Conduction-cooled High-temperature Superconducting Coil Research Articles

A Quench Protection Method for HTS Coils Based on Temperature Rise Forecast

When the conduction-cooled superconducting energy storage magnet performs power response, the magnet will generate a temperature rise due to the existence of the ac loss of the magnet and the eddy current loss of the cold-conducting plate. If the temperature rise is too high, the magnet will be quenched, and the magnet will burn out in severe cases. Therefore, it is important to attain real-time temperature rising of high-temperature superconducting (HTS) magnet for its safe operation. However, because the temperature change has a certain hysteresis compared with the current change, the temperature rising of the HTS magnet cannot be detected immediately by temperature sensors. In this article, a quench protection method based on temperature rise forecast was proposed, through which the temperature rising of HTS coils cooled by conduction can be attained immediately. The experimental results proved that the quench protection method based on temperature rise forecast could protect conduction cooled HTS coils under overcurrent or poor refrigeration conditions.

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.

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.

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.

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.

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.

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.

A study on I–V characteristics of conduction-cooled HTS coil

A conduction-cooled high-temperature superconducting (HTS) coil consisting of two double pancake coils was fabricated. The current–voltage (I–V) characteristics of HTS coil were obtained at different temperatures by using GM cryocooler. Using OPERA 2D, magnetic field (Br) perpendicular to the surface of the tape at each layer of HTS coil was obtained. In order to compare with the measured values, I–V curve of the HTS coil was simulated, in the basis of the magnetic field dependence of the critical current (Ic) in Bi-2223 tapes and index of n-value. Results showed pretty good agreement between the simulated and the measured critical currents at 77.3K and 40K, with a bit difference under simple assumption of index of n value.

Investigation of cooling effects on conduction cooled HTS tape due to high thermal conduction plastics

Local temperature rise due to insufficient cooling is one of the important problems in conduction-cooled high-temperature superconducting (HTS) coils. Assuming two types of plastics having high thermal conductivity as spacers in HTS coils, we measured temperature rises of a conduction-cooled HTS tape under various contact conditions between the plastics and the HTS tape. The cooling performance from the Zylon fiber reinforced plastic to the HTS tape was not so good, since the fibers in the plastic were oriented in one direction. The Dyneema fiber reinforced plastic effectively cooled the HTS tape, because the fibers in the plastic were two dimensions. The possibility to use the plastics with high thermal conductivity as the spacers in the conduction-cooled HTS coils was demonstrated.