Conduction-cooled High-temperature Superconducting Research Articles

Thermal Design of the Conduction-Cooled High Temperature Superconducting Magnet for Pulsating Magnetic Field

A conduction-cooled high temperature super-conducting (HTS) solenoid is fabricated to produce pulsat-ing 4 T magnetic field for magnetic refrigeration. HTS con-ductor of 245 m is wound as a solenoid with 112 layers each of which consists of 12 turns of the conductor. For a stable ramping operation of the magnet, AC loss inside the wind-ing pack should be effectively rejected to the heat sink. The thermal drains are inserted between the layers and con-nected to the magnet flange which is cooled by a commercial two-stage Gifford-McMahon cooler. They are made of Ox-ygen-Free High Conductivity (OFHC) copper, and has a width of 6 mm and a thickness of 50 μm. Thermal analysis is conducted to confirm the effect of the thermal drain on the AC loss heat rejection. AC loss of the magnet is esti-mated by using T-A formulation (current vector potential T and magnetic vector potential A). The ramp rate of the mag-net and the heat sink temperature are set to 0.1 T/s and 4.5 K, respectively. In the thermal diffusion calculation, each winding layer is considered as the lumped capacitance. In addition, thermal conduction between the adjacent lay-ers and that through the thermal drains are considered. The thermal analysis confirms that the conductor temperature of the winding is maintained lower than the operating limit of 20 K. Based on the results of the thermal analysis, the stable pulsating operation of the conduction-cooled super-conducting magnet is predicted to be feasible and is also confirmed by the experiment.

Increase of Thermal Stability Caused by High Thermal Conduction Fiber and Sheet in Conduction-Cooled HTS Coils

We have examined thermal stability in high-temperature superconducting (HTS) coils, focusing on thermal properties of coil's structural components. As the component materials, high thermal conduction fibers and sheets made from a liquid crystal polymer (LCP) were used. First, we fabricated an LCP fiber reinforced plastic (LCP-FRP), and experimentally estimated thermal conductivity of the LCP-FRP round rod at cryogenic temperature, and made sure the conductivity was high. Next, thermal strain of the LCP-FRP rod from room temperature to 77 K was measured. From the results, the LCP-FRP bar expanded to a longitudinal direction of the bar with the cool down. Finally, we made an experimental apparatus which simulated a part of the HTS coil, and inserted the LCP sheet or a GFRP sheet in the HTS coil model. When the model was heated by a local heater, temperature rise of the model having the LCP sheet was lower than that having the GFRP sheet. From those experimental results, we think that thermal stability of the HTS coil becomes better by using the LCP sheet for non-metallic structural component in the coil.

Analysis of Eddy-Current Loss and its Reduction in a Conduction-Cooled HTS Magnet

Since conduction cooling system is environmental friendly and easy operating, the conduction-cooled method is widely used in cryogenic cooling system of high-temperature superconducting (HTS) magnet. The eddy-current losses are generated in the cooling structures when HTS magnet is charging and discharging or the current in the magnet is alternating. In this paper, a 3D model of analyzing the eddy-current loss was built. The eddy-current losses of a HTS magnet with different cooling structures were calculated. Thermal analysis of the HTS magnet was carried out together with the electromagnetic analysis. The effect of the cooling structures’ geometric parameters on the eddy-current losses was discussed. The methods of reducing the eddy-current loss were proposed. The analysis results demonstrate that the optimization of the geometric parameters would greatly benefit the reduction of the eddy-current losses.

Study on the Thermal Characteristic of a 150 kJ/100 kW Conduction-Cooled HTS Magnet

A 150 kJ/100 kW conduction-cooled HTS superconducting magnetic energy storage magnet, which consists of 17 double pancake coils and operates at 20 K, was developed and manufactured in the Huazhong University of Science and Technology. To evaluate the thermal characteristic of the magnet, this paper conducts a series of experiments and carries out the loss analyses. When the maintaining time and peak value of the flat top current in the magnet are constant, the maximum temperature rise will first decrease and then increase with the continuous augment of the current ramp rate, and it means that there is a valley value for the maximum temperature rise during an operational cycle time including charging, standby, and discharging stages. Similarly, the total losses, which include the ac heat loss and the resistive loss caused by the joints between every two double pancakes and extra resistance of the high temperature superconducting (HTS) coils, will fall with the increase of the ramp rate to its valley value, and then it will increase with the ramp rate. Experimental results show that the temperature variation of the magnet is in keeping with the heat loss variation.

