Conduction-cooled Superconducting Magnet System Research Articles

Optimization of thermal link for a conduction-cooled superconducting magnet system

The thermal link between the second stage of cryocooler and the superconducting magnet has a significant influence on the initial cooldown time of conduction-cooled superconducting magnet. In this paper, a numerical investigation is carried out on the initial cooldown characteristics of a superconducting magnet system used flexible copper strap as the thermal link. The optimal shape ratio (cross-sectional area to length) of the copper strap is obtained to minimize the cooldown time. The results show that optimal shape ratio decreases with increasing the length. In addition, the effect of copper purity on the cooldown time is little at the optimal shape ratio or at RRR (Residual Resistive Ratio) > 100.

Experimental investigation of a serial-parallel configuration helium pulsating heat pipe

Cryogenic pulsating heat pipe with the features of exceptional heat transfer performance and easy manufacturing is a good candidate to effectively transfer the cooling power energy in the conduction-cooled superconducting magnet system. In this work, a new type of serial-parallel 24-turn configuration pulsating heat pipe with helium as working fluid has been investigated experimentally under various working conditions by changing heat loads, inclination angles, and filling ratios. Experimental results demonstrate that for this pulsating heat pipe, working in vertical orientation is more appropriate. Additionally, the optimum filling ratio changes with the heat load input. The maximum effective thermal conductivity is 12357 W/(m∙K), obtained at the inclination angle of 90° and the filling ratio of 66.1%. The new configuration demonstrates high thermal performance well above the critical point of helium. At the filling ratio of 94.2% and the vertical position, this pulsating heat pipe can transport up to 25 W heat load without reaching dry-out occurrence. The influences of the number of turns and tube structure are discussed by comparing the serial-parallel 24-turn helium pulsating heat pipe with the 4-turn helium pulsating heat pipe and the serial 24-turn helium pulsating heat pipe with the same dimensions we tested previously. The result shows that the serial-parallel 24-turn helium pulsating heat pipe is more efficient concerning heat transfer capacity and thermal resistance. Increasing the number of turns can reduce the temperature gradient of the helium pulsating heat pipe, and changing the tube structure from series to series–parallel can greatly improve the heat transfer limit of the helium pulsating heat pipe. Moreover, another novel parallel 24-turn helium pulsating heat pipe is proposed. According to the theoretical analysis, The parallel 24-turn helium pulsating heat pipe could reach a higher maximum effective thermal conductivity, whereas the serial-parallel 24-turn helium pulsating heat pipe is outstanding in heat transfer capacity.

A Conduction-Cooled Superconducting Magnet System - Design, Fabrication and Thermal Tests

A conduction-cooled superconducting magnet system with an operating current of 105.5 A was designed, fabricated and tested for material processing applications. The magnet consists of two coaxial NbTi solenoid coils with an identical vertical height of 300 mm and is installed in a high-vacuumed cryostat. A two-stage GM cryocooler with a cooling power of 1.5 W at 4.2 K in the second stage is used to cool the system from room temperature to 4.2 K. In this paper, the detailed design, fabrication, thermal analysis and tests of the system are presented.

Interfacial Contact Resistance Along the Thermal Link in Conduction-Cooled Low-Temperature Superconducting Magnet System

We have developed a conduction-cooled superconducting magnet system (CCSMS) using a low-temperature superconducting (SC) coil. The CCSMS consists of several metal components in contact with each other between the cryocooler and SC magnet. The thermal contact resistance (TCR) occurring at these metal interfaces may affect the CCSMS. Therefore, accurate measurement of TCR is essential for the design and development of CCSMS. We fabricated a TCR measurement system using a cryocooler as a heat sink. The TCR was calculated using the temperature gradient across the metal interface measured in the steady state. The temperature distribution along the thermal link in the CCSMS was estimated using the measured TCR and compared to the experimental values. The influence of surface roughness on the temperature distribution in the CCSMS is also discussed.

Intermediate Joint of Current Lead in Conduction-Cooled Superconducting Magnet System

The semi-retractable current lead is composed of a normal metal element, conducting the current from room temperature to intermediate temperature, and a high-temperature superconducting element, conducting the current down to liquid helium temperature. The normal metal element is disengaged from the high-temperature superconducting element through the multicontact connector without disturbance to the insulating vacuum space and without requiring complete removal of the normal metal element. The current lead is applied into the conduction-cooled superconducting magnet system developed by ourselves. The intermediate joint with a lockable set point is thermally connected to the first-stage of a cryocooler and carries current through a strip of louvered material. The performance of current lead is investigated with respect to the electrical contact resistance during the magnet charging process. The effects of current level and operating temperature on the heat generation in the intermediate joint are also discussed.

