may be wrapped with the superconducting layer on the edges during mass production. In order to prevent current conduction on superconducting and non-superconducting sides at temperatures below the superconducting critical temperature through the superconducting layer wrapped on the edges, it is necessary to remove the superconducting layer that may be covered on the edges of the HTS tape. It is achieved in the third step by reacting them with an acetic acid solution. In this step, it is necessary to cover the GdBCO layer that has been exposed in the middle section of the tape with a Kapton tape to prevent this section from being corroded by acetic acid, and then the tape is put into the acetic acid solution to react. The reaction removes the GdBCO that may be wrapped around the buffer layer edges of the HTS tape. As a result, the electrical connection between the two surfaces of the tape is cut off, ensuring that almost all of the current flows on the GdBCO layer. Due to the buffer layer beneath the GdBCO layer being essentially insulating and the relatively high resistance of the GdBCO layer when not in the superconducting state, the entire semi-PCS shows a high switching-off resistance.

Figure 3 shows the semi-PCS configurations for this study. The dark part in the middle of the tape is the GdBCO layer, where Cu and Ag layers have been removed. The shadow part around the tape is the buffer layer, where Cu, Ag and GdBCO layers have been removed. The critical temperature of the HTS tape is approximately 93 K. We defined that semi-PCS 'off' when it is at 110 K. A type E thermocouple in the center of the semi-PCS is used to collect temperature data, which is connected to the temperature controller to determine whether it is 'on' or 'off.' A nickel–chromium wire (Ni₈₀Cr₂₀) is used as the heating source and is connected to a DC power supply. The nickel–chromium wire transfers the heat generated to the HTS tape by conductive heat transfer. Voltage taps were installed near the ends of the foam to monitor the semi-PCS resistance during the heating experiments. To improve the heat transfer efficiency, the HTS tape and the nickel–chromium wire were uniformly coated with cryogenic thermally conductive grease (i.e. Apiezon N grease). The HTS tape and the nickel–chromium heater were closed by two extruded polystyrene (XPS) foams with poor thermal conductivity to create a 'heat trap' to store the heat. The XPS foams were sealed by low-temperature adhesive. We placed the semi-PCS unit in a large foam container for 77 K operation in liquid nitrogen (LN₂).

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Figure 4 shows the general arrangement of our insulated coil with designed high switching-off resistance semi-PCS, which was used for the excitation experiments to verify the effectiveness of the semi-PCS. The HTS coils were wound using HTS tapes with an average critical current of 198 A. The HTS tape is 6 mm in width, 180 μm in thickness, and the total length of the coil is 128 m. In this study, Kapton tapes with a width of 6 mm and a thickness of 50 μm were used for insulation. After the HTS coil was wound, its inductance was measured to be 5.6 mH using a TH2811D LCR Meter. Note that the HTS coil and semi-PCS are connected via two soldered joints. The superconducting side of the semi-PCS tape and the superconducting side of the coil tape are soldered by a thin layer of low-temperature soldering tin between them to form a joint with a resistance of about 10 nΩ. Since superconducting joint technology was not adopted in making this HTS coil, the coil operates in a semi-persistent mode. To ensure no accidental burning of the semi-PCS during switching, we chose a liquid nitrogen bath (LN₂, 77 K) as the experimental environment. An Agilent 34 420 A digital nano voltmeter was connected to both ends of the coil to measure the voltage, including the voltage generated by the semi-PCS and the joints. The source current, controlled by a LabVIEW program, was generated by an Agilent 6680 A power supply. The magnetic field was monitored using a Gaussmeter (Lake Shore Model 425 with a resolution of 0.01 mT) with a Hall sensor (Lake Shore HGCA-3020) inserted in the center of the coil.

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The HTS tape specimen subjected to a three-step treatment described in section 2.2 is shown in figure 5(a). The length of the specimen is approximately 17 cm, with an exposed GdBCO layer measuring around 3 cm in length and 4.5 mm in width. In order to examine the microstructures of the treated tape, a section was taken out from it, shown in figure 5(b), and observed under scanning electron microscopy (SEM, NOVA NanoSEM 230). Figure 5(c) illustrates the three areas (A, B, and C) at the tape's edge. To further verify the elemental composition of each area, energy dispersive spectroscopy (EDS) was used to analyze the elemental compositions. As shown in figure 6, the tape's edge layer area A has the main composition of Ni, Cr, and Mo, so it is the Hastelloy substrate layer. Area B is the main composition of Mg and O, so it is the buffer layer, and area C is the copper layer. The results show that the copper, silver and superconducting layers have been clearly removed, which can prevent current flow between the superconducting and non-superconducting sides of the tape. This guarantees that the semi-PCS has high resistance in off-state.

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In order to determine the electrical characteristics of the HTS tapes treated in the three steps mentioned in section 2.2 and assess the reliability of the production process, we fabricated a semi-PCS sample. It was then subjected to three cycles of heating up to 110 K and cooling down to 77 K. The critical current of the semi-PCS was measured using the four-probe method. Figure 7 shows representative I–V curves for the semi-PCS and the original tape used as a reference. The n-value of the treated tape did not change compared to the original tape, but the critical current reduced by approximately 26%. This reduction can be attributed to the shorten in the width of the superconducting layer after etching with acetic acid, from 6 mm to around 4.5 mm. So, the reduction of the critical current can be considered reasonable, indicating that the chemical etching in steps 1, 2 and 3 did not cause damage to the GdBCO layer of the HTS tape. It is necessary to point out that even though the critical current of the tape decreases, the semi-PCS is typically located far from the center of coils and experiences a relatively weak magnetic field, so the critical current of the tape for making semi-PCS would be much larger than the critical current of the HTS coil. Furthermore, the semi-PCS exhibited no critical current decay across multiple warming and cooling thermal cycles, suggesting it suitability and reliability for applications in the HTS magnets.