An Effective Cryostat Design of Conduction-Cooled HTS Magnets for a 300-kW-Class Superconducting Induction Heater

A superconducting magnet is operated under cryogenic conditions. To effectively design a cryostat for high-temperature superconducting (HTS) magnets, thermal loads—including conduction and radiation—should be minimized. These heat loads are closely related to the shape of both cryostat and HTS magnets, metal current lead, and cooling method used. A 300-kW-class superconducting induction heater (SIH) is being developed to design an effective cryostat. This paper presents two different cryostat designs (with different shapes) for conduction-cooled HTS magnets. The thermal stability of the HTS magnet was analyzed by considering heat generation and temperature increase and how these were related to quench phenomena. The cooling characteristics of single- and two-stage cryocoolers were analyzed and compared using the finite-element method. The study results will be applied to the fabrication of the 300-kW-class SIH.

Influence of High-Thermal-Conductivity Plastic With Negative Thermal Expansion Coefficient on Cooling Performance in Conduction-Cooled HTS Coils

We evaluated an insulative-high-conduction plastic (IHCP) sheet, which had high thermal conductivity perpendicular to its plane, as an electrical insulation sheet in a high-temperature superconducting (HTS) coil. Thermal conductivity of the IHCP was measured at cryogenic temperatures and was found to be 20–40 times higher than that of a glass-fiber-reinforced plastic (GFRP). Next, the thermal strain of the IHCP was measured, and a negative thermal expansion coefficient was found along the fiber direction. The IHCP expanded to produce a strain of approximately 0.4% during cooling from room temperature to liquid nitrogen temperature. Finally, a short HTS tape was sandwiched by the IHCPs and was quenched with a local heater placed on the tape. The time required for quenching with the IHCP was several times longer than that with the GFRP. From numerical simulations of the quench tests, the IHCP's thermal contact resistance with the HTS tape was estimated to be approximately 35–40% lower than that for the GFRP. We think the IHCP is effective for increasing the thermal stability of an HTS coil.

Study on Thermal Measurement Method of AC Loss in a Conduction-Cooled HTS Coil

The miniaturization of a synchrotron accelerator can be realized by applying a conduction-cooled high-temperature superconducting (HTS) bending magnet with a cryocooler. A variation of an exciting current through the HTS magnet generates the ac loss, and the ac loss causes a temperature rise of the HTS magnet. AC loss in HTS coils is typically measured using the electromagnetic method or thermal measurement method. However, the measurement method of the ac loss of the conduction-cooled HTS coil under non-sinusoidal current excited has not been established because of the very large inductive voltage of the coil, the very low power factor, and no refrigerant liquid or gas. This study reports a novel thermal measurement method of ac loss in a conduction-cooled HTS coil using a heater and a cryogenic temperature controller. The validity of the proposed thermal measurement method was verified by measuring the loss in a copper coil which was cooled by conduction cooling with a 4-K Gifford-McMahon cryocooler. The measured electrical loss in the coil was good agreement with analytical one. Moreover, the validity of our method was verified by measuring the ac loss in a conduction-cooled small double-pancake HTS coil wounded by a rare-earth barium copper oxide (REBCO) tape conductor at 20 K. The measurement results of ac losses in the HTS coil show that our proposed measuring method is very useful for evaluation of the ac loss in the conduction-cooled HTS coil.

Fast Charging and Thermal Stability Improvement of a Conduction Cooled HTS Coil Wound by Graphene Oxide Coated HTS Tape

The equivalent thermal conductivity of the conduction cooled high-temperature superconducting (HTS) coil is one among important design parameters. It is possible to rapidly rise the hot spot temperature of HTS coil in quench, because the normal propagation velocity of HTS wire is lower than 100 times one of low-temperature superconducting wire such as NbTi or Nb3Sn alloy, and the conduction cooled HTS coil is operated on vacuum adiabatic condition. Generally, polyimide tapes for electrical insulation are inserted between layers of HTS coil but initial cool-down time due to low equivalent thermal conductivity of HTS coil get longer and the thermal runaway current in quench is decreased. Recently, the researches on HTS coil without insulation to improve the equivalent thermal conductivity have been performed. However, because HTS coil without insulation are short-circuit, the charging time to reach the designed magnet flux density of HTS coil is too long. In this paper, turn to turn insulation of HTS coil wound by HTS tapes with graphene-oxide coated Cu stabilizer is presented. The graphene oxide has relatively high resistivity and can be coated with a thickness of less than 10 μ m. To confirm the effect of the proposed method, two HTS coil samples that were insulated with polyimide tape and coated with graphene oxide were fabricated. Based on the results of thermal analysis and characteristics tests, the equivalent thermal conductivity, initial cool-down time, critical current, and thermal runaway current of HTS coils with different turn to turn insulation were compared.

