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A conduction-cooled superconducting split magnet system with large crossing warm bore has been successfully constructed in our laboratory for material processing applications. The magnet system design was described in a previous paper. The magnet is composed of six NbTi low temperature superconducting coils, which generate 5.5-T central magnetic field and two Bi2223/Ag high temperature superconducting (HTS) insert coils, which generate 2.5-T central magnetic field and assembled in the form of split coil groups. The magnet has a 136-mm split gap to accommodate the crossing warm bore of 100 mm in diameter. The magnet system is cooled by two GM cryocoolers. The initial cooldown takes 9.2 days and the final temperature of the magnet is about 4.0 K. The HTS coils and NbTi coils are to be operated in the driven mode with two independent power supplies, under the operating currents of 200 A (HTS) and 139 A (NbTi), respectively. The magnet is successfully powered up to 8 T with a the ramp time of 290 min. In this paper, the fabrication and test of the superconducting split magnet system are presented.

A passive quench protection design of the 9.4 T whole-body magnetic resonance imaging superconducting magnet is proposed. The design of the coil subdivision with shunt resistors is introduced. The selection of the configuration of the heater network is detailed. The optimization of the geometric parameter of the heater strips and the thickness of the insulation binding outside the heater strips are discussed. A winding sequence that could accelerate the quench propagation in the compensating coil is proposed. The calculation results show that the optimized quench protection design can guarantee the safety of the magnet in the case of any coil as the quench initiation coil.

The Al2O3 machinable ceramic material, which has excellent insulation property and good thermal conductivity at low temperature, was used to make a coil former of a Nb3Sn superconducting coil. Multifilament Nb3Sn superconducting wire was wound on the ceramic former to form a Nb3Sn superconducting coil. Afterwards, the coil was heat treated in vacuum, impregnated, and solidified. Finally, the coil was placed into a special cryostat to exam the critical current of the coil. The test results show that the operation current of the coil is no less than 500 A under 4 T background field. The coil can be charged at ramp rate of 10 A/s without quenching.

A superconducting rotor with the high-speed rotation is driven by the torque generated from stator coils at low temperature in 4.2 K. It is necessary to fill the rotor housing with cold helium gas to remove the heat from the superconducting rotor to the support structure. However, it will bring the drag force from the gas, which has a negative effect on the rotating speed of the rotor. The drag torque of gas to act on the rotating rotor was calculated, and the experiment of deceleration of the rotating rotor was performed. The results show that the angular velocity is decaying with exponential function when the rotor rotates in the gas. It is necessary to calculate rotor deceleration at different gas pressures with different analysis methods.

The quench protection system with the circuit subdivision and heater network for a whole-body magnetic resonance imaging superconducting magnet has been designed. A test magnet with the similar quench protection design was built. Quench experiments for the test magnet have been conducted to verify a quench simulation code that is used to optimize the quench protection design. The fabrication of the test magnet has been demonstrated. The experimental results have been analyzed. The comparison between the experimental data and the calculated results has been discussed. The quench protection method with the heater network proves to work well, and the quench simulation code is proved to be reliable.

Quench protection design is the key task of the manufacture of a superconducting magnet. The analysis of quench propagation of the superconducting coils is very important for the quench protection design. A numerical model based on the control volume method has been built to simulate the quench propagation. An experiment with five test superconducting coils and an ambient coil was performed to verify the numerical code. The quench protection method of using the heater is proved effective on accelerating the quench propagation by both the simulation and the experiment.

A superconducting magnet with the center magnetic field of 10 T and warm bore size of 100 mm was designed and fabricated. The magnet coils were fabricated with NbTi and Nb3Sn superconducting wires. The NbTi and Nb3Sn coils were manufactured with inter-winding support structure. A cryostat was designed and fabricated to accommodate the magnet with an outer diameter of 740 mm and height of 560 mm. The magnet system was cooled by a GM cryocooler with cooling capacities of 35 W at the first stage and 1.5 W at the second stage cold head. A pair of Bi2223 bulk high temperature current leads were employed to reduce heat leakage to the magnet. Total weight of the magnet system is about 230 kg. The superconducting magnet can be cooled down to 4.1 K in 90 hours with the cryocooler. The magnet was tested and reached the maximum operating current of 119.4 A, corresponding to a center magnetic field of 10.25 T in the warm bore. No quench was observed during the test process. It shows that the magnet stability is improved with the inter-winding support structure in magnet coil fabrication. The superconducting magnet system has been operating stably for several months. Extensive experiments on magnetic separation have been done on the magnet setup. In this paper, the detailed information of magnet design, fabrication and test are presented.