Practical Application of Gas-Gap Thermal Switch in Conduction Cooled Superconducting Magnet System

A gas-gap thermal switch is developed to reduce the cool-down time of conduction cooled superconducting magnet. The main feature of the thermal switch is that it is sealed naturally at atmospheric environment in comparison with other gas-gap thermal switch, thereby it is much greater convenient during assembly. The switch is a closed container that contains several pairs of concentric tube-fins arranged in staggered way and it is located between first stage and second stage of GM cryocooler. At the beginning of the cool down, the thermal switch can transfer power from the first stage with the higher refrigeration capacity to the magnet. So the cool-down rate of the magnet is increased. To check the performance of the switch, we use a superconducting magnet for gyrotron whose weight is 41.5 kg as background device. The experiment shows that the cool-down time is shortened from 25 hours to 20 hours.

Optimal cool-down time of a 4 K superconducting magnet cooled by a two-stage cryocooler

A cool-down time is one of the major factors in many cryocooler applications, especially for the design of conduction-cooled superconducting devices. Cool-down time means a time cooling a thermal mass from a room-temperature to cryogenic-temperature within a stipulated amount of time. The estimation of cool-down time seeks the elapsed time to cool the thermal object by a cryocooler during initial cool-down process. This procedure includes the dimension and properties of thermal object, heat transfer analysis for cryogenic load, thermal interface between cold mass and cryocooler, and available refrigeration capacity of cryocooler. The proposed method is applied to the specific cooling system for 3 T superconducting magnet cooled by a two-stage GM cryocooler. The result is compared with that of experiment, showing that proposed method has a good agreement with experiment. In addition, the initial cool-down time can be shortened by employing thermal link between the cold mass and first-stage of cryocooler. Through a rigorous modeling and analysis taking into account the effect of thermal link size, it is concluded that there exists an optimal cool-down time during initial cooling in conduction-cooled superconducting magnet system.

Initial Cooldown Characteristic of Conduction-Cooled High Field Magnet System

Cooldown time is one of the major factors in the conduction-cooled superconducting magnet system. Cooldown time means a time cooling a superconducting magnet from room temperature to cryogenic temperature within a stipulated amount of time. The estimation of cooldown time seeks the elapsed time to cool the superconducting magnet by cryocooler during initial cooldown process. This procedure includes the modeling of properties of low temperature superconductor, the dimensions of superconducting magnet, the cryogenic loads, the thermal link between magnet and cryocooler, and the available refrigerating capacity of cryocooler. This method is applied to the conduction-cooled superconducting magnet system developed by ourselves for accuracy confirmation. The effects of magnetic field, size of magnet and thermal link on the initial cooldown are also discussed.

Cool-down characteristic of conduction-cooled superconducting magnet by a cryocooler

The initial cooling is crucial in conduction-cooled superconducting magnet system. Since a cryocooler is the only heat sink in conduction-cooled superconducting magnet system, the initial cool-down time is strongly dependent upon the refrigeration capacity of cryocooler. In this paper, the process of estimation of initial cool-down time is presented. The procedure includes the modeling of properties of low temperature superconductor, the dimensions of superconducting magnet, the cryogenic loads, the thermal link between magnet and cryocooler, and the available refrigerating capacity of a cryocooler. The method is applied to the 3T superconducting magnet cooled by a two-stage GM cryocooler, showing that the results of proposed method have a good agreement with those of experiment. The effects of superconducting magnet size and secondary thermal link on the cool-down characteristic are also discussed.

Development of Wide-Bore Conduction-Cooled Superconducting Magnet System for Material Processing Applications

The application of high magnetic field to material processing, so called electromagnetic processing of material (EPM) has been recognized as cutting edge technology, especially in the field of advanced material processing. It is the most effective methods to control thermal, mass and energy transfer during material solidification. A wide-bore conduction-cooled superconducting magnet with operating current of 116 A was designed, fabricated and tested for the material processing devices. The superconducting magnet has the effective warm hole of 18 cm, the maximum center field of 6 T and homogeneity of 5% in diameter of 5 cm. After the Nb/sub 3/Sn coil insert installed, the magnet can provide the maximum center field of 10 T with effective warm bore of 10 cm. A second-stage GM cryocooler with the second-stage cooling power of 1 W is used to cool the superconducting magnet from room temperature to 4.2 K. In this paper, the design, fabrication, test, stress analysis and quench protection characteristics are presented.