Numerical Simulation and Experimental Validation of a Cooling Process in a 150-kJ SMES Magnet

The Superconducting Magnetic Energy Storage (SMES) system is expected to keep the power balance and increase the stability in an electric power system. During operation, heat may be dynamically generated in the superconducting magnet when the SMES system exchanges energy with the electric grid due to ac losses in superconductors and eddy losses in metal parts. To design a suitable cooling system for an SMES system, it is necessary to find out the heat transfer process from a superconducting magnet to a cryorefrigerator. This paper presents a model to analyze the heat transfer in a 150-kJ/100-kW conduction-cooled high-temperature superconducting (HTS) SMES system. The simulation results were identical with the experimental data. Thus, it is verified that the analysis method proposed here is useful for designing a cooling system for a conduction-cooled HTS SMES system.

Ramping Operation of the Conduction-Cooled High-Temperature Superconducting Magnet for an Active Magnetic Regenerator System

An active magnetic regenerator (AMR) system requires fast ramp rate of magnetic field variation. The magnetic field change enables a magnetic refrigerant to create magnetocaloric effect for magnetic refrigeration. In this paper, a conduction-cooled high-temperature superconducting (HTS) magnet has been thermally analyzed and tested for an AMR system. A GdBCO conductor insulated with polyimide tape was wound by dry winding and standard double-pancake coil method. Stycast 2850FT was applied to the edge of the tape conductor so that the cooling plates, which were located at the top and bottom of the coil, should effectively cool the magnet. The whole thermal bridging between the magnet and a two-stage GM cryocooler is analyzed by the numerical simulation and a Joule heating experiment of the HTS magnet at normal state. The inductance of the HTS magnet was measured as 350 mH. The HTS magnet is intended to be operated with maximum 1 T/s at 20 K, cooled by the cryocooler. A solenoid switch and an external dump resistor were employed in order to discharge the HTS magnet. A continuous ramping operation test was conducted with a 60-A peak current and a maximum 20-A/s ramp rate by 3-kW dc power supply. This paper describes the experimental results of the ramping operation of the HTS magnet for an AMR system, and the technical issues on the results are discussed.

Thermal Stability of Conduction-Cooled HTS Magnets for Rotating Gantry

Carbon ion cancer therapy is becoming more widespread due to its high curative effects and low burden on patients. In particle cancer therapy, it is desirable that the treatment beam irradiates the tumor from different directions in order to reduce the dose on normal cells. A rotating gantry is a suitable apparatus for meeting this requirement. Since the irradiation device orbits a patient inside the gantry, the beam can irradiate the tumor from any direction without changing the posture of the patient. Rotating gantries are already commonly used for proton cancer therapy. On the other hand, those designed for carbon ion cancer therapy have not yet been adopted because they are too large and heavy for installation in general hospitals. An R&D project to reduce the size of a rotating gantry for carbon ion cancer therapy by applying high-temperature superconducting (HTS) magnets is now in progress. It is difficult to use a coolant for the magnets mounted on the rotating gantry because they are rotating. Therefore, conduction cooling should be employed. On the other hand, the magnetic field generated by the magnets of the rotating gantry should be changed depending on the energy of the carbon ion beam in raster-scanning irradiation, and there is some possibility that the carbon ion beam will collide with the coils. This causes anomalous thermal inputs, such as those due to ac loss and beam loss, resulting in the problem of poor thermal stability of the conduction-cooled HTS coils heated by such thermal inputs. Therefore, it is important to estimate the thermal stability of the conduction-cooled HTS coils correctly. In this paper, a saddle-shaped HTS coil was designed, and the thermal runaway current of the HTS coil was numerically simulated.

Stability and quench behaviors of conduction-cooled 2G HTS coil co-wound with SS tape

We fabricated a second-generation (2G) high-temperature superconducting (HTS) model coil cowound by a 2G HTS tape and a stainless steel (SS) tape. The inner diameter of the model coil was 120 mm, and the number of turns was 100. A 2G HTS tape 12 mm in width manufactured by SuNAM Co., Ltd. in South Korea was used. The total length of the coil was 44 m, and the critical current $(I_{{c}})$ of the model coil was about 174 A in liquid nitrogen. The minimum quench energy (MQE) was 19.1–22.6 J, and the normal zone propagation velocity (NZPV) was 2.4–2.7 mm/s at 0.977 $I_{{c}}$ in liquid nitrogen. MQE was 2.85 J at 0.86 $I_{{c}}$ , and NZPV was 2.7 mm/s at 77 K in conduction cooling. A 2G HTS coil cowound with an SS tape was robust in the event of the quench induced by heater and overcurrent flow above critical current.