A conduction-cooled superconducting magnet was designed, fabricated and tested. A two-stage Gifford-McMahon (GM) cryocooler with the second-stage cooling power of 1.5 W at 4.2 K is used to cool the superconducting magnet from room temperature to 4.2 K. The superconducting magnet has the operating current of 105.5 A, the effective warm bore size of 250 mm, the maximum center field of 4.5 Tesla. The superconducting magnet will be used for the material processing. In this paper, the detailed design, fabrication and test are presented.

A superconducting magnet with a center field of 4.5 T cooled by GM cryocooler and operated in the persistent current mode has been designed, fabricated and tested for gyrotron. The superconducting magnet has a warm bore with diameter of 90 mm, the homogenous region with the diameter of 40 mm and length of 230 mm. The ratio of the axial field to the center field located at 180 mm is lower than 88%. In the other special points, the ratios of the radial field to the axial field should be less than from 3% to 11%. The thermally-controlled NbTi/CuNi switch with superconducting joint is connected to the conduction cooled magnet. In this paper, the detailed design, fabrication and test are reported.

A superconducting magnet with the center field of 9.4 T is designed and fabricated for 400 MHz Nuclear Magnetic Resonance. Superconducting coil with NbTi/Cu superconducting wire is employed and cooled by re-condensed liquid helium and the magnet system with the clear-bore of 54 mm. The pulsed tube refrigerator with separated valve is employed to cool the magnet system. The superconducting magnet has an active shield with high pure copper shield to protect during quench of the shielding coil. The paper reports the electromagnetic design, and fabrication is detailed.

In this paper, the technique of ultrasonic flexural vibration assisted chemical mechanical polishing (UFV-CMP) was used for sapphire substrate CMP. The functions of the polishing pad, the silica abrasive particles, and the chemical additives of the slurry such as pH value regulator and dispersant during the sapphire's UFV-CMP were investigated. The results showed that the actions of the ultrasonic and silica abrasive particles were the main factors in the sapphire material removal rate (MMR) and the chemical additives were helpful to decrease the roughness of sapphire. Then the effects of the flexural vibration on the interaction between the silica abrasive particles, pad and sapphire substrate from the kinematics and dynamics were investigated to explain why the MRR of UFV-CMP was bigger than that of the traditional CMP. It indicated that such functions improved the sapphire's MRR: the increasing of the contact silica particles’ motion path lengths on the sapphire's surface, the enhancement of the contact force between the contact silica particles and the sapphire's surface, and the impaction of the suspending silica particles to the sapphire's surface.

In order to increase energy dumping speed, IGBT (insulated gate bipolar transistor) is used in quench protection of a 2 MJ superconducting magnet. 88 resistors, each connects in series with an IGBT, are connected in series-parallel connection to form a controllable varistor. On/off of the IGBTs are controlled by two single chip processors. Energy dumping with nearly constant magnet terminal voltage is realized in training quenches of the magnet. The necessary breaking capacity of the circuit breaker is also lowered. Details of quench protection electric circuit design and experimental results are presented.

A digital quench detection and protection system has been developed for a micro-sized superconducting magnetic energy storage system (SMES). The digital quench detection system based on a high-performance NI-6224 digital acquisition card accepts a standard set of input signal such as magnet induced voltage, current, pressure of cryostat. It also provides improved signal processing for sensitive and exact quench detection and data logging function. Once the superconducting magnet quenches, the quench detection system activates immediately the protection circuit. Some novel protection circuits are developed to dump the electromagnetic energy stored in the magnet; these greatly enhance the dump efficiency by maintaining the magnet terminal voltage constant during the dumping energy process. This paper detailedly describes the system and presents results.

The sapphire substrates are polished by traditional chemical mechanical polishing (CMP) and ultrasonic flexural vibration (UFV) assisted CMP (UFV–CMP) respectively with different pressures. UFV–CMP combines the functions of traditional CMP and ultrasonic machining (USM) and has special characteristics, which is that ultrasonic vibrations of the rotating polishing head are in both horizontal and vertical directions. The material removal rates (MRRs) and the polished surface morphology of CMP and UFV–CMP are compared. The MRR of UFV–CMP is two times larger than that of traditional CMP. The surface roughness (root mean square, RMS) of the polished sapphire substrate of UFV–CMP is 0.83 A measured by the atomic force microscopy (AFM), which is much better than 2.12 A obtained using the traditional CMP. And the surface flatness of UFV–CMP is 0.12 μm, which is also better than 0.23 μm of the traditional CMP. The results show that UFV–CMP is able to improve the MRR and finished surface quality of the sapphire substrates greatly. The material removal and surface polishing mechanisms of sapphire in UFV–CMP are discussed too.