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The resistance of the semi-PCS in the off-state was tested subsequently. Figure 8(a) shows the transient response of the prototype semi-PCS with the heater submerged in the LN₂ bath. Applying 1.67 W of power to nickel–chromium wire in the semi-PCS required a hold time of about 2 min to open it. The temperature measured by thermocouples during the test was about 111 K. By soldering two voltage leads, the resistance of the semi-PCS at 111 K can be measured to be 1.18 Ω, which is much larger than the resistance in the common heater-triggered semi-PCS [8, 13, 17], which are normally in the mΩ level. The change of resistance exhibits a delay during both the warming-up and cooling-down stages. Specifically, the resistance begins to change when the temperature reaches around 100 K during warming up, while it rapidly returns to zero resistance during cooling down. The delay is caused by the thermocouple being in contact with the nickel–chromium heater (i.e. the thermocouple does not contact to the HTS tape directly). This contact may lead to a higher measured temperature than the actual temperature of the HTS tape during the warming-up stage.

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Furthermore, a semi-PCS of the same size as the previous one using the same THEVA tape without etching was also made, and the voltage taps and heater arrangements were the same. Figure 8(b) shows the I–V curve of this semi-PCS at 111 K. The voltage is linearly proportional to the current, indicating that the semi-PCS is resistive. By determining the slope of the line, we can calculate the resistance of the semi-PCS made of the unetched THEVA tapes at 111 K as 0.28 mΩ. So, the switching-off resistance of the semi-PCS made of etched tapes in the off-state is improved by over 4000 times compared to that of unetched tapes.

Figure 9 shows the equivalent circuit model corresponding to this experiment. I_op is the source current supplied by the DC power supply; R_sc and R_PCS denote the coil resistance and the semi-PCS resistance, respectively; the voltages generated by R_sc and R_pcs are described by the E-J power law equation; I_PCS , I_sc , L_sc, and R_joint are the currents in the semi-PCS, the currents charging the coils, the coil inductance, and the joint resistance, respectively. The voltage between points A and B is defined as U_PCS.

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Figure 10 shows the experimental and simulation results of insulated coil excitation. The excitation rate is set to 1 A s⁻¹, and the excitation target current is 20 A for test. At the beginning of the charging process, since the impedance of the semi-PCS is much larger than that of the coil, only a minimal amount of I_op flows through the semi-PCS, and most of the I_op flows into the coil. The distribution of the currents in the semi-PCS and the coil is inversely proportional to the ratio of their impedances. After about 20 s, the I_PCS and I_sc stabilize. The observed voltage across the coil of 5.7 mV during charging corresponds to the calculated coil inductance multiplied by the current ramp rate, and the measured magnetic induction of 14.5 mT at a current of 20 A is within 1% different from the expected value. The difference between the experimental and simulation results may be due to measurement errors because I_PCS is calculated from U_PCS, and I_sc is calculated by subtracting I_PCS from I_op. The experiment is used to verify the validity of the high switching-off resistance semi-PCS proposed in this paper and the correctness of the simulation model.

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In order to further describe the advantages of the semi-PCS proposed in this paper, the simulation model mentioned above is utilized on a large insulated coil. The data used in this study is obtained from [18], where the HTS insulated coil was constructed by connecting six single pancake coils in series. Each coil consists of 335 turns, resulting in a total of 2010 turns and an inductance of 1.52 H. The excitation target is set to 125 A for safety. Figure 11 shows the excitation results of the proposed semi-PCS and the conventional PCS for generating the same power dissipation. The results show that when both the PCSs generate 0.19 W power dissipation, the semi-PCS proposed in this paper takes only 400 s to complete the excitation, and the excitation speed reaches 0.31 A s⁻¹, while the conventional PCS takes 5000 s, and the excitation speed can only reach 0.025 A s⁻¹. Notably, the heater-triggered PCS requires continuous heating to sustain its off-state. Therefore, increasing the speed of excitation can minimize heat input, effectively reducing the thermal load of the magnet system.

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In addition, the size of the semi-PCS in this paper is also much smaller than the PCS in [18], which uses a 1300 mm HTS tape, while only a 170 mm HTS tape is used in this paper, and the tape encapsulated in the adiabatic foam is only 50 mm. The smaller PCS implies a minor enthalpy change from 'on' to 'off,' which can shorten the switching time, which is also crucial for the efficiency of operations of the HTS magnets.

Advantages of this proposed semi-PCS include (1) a simple fabrication process, (2) very high resistance in the off-state (i.e. ohmic level), and (3) minimizing the heat burden while charging the coil. Nevertheless, the semi-PCS proposed in this paper has its limitations. It requires a high level of tape manufacturing process. Some tapes have defects in the buffer layer [19], which can lead to current flow from the superconducting side to the non-superconducting side through buffer layer and cause the switching-off resistance of the semi-PCS to be dominated by the resistance of the Hastelloy layer. In addition, a parallel protection resistor alongside the semi-PCS is still necessary in practical applications, which helps to release the energy in the magnet coils in case of an unexpected quench in the semi-PCS, enhancing the safety of the magnet's closed-loop operation [20].