Development of a Movable HTS SMES System

A 600-V/150-kJ/100-kW conduction-cooled high-temperature superconducting (HTS) magnetic energy storage (SMES) system is developed. In this paper, the configuration of the HTS SMES is introduced. The magnet is a solenoid type, which uses two kinds of HTS tapes, and cooled to about 20 K. A series of laboratory experiments and field tests are carried out to evaluate the performance of the SMES system, including the current-carrying ability of a magnet, the active and reactive power exchange capability between the SMES and an alternating-current power system, power oscillation damping, the improving electrical energy quality, etc. The results show that the SMES system meets the design requirements and can maintain a long-term stable operation in a power system.

Development and parametric study of the convection-type stationary adiabatic demagnetization refrigerator (ADR) for hydrogen re-condensation

The adiabatic demagnetization refrigerator (ADR) system in this paper is composed of a conduction-cooled current cycling high-temperature superconducting (HTS) magnet system, a magnetic bed assembly, its heat exchange parts and an auxiliary precooling stage (a commercial GM cryocooler and a liquid nitrogen vessel). The whole magnetic refrigeration system including the conduction-cooled HTS magnet is cooled by the precooling stage to absorb the rejection heat of the ADR cycle. The packed bed type magnetic bed consists of tiny irregular powders of Dy0.9Gd0.1Ni2 enclosed in a thin walled stainless steel container (22.2mm in O.D., 0.3mm in thickness and 40.0mm in height). The precooled heat transfer fluid (helium) travels through the magnetic material when heat rejection is required; otherwise the helium stagnates within its pores (pseudo-adiabatic process). Flow of the heat transfer fluid substitutes for the function of a traditional heat switch, creating, essentially, a forced-convection type heat switch. The magnetic bed assembly is periodically magnetized and demagnetized at the center of the conduction-cooled HTS magnet which can stably generate both strong and alternating magnetic field from 0T to 3.0T (0–130A) with an average ramp rate of 0.24Ts−1. The cooling capacities of the ADR system at 20K which is the normal boiling point (NBP) of hydrogen, are 11.1Jcycle−1, 6.3Jcycle−1 and 1.9Jcycle−1 when the temperature spans are 1K, 2K and 3K, respectively. We describe the detailed construction of the ADR system and discuss the test results with the operational parameters (the entrained helium pressure, the mass flow rate of helium and the operating temperature span) in the 20K region.

Design of Conduction-cooled HTS Coils for a Rotating Gantry

Carbon ion cancer therapy is becoming more widespread due to its high curative effects and low burden on patients. Carbon ions are delivered to patients through electromagnets on a rotating gantry.A rotating gantry is attractive because it allows carbon ions to irradiate a tumor from any direction without changing the posture of the patient. On the other hand, because of the high magnetic rigidity of carbon ions, the weight of a rotating gantry for carbon cancer therapy is about three times higher than one for proton cancer therapy, according to our estimation. The use of high-temperature superconducting (HTS) magnets has been considered for reducing the size of the rotating gantry for carbon cancer therapy. The target weight is 200 t or less,which is equivalent to the weight of a typical rotating gantry for proton cancer therapy.In this study, the magnet layout of the rotating gantry and the superconducting magnets were designed from the viewpoint of beam optics.When applying high-temperature superconductors to accelerator magnets, there are some issues that should be considered, for example, the influence of tape magnetization and manufacturing accuracy on the field quality, the thermal stability of the conduction-cooled HTS coils under an alternating magnetic field, and methods to protect the coils from thermal runaway caused by an anomalous thermal input such as that due to beam loss. First, the thermal stability of the conduction-cooled HTS coils was simulated numerically, and the thermal runaway current was calculated in a static situation.

HTS Current Leads for the NHMFL Series-Connected Hybrid Magnet

A design for the current leads for the National High Magnetic Field Laboratory series-connected hybrid magnet is described. The leads are comprised of a nitrogen vapor cooled resistive heat exchanger section, operating between 296 and 79 K. The heat exchanger uses the “pierced jelly roll” lead technology proven at the National High Magnetic Field Laboratory. A conduction-cooled high-temperature superconducting (HTS) section, operating between 79 and 6 K, uses Sumitomo Type G Bi-2223/Ag5%Au conductors, stacked and soldered into a stainless steel and copper shunt brazement. The leads are located in close proximity to the magnet, and the magnitude of the fringe field on the HTS tapes is 0.37 tesla. The configuration of the HTS section is determined by careful analysis of the self-field and fringe field at the warm end of the HTS section, and by findings from a program of conductor testing.