For the goal of superconducting magnet applications in the advanced testing device for high temperature superconducting (HTS) wire and sample coils, a wide bore conduction-cooled superconducting magnet with available warm bore of phi 186 mm and center field of 5 T for the background magnetic field applications was designed and fabricated and tested. A sample cryostat with two GM cryocoolers is inserted in the background magnet. The system allows measurements to be performed in a repeatable and reliable fashion. The detailed design, fabrication and thermal analysis are presented in the paper.

A conduction-cooled superconducting magnet with the warm room of Phi 80 mm and the center field of 0~4 T was designed, fabricated and tested. The magnet can be operated for two different sets of coils which have different homogenous regions with lengths of 150 mm and 250 mm. The homogeneity of magnetic field is about plusmn0.25%. All the homogeneous regions are with the same starting point. The center field is decayed to 1/6-1/7 from the original point to 195 mm. The operating temperature of the magnet is defined at the 5.5 K for the conduction-cooled magnet to take into account the temperature rise during charging current. The thermal equilibrium of the superconducting magnet and cryogenic system is analysed to define ramping rate, operating current and margin of superconducting magnet. The detailed design and fabrication of the superconducting magnet for gyrotron are discussed. The test results show that the superconducting magnet can generate the requirement of magnetic field distribution.

We are carrying out a project to develop a conduction-cooled high temperature superconducting magnet system with an energy storage of 25 kJ. This magnet system will be used to investigate fast discharging performances. The magnet is constructed with 20 double pancakes. Each pancake is wound with Bi2223 tape. The magnet has an outer diameter of 212 mm and a clear bore of 108 mm. Cryogenics for the magnet system is designed to cool the coils with a GM cryocooler together with solid nitrogen. It is expected to operate the magnet system stand-alone. The operating current is designed as 110 A. It can generate a central magnetic field of 3.8 T at a temperature of 20 K. In this paper, the magnet design, coil fabrication and cryogenic system are presented.

The development of a high frequency, high harmonic gyrotron based on a superconducting magnet is described. A conduction-cooled superconducting magnet with the warm room of Phi 80 mm and the center field of 4 T was designed and fabricated. The superconducting magnet has two homogenous regions with length of 150 mm and 250 mm. The homogeneity of magnetic field is about plusmn0.25%. All the homogeneous regions are with the same starting point and the field is decayed to 1/6-1/7 from the front point of the homogeneous region to 195 mm. This paper describes the design and fabrication of the superconducting magnet for Gyrotron

A high temperature superconducting (HTS) magnet for 10 kJ superconducting magnetic energy storage (SMES) system is designed by an improved optimal algorithm and cooled through GM cryocooler. In order to demonstrate the technology, a model HTS magnet made of Bi-2223/Ag tape to fabricate double-pancakes with the inner and outer diameters of 120 mm, 207.1 mm and height of 119 mm was fabricated and tested. The magnet is operating with GM cryocooler at 10 K. We have tested the Bi2223 HTS magnet at various ramping rates with 0.1-5.06 A/s. The experimental results show that the model HTS magnet is operating very stable. In the paper, we report the design of the HTS-SMES and fabrication technique for the model HTS magnet

A 6 T conduction-cooled superconducting magnet was designed, fabricated and tested. The magnet is composed of two coaxial NbTi solenoid coils with identical axial length. Clear bore of the magnet is phi 226 mm. The magnet is installed in a vacuum cryostat with a phi 100 mm room temperature bore. The cryostat is designed in a support frame to be rotatable in a horizontal or vertical direction. A two-stage 4 K Gifford-McMahon (GM) cryocooler is used to cool down the superconducting magnet from room temperature to 4 K. The cooling power of the 4 K cold head is 1 W. A pair of Bi-2223 high temperature superconducting current leads was employed to reduce heat leakage into 4 K cold mass. Total cold mass of the superconducting magnet is about 115 kg. It takes 82 hours to cool down the magnet from 300 K to 4 K directly through the cryocooler. The superconducting magnet reached the designed central magnetic field of 6 T in the warm bore when a 115 A energizing current is applied. The superconducting magnet was stably operating more than 275 hours continuously in full field. Further, a Nb3Sn coil insert to be installed, the magnet can provide the maximum center field of 10 T with effective warm bore of phi 100 mm. In this paper, the detailed design, fabrication and test are presented

Influence of inter-strand electrical and thermal conductivity on stability of multi-strand superconducting cables is studied theoretically based on a 3-strand cable model. The simulation model takes into account transient heat-transfer characteristics between strand surface and helium, inter-strand current sharing, heat conduction as well as variation of thermal physical properties with temperature and magnetic field. Minimum quench energy is calculated both in case of uniform current distribution and in the case of nonuniform current distribution. The results show that the influence of inter-strand electrical conductivity on minimum quench energy increases as the nonuniformity of current distribution increases. Calculations were performed in view of the special situation of Rutherford cables in dipole magnets for particle accelerators. The results demonstrated that differences in inter-strand contact properties could lead to evident different ramp-rate limitation behavior. It is concluded that serious ramp-rate limitation problem could be avoided by keeping inter-strand electrical and thermal conductivity in a proper range.

A new apparatus has been developed to investigate interstrand contact properties of multistrand superconducting cables. The apparatus can measure interstrand electrical and thermal conductance simultaneously. Two NbTi samples were measured and influences of surface coating, contact force and pressing history on contact properties were investigated. These measurements provide essential data for theoretical analyses of stability of multistrand superconducting cable against local disturbances.

Fiber optic sensors for micro-displacement measurement are applied to displacement measurement of superconductors. Output characteristics of a fiber optic sensor were measured at 77 K and 4.2 K. The results show that the linearity between output voltage ratio and displacement is good both at 77 K and at 4.2 K, and the sensitivity at 77 K is higher than that at 4.2 K.

The nuclear magnetic resonance superconducting magnet technology is being developed in our laboratory. Because the upper critical magnetic field of NbTi alloy is not high enough, an Nb3Sn superconductor has to be used in the manufacture of magnets for 500 MHz and above. In this paper, an experimental Nb3Sn superconducting coil was manufactured to study the fabrication technology. The bobbin with Al2O3 ceramic coating is made of stainless steel. A vertical tube type vacuum heat treatment furnace was established for Nb3Sn formation. Heat treatment parameters for the Nb3Sn coil were done at 670°C in vacuum. Vacuum pressure impregnation was done to strengthen the assembly. The Nb3Sn coil, the switch, and the current leads were joined together using superconducting solder matrix replacement method. In the test the maximum current of the Nb3Sn coil was 263 A, corresponding to a central magnetic field of 3.47 T. Persistent current mode operation was also successfully performed.

In order to reliably protect the superconducting MRI magnets as soon as possible, a hybrid protection system is developed. The protection system, which combines the both merits of active and passive protection schemes, includes a quench detection controller, a protection superconducting switch, a capacitor bank, a strip heaters network and a protection circuitry with back to back diode sets and shunting resistance coils. The quench detection controller is employed to detect the start of a quench and to subsequently switch on the capacitor bank to quench the protection superconducting switch. The shunting current in the coil subdivision flows through the shunting resistance coil also to quench the protection superconducting switch. The voltage across the protection superconducting switch energizes the strip heaters. Once a quench occurs in a magnet, the protection system immediately initiates all superconducting coils to quench. The hybrid protection system is successfully applied to a 1.5 T whole-body MRI magnet. This paper describes the details of the hybrid protection system and presents the quench simulation results.

A superconducting switch with an off-state resistance of 10 Ω, operating current of about 67 A, for a NMR magnet in persistent current operation to obtain higher field stability is designed and manufactured. The superconducting switch is solenoid type which consists of four parts: bobbin, superconductor, heater and adiabatic part. And NbTi/CuNi superconducting wire with higher resistance is used and wound on the switch bobbin in a bifilar manner for low inductance and minimum field perturbation. Numerical simulations by FEM method for the switching performance of temperature distribution during heating and cooling cycles are performed. Finally the switching performance tests were carried out and the results show that the minimum heating power is about 0.69 W and the off-time is less than 10 s.

A project to develop a 9.4 T magnetic resonance imaging system is proposed for bioscience research applications. A whole body superconducting magnet system will be manufactured and test in the Institute of Electrical Engineering, Chinese Academy of Sciences (IEE, CAS). This magnet system features a room temperature bore of 800 mm in diameter, helium bath cooing, 9.4 T center magnetic field and passive iron shielding. The magnet is designed with radial layer-winding method. Five coaxial coils will be wound independently and assembled together as the main magnet. Coil length of the magnet is 3000 mm. In the magnet design, current density grading is performed to optimize the magnetic field distribution and stress level in the coil windings. The maximum magnetic field is 9.505 T, corresponding to an operating current of 224.515 A. The total magnetic energy storage is 138 MJ. Detailed magnetic and mechanic structure design as well as structure stress analysis are presented in this paper.

Using graded current density in the design of multilayer-solenoid superconducting magnets is an effective way to save on the total weight of superconducting materials. Following a discussion of the relationship between quench propagation velocity and the geometrical location of the windings involved, the authors propose a mathematical model applicable to both magnets with graded current density and to magnets protected by multishunt resistors. Several numerical examples were calculated using a computer program based on this model. The selection of shunt resistors for protecting the superconducting magnet is also discussed. >