De-Lamination Characteristics of Coated Conductor for Conduction Cooled HTS Coil

According to the continuous development of coated conductors and compact cryocoolers, research and development efforts for High Temperature Superconducting (HTS) magnets are increasing using conduction cooling method. To increase the cooling efficiency and thermal stability of the HTS magnet, the coated conductor is wound by wet-winding or epoxy impregnating in vacuum after dry-winding. Due to the large Lorentz force and thermal contraction, stress analysis of the composite material, which is composed of HTS conductor, insulation layer and epoxy layer, is necessary to assure the mechanical stability of the HTS magnets. Mechanical strength for a/b axis, which is parallel to the conductor surface, is usually strong enough to endure the large tensile stress due to the tough substrate material. However, c-axis strength, which is perpendicular to the conductor surface, is not strong enough to ensure the large Lorentz force. The de-lamination of the multi-layered HTS tape in a coil structure can occur and the results were previously reported. Therefore, the test data for allowable c-axis strength is necessary to design the mechanical stability of the HTS coil. This paper describes the experimental results for the c-axis tensile strength of various coated conductors. The results show the wide divergence of the c-axis tension force from 18 MPa to 53 MPa. Through the FEM analysis for multi-layered structure of the HTS tape, concept design for HTS tape of enhanced c-axis strength is suggested.

Development of a 5.1 T conduction-cooled YBCO coil composed of a stack of 12 single pancakes

We fabricated and tested a 5T-class conduction-cooled high-temperature superconducting (HTS) coil composed of a stack of 12 single pancake coils wound with YBCO-coated conductors. The length of each single pancake coil was 25m, and the inner diameter of the coil was 50mm. The voltage–current characteristics were measured in liquid nitrogen and under conduction-cooled conditions at 10–60K. At 10K, the central magnetic field of the 12 stacked pancake coils was as high as 5.1T at 305A. We also tested the coil operation in a 4T background magnetic field.

Thermal Conductivity of High-Strength Polyethylene Fiber and Applications for Cryogenic Use

The local temperature rise of the tape is one of instabilities of the conduction-cooled high temperature superconducting (HTS) coils. To prevent the HTS tape from locally raising a temperature, high thermal conductive fiber reinforced plastic was applied to coil bobbin or spacer for heat drain from HTS tape. The thermal conductivity of ramie fibers increases by increasing orientation of molecular chains with drawing in water, and decreases by chain scission withγ-rays irradiation or by bridge points in molecular chains with vapor-phase-formaldehyde treatments. Thermal conductivity of high strength ultra-high-molecular-weight (UHMW) polyethylene (PE) fiber increases lineally in proportion to tensile modulus and decreases by molecular chain scissions withγ-rays irradiation. This result suggested the contribution of the long extended molecular chains due to high molecular weight on the high thermal conductivity of high strength UHMW PE fiber. Thermal conductivity of high strength UHMW PE fiber reinforced plastics in parallel to fiber direction is proportional to the cross sectional ratio of reinforcement oriented in the conduction direction. Heat drain effect of high strength UHMW PE fiber reinforced plastic from HTS tape is higher than that of glass fiber reinforced plastic (GFRP) and lower than that of aluminum nitride (AlN). In the case of HTS coil, the thermal stability wound on coil bobbin made of high strength UHMW PE fiber reinforced plastic is good as that of AlN, and better than that of GFRP.

Thermal Stability of Conduction-Cooled YBCO Pancake Coil

When a high-temperature superconducting (HTS) coil is operated using a conduction-cooling technique, the coil should be impregnated with epoxy resin because the coil is not sufficiently cooled without it. On the other hand, it is important that the coil should have no damaged area in the winding because the damaged area would generate heat locally and the coil would eventually quench. However, whenever we fabricated an impregnated HTS coil wound with YBCO tapes and evaluated its V-I characteristics, the n-value of the coil was much lower than we expected, indicating that the critical currents of some areas in the winding drastically decreased. Therefore, we have started to improve the performance of an impregnated HTS coil wound with YBCO tapes. In this paper, we investigated the cause of the degradation and found that the degradation did not occur when we decreased the radial tensile stress in the windings. Then we fabricated four single-pancake coils, stacked them, and tested them in a conduction-cooled condition. The measured V-I curves were in good agreement with the calculated ones, suggesting that we successfully developed a technique of fabricating an impregnated HTS coil wound with YBCO tapes with no degradation. We also measured thermal runaway currents of a conduction-cooled HTS coil composed of two single-pancake coils wound with YBCO tapes and numerically simulated the thermal properties by using a three-dimensional heat conduction equation in order to study the thermal stability of the YBCO coil. The measured thermal runaway currents were in good agreement with the calculated